This is libtool.info, produced by makeinfo version 7.1.1 from
libtool.texi.

This manual is for GNU Libtool (version 2.6.1, 3 June 2026).

   Copyright © 1996-2019, 2021-2026 Free Software Foundation, Inc.

   Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, with no Front-Cover Texts, and with no Back-Cover
Texts.  A copy of the license is included in the section entitled "GNU
Free Documentation License".
INFO-DIR-SECTION Software development
START-INFO-DIR-ENTRY
* Libtool: (libtool).           Generic shared library support script.
END-INFO-DIR-ENTRY

INFO-DIR-SECTION Individual utilities
START-INFO-DIR-ENTRY
* libtool-invocation: (libtool)Invoking libtool. Running the ‘libtool’ script.
* libtoolize: (libtool)Invoking libtoolize.      Adding libtool support.
* libtool-next-version: (libtool)Invoking libtool-next-version. Running the ‘libtool-next-version’ wizard.
END-INFO-DIR-ENTRY


File: libtool.info,  Node: Top,  Next: Introduction,  Prev: (dir),  Up: (dir)

Shared library support for GNU
******************************

This file documents GNU Libtool, a script that allows package developers
to provide generic shared library support.  This edition documents
version 2.6.1.

   *Note Reporting bugs::, for information on how to report problems
with GNU Libtool.

* Menu:

* Introduction::                What the heck is libtool?
* Libtool paradigm::            How libtool's view of libraries is different.
* Using libtool::               Example of using libtool to build libraries.
* Invoking libtool::            Running the ‘libtool’ script.
* Integrating libtool::         Using libtool in your own packages.
* Other languages::             Using libtool without a C compiler.
* Versioning::                  Using library interface versions.
* Library tips::                Tips for library interface design.
* Inter-library dependencies::  Libraries that depend on other libraries.
* Dlopened modules::            ‘dlopen’ing libtool-created libraries.
* Using libltdl::               Libtool's portable ‘dlopen’ wrapper library.
* Trace interface::             Libtool's trace interface.
* FAQ::                         Frequently Asked Questions
* Troubleshooting::             When libtool doesn't work as advertised.
* Maintaining::                 Information used by the libtool maintainer.
* GNU Free Documentation License::  License for this manual.
* Combined Index::              Full index.

 -- The Detailed Node Listing --

Introduction

* Motivation::                  Why does GNU need a libtool?
* Issues::                      The problems that need to be addressed.
* Other implementations::       How other people have solved these issues.
* Postmortem::                  Learning from past difficulties.

Using libtool

* Creating object files::       Compiling object files for libraries.
* Linking libraries::           Creating libraries from object files.
* Linking executables::         Linking object files against libtool libraries.
* Debugging executables::       Running GDB on libtool-generated programs.
* Installing libraries::        Making libraries available to users.
* Installing executables::      Making programs available to users.
* Static libraries::            When shared libraries are not wanted.

Linking executables

* Wrapper executables::         Wrapper executables for some platforms.

Invoking ‘libtool’

* Compile mode::                Creating library object files.
* Link mode::                   Generating executables and libraries.
* Execute mode::                Debugging libtool-generated programs.
* Install mode::                Making libraries and executables public.
* Finish mode::                 Completing a library installation.
* Uninstall mode::              Removing installed executables and libraries.
* Clean mode::                  Removing uninstalled executables and libraries.

Integrating libtool with your package

* Autoconf macros::             Autoconf macros exported by libtool.
* Makefile rules::              Writing ‘Makefile’ rules for libtool.
* Using Automake::              Automatically supporting libtool.
* Configuring::                 Configuring libtool for a host system.
* Distributing::                What files to distribute with your package.
* Static-only libraries::       Sometimes shared libraries are just a pain.

Configuring libtool

* LT_INIT::                     Configuring ‘libtool’ in ‘configure.ac’.
* Configure notes::             Platform-specific notes for configuration.

Including libtool in your package

* Invoking libtoolize::         ‘libtoolize’ command line options.
* Autoconf and LTLIBOBJS::      Autoconf automates LTLIBOBJS generation.

Using libtool with other languages

* C++ libraries::               Writing libraries for C++
* Tags::                        Tags

Library interface versions

* Interfaces::                  What are library interfaces?
* Libtool versioning::          Libtool's versioning system.
* Updating version info::       Changing version information before releases.
* Release numbers::             Breaking binary compatibility for aesthetics.

Tips for interface design

* C header files::              How to write portable include files.

Dlopened modules

* Building modules::            Creating dlopenable objects and libraries.
* Dlpreopening::                Dlopening that works on static platforms.
* Linking with dlopened modules::  Using dlopenable modules in libraries.
* Finding the dlname::          Choosing the right file to ‘dlopen’.
* Dlopen issues::               Unresolved problems that need your attention.

Using libltdl

* Libltdl interface::           How to use libltdl in your programs.
* Modules for libltdl::         Creating modules that can be ‘dlopen’ed.
* Thread Safety in libltdl::    Registering callbacks for multi-thread safety.
* User defined module data::    Associating data with loaded modules.
* Module loaders for libltdl::  Creating user defined module loaders.
* Distributing libltdl::        How to distribute libltdl with your package.

Frequently Asked Questions about libtool

* Stripped link flags::         Dropped flags when creating a library

Troubleshooting

* Libtool test suite::          Libtool's self-tests.
* Reporting bugs::              How to report problems with libtool.

The libtool test suite

* Test descriptions::           The contents of the old test suite.
* When tests fail::             What to do when a test fails.

Maintenance notes for libtool

* New ports::                   How to port libtool to new systems.
* Tested platforms::            When libtool was last tested.
* Platform quirks::             Information about different library systems.
* libtool script contents::     Configuration information that libtool uses.
* Cheap tricks::                Making libtool maintainership easier.

Porting libtool to new systems

* Information sources::         Where to find relevant documentation
* Porting inter-library dependencies::  Implementation details explained

Platform quirks

* Compilers::                   Creating object files from source files.
* Reloadable objects::          Binding object files together.
* Multiple dependencies::       Removing duplicate dependent libraries.
* Archivers::                   Programs that create static archives.
* Cross compiling::             Issues that arise when cross compiling.
* File name conversion::        Converting file names between platforms.
* Windows DLLs::                Windows header defines.

File name conversion

* File Name Conversion Failure::  What happens when file name conversion fails
* Native MinGW File Name Conversion::  MSYS file name conversion idiosyncrasies
* Cygwin/Windows File Name Conversion::  Using ‘cygpath’ to convert Cygwin file names
* Unix/Windows File Name Conversion::  Using Wine to convert Unix paths
* LT_CYGPATH::                  Invoking ‘cygpath’ from other environments
* Cygwin to MinGW Cross::       Other notes concerning MinGW cross



File: libtool.info,  Node: Introduction,  Next: Libtool paradigm,  Up: Top

1 Introduction
**************

In the past, if you were a source code package developer and wanted to
take advantage of the power of shared libraries, you needed to write
custom support code for each platform on which your package ran.  You
also had to design a configuration interface so that the package
installer could choose what sort of libraries were built.

   GNU Libtool simplifies your job by encapsulating both the
platform-specific dependencies, and the user interface, in a single
script.  GNU Libtool is designed so that the complete functionality of
each host type is available via a generic interface, but nasty quirks
are hidden from the programmer.

   GNU Libtool's consistent interface is reassuring... users don't need
to read obscure documentation to have their favorite source package
build shared libraries.  They just run your package ‘configure’ script
(or equivalent), and libtool does all the dirty work.

   There are several examples throughout this document.  All assume the
same environment: we want to build a library, ‘libhello’, in a generic
way.

   ‘libhello’ could be a shared library, a static library, or both...
whatever is available on the host system, as long as libtool has been
ported to it.

   This chapter explains the original design philosophy of libtool.
Feel free to skip to the next chapter, unless you are interested in
history, or want to write code to extend libtool in a consistent way.

* Menu:

* Motivation::                  Why does GNU need a libtool?
* Issues::                      The problems that need to be addressed.
* Other implementations::       How other people have solved these issues.
* Postmortem::                  Learning from past difficulties.


File: libtool.info,  Node: Motivation,  Next: Issues,  Up: Introduction

1.1 Motivation for writing libtool
==================================

Since early 1995, several different GNU developers have recognized the
importance of having shared library support for their packages.  The
primary motivation for such a change is to encourage modularity and
reuse of code (both conceptually and physically) in GNU programs.

   Such a demand means that the way libraries are built in GNU packages
needs to be general, to allow for any library type the package installer
might want.  The problem is compounded by the absence of a standard
procedure for creating shared libraries on different platforms.

   The following sections outline the major issues facing shared library
support in GNU, and how shared library support could be standardized
with libtool.

   The following specifications were used in developing and evaluating
this system:

  1. The system must be as elegant as possible.

  2. The system must be fully integrated with the GNU Autoconf and
     Automake utilities, so that it will be easy for GNU maintainers to
     use.  However, the system must not require these tools, so that it
     can be used by non-GNU packages.

  3. Portability to other (non-GNU) architectures and tools is
     desirable.


File: libtool.info,  Node: Issues,  Next: Other implementations,  Prev: Motivation,  Up: Introduction

1.2 Implementation issues
=========================

The following issues need to be addressed in any reusable shared library
system, specifically libtool:

  1. The package installer should be able to control what sort of
     libraries are built.

  2. It can be tricky to run dynamically linked programs whose libraries
     have not yet been installed.  ‘LD_LIBRARY_PATH’ must be set
     properly (if it is supported), or programs fail to run.

  3. The system must operate consistently even on hosts that don't
     support shared libraries.

  4. The commands required to build shared libraries may differ wildly
     from host to host.  These need to be determined at configure time
     in a consistent way.

  5. It is not always obvious with what prefix or suffix a shared
     library should be installed.  This makes it difficult for
     ‘Makefile’ rules, since they generally assume that file names are
     the same from host to host.

  6. The system needs a simple library version number abstraction, so
     that shared libraries can be upgraded in place.  The programmer
     should be informed how to design the interfaces to the library to
     maximize binary compatibility.

  7. The install ‘Makefile’ target should warn the package installer to
     set the proper environment variables (‘LD_LIBRARY_PATH’ or
     equivalent), or run ‘ldconfig’.


File: libtool.info,  Node: Other implementations,  Next: Postmortem,  Prev: Issues,  Up: Introduction

1.3 Other implementations
=========================

Even before libtool was developed, many free software packages built and
installed their own shared libraries.  At first, these packages were
examined to avoid reinventing existing features.

   Now it is clear that none of these packages have documented the
details of shared library systems that libtool requires.  So, other
packages have been more or less abandoned as influences.


File: libtool.info,  Node: Postmortem,  Prev: Other implementations,  Up: Introduction

1.4 A postmortem analysis of other implementations
==================================================

In all fairness, each of the implementations that were examined do the
job that they were intended to do, for a number of different host
systems.  However, none of these solutions seem to function well as a
generalized, reusable component.

   Most were too complex to use (much less modify) without understanding
exactly what the implementation does, and they were generally not
documented.

   The main difficulty is that different vendors have different views of
what libraries are, and none of the packages that were examined seemed
to be confident enough to settle on a single paradigm that just _works_.

   Ideally, libtool would be a standard that would be implemented as
series of extensions and modifications to existing library systems to
make them work consistently.  However, it is not an easy task to
convince operating system developers to mend their evil ways, and people
want to build shared libraries right now, even on buggy, broken,
confused operating systems.

   For this reason, libtool was designed as an independent shell script.
It isolates the problems and inconsistencies in library building that
plague ‘Makefile’ writers by wrapping the compiler suite on different
platforms with a consistent, powerful interface.

   With luck, libtool will be useful to and used by the GNU community,
and that the lessons that were learned in writing it will be taken up by
designers of future library systems.


File: libtool.info,  Node: Libtool paradigm,  Next: Using libtool,  Prev: Introduction,  Up: Top

2 The libtool paradigm
**********************

At first, libtool was designed to support an arbitrary number of library
object types.  After libtool was ported to more platforms, a new
paradigm gradually developed for describing the relationship between
libraries and programs.

   In summary, "libraries are programs with multiple entry points, and
more formally defined interfaces."

   The best way to introduce the libtool paradigm is to contrast it with
the paradigm of existing library systems, with examples from each.  It
is a new way of thinking, so it may take a little time to absorb, but
when you understand it, the world becomes simpler.


File: libtool.info,  Node: Using libtool,  Next: Invoking libtool,  Prev: Libtool paradigm,  Up: Top

3 Using libtool
***************

It makes little sense to talk about using libtool in your own packages
until you have seen how it makes your life simpler.  The examples in
this chapter introduce the main features of libtool by comparing the
standard library building procedure to libtool's operation on two
different platforms:

‘a23’
     An Ultrix 4.2 platform with only static libraries.

‘burger’
     A NetBSD/i386 1.2 platform with shared libraries.

   Source files for the following examples are taken from the Autotest
file ‘tests/demo.at’.  The files can be extracted by running a demo test
and preserving the artifacts:

     burger$ make check TESTSUITEFLAGS="-d -k 'preloaded static library'"
     burger$ cp -r tests/testsuite.dir/027 demo/

   You can follow these examples on your own platform, using the
preconfigured libtool script that was installed with libtool (*note
Configuring::).  Assume that we are building a library, ‘libhello’, out
of the files ‘foo.c’ and ‘hello.c’.

   Note that the ‘foo.c’ source file uses the ‘cos’ math library
function, which is usually found in the standalone math library, and not
the C library (*note Trigonometric Functions: (libc)Trig Functions.).
So, we need to add ‘-lm’ to the end of the link line whenever we link
‘foo.lo’ into an executable or a library (*note Inter-library
dependencies::).

   The same rule applies whenever you use functions that don't appear in
the standard C library... you need to add the appropriate ‘-lNAME’ flag
to the end of the link line when you link against those objects.

   After we have built that library, we want to create a program by
linking ‘main.o’ against ‘libhello’.

* Menu:

* Creating object files::       Compiling object files for libraries.
* Linking libraries::           Creating libraries from object files.
* Linking executables::         Linking object files against libtool libraries.
* Debugging executables::       Running GDB on libtool-generated programs.
* Installing libraries::        Making libraries available to users.
* Installing executables::      Making programs available to users.
* Static libraries::            When shared libraries are not wanted.


File: libtool.info,  Node: Creating object files,  Next: Linking libraries,  Up: Using libtool

3.1 Creating object files
=========================

To create an object file from a source file, the compiler is invoked
with the ‘-c’ flag (and any other desired flags):

     burger$ gcc -I. -g -O -c main.c
     burger$

   The above compiler command produces an object file, usually named
‘main.o’, from the source file ‘main.c’.

   For most library systems, creating object files that become part of a
static library is as simple as creating object files that are linked to
form an executable:

     burger$ gcc -I. -g -O -c foo.c
     burger$ gcc -I. -g -O -c hello.c
     burger$

   Shared libraries, however, may only be built from
“position-independent code” (PIC). So, special flags must be passed to
the compiler to tell it to generate PIC rather than the standard
position-dependent code.

   Since this is a library implementation detail, libtool hides the
complexity of PIC compiler flags and uses separate library object files
(the PIC one lives in the ‘.libs’ subdirectory and the static one lives
in the current directory).  On systems without shared libraries, the PIC
library object files are not created, whereas on systems where all code
is PIC, such as AIX, the static ones are not created.

   To create library object files for ‘foo.c’ and ‘hello.c’, simply
invoke libtool with the standard compilation command as arguments (*note
Compile mode::):

     a23$ libtool --mode=compile gcc -I. -g -O -c foo.c
     gcc -I. -g -O -c foo.c -o foo.o
     a23$ libtool --mode=compile gcc -I. -g -O -c hello.c
     gcc -I. -g -O -c hello.c -o hello.o
     a23$

   Note that libtool silently creates an additional control file on each
‘compile’ invocation.  The ‘.lo’ file is the libtool object, which
Libtool uses to determine what object file may be built into a shared
library.  On ‘a23’, only static libraries are supported so the library
objects look like this:

     # foo.lo - a libtool object file
     # Generated by ltmain.sh (GNU libtool) 2.6.1
     #
     # Please DO NOT delete this file!
     # It is necessary for linking the library.

     # Name of the PIC object.
     pic_object=none

     # Name of the non-PIC object.
     non_pic_object='foo.o'

   On shared library systems, libtool automatically generates an
additional PIC object by inserting the appropriate PIC generation flags
into the compilation command:

     burger$ libtool --mode=compile gcc -I. -g -O -c foo.c
     mkdir .libs
     gcc -I. -g -O -c foo.c  -fPIC -DPIC -o .libs/foo.o
     gcc -I. -g -O -c foo.c -o foo.o >/dev/null 2>&1
     burger$

   Note that Libtool automatically created the ‘.libs’ directory upon
its first execution, where PIC library object files will be stored.

   Since ‘burger’ supports shared libraries, and requires PIC objects to
build them, Libtool has compiled a PIC object and made a note of it in
the libtool object:

     # foo.lo - a libtool object file
     # Generated by ltmain.sh (GNU libtool) 2.6.1
     #
     # Please DO NOT delete this file!
     # It is necessary for linking the library.

     # Name of the PIC object.
     pic_object='.libs/foo.o'

     # Name of the non-PIC object.
     non_pic_object='foo.o'

   Notice the second run of GCC has its output discarded.  This is done
so the compiler warnings aren't annoyingly duplicated.  If you need to
see both sets of warnings (you might have conditional code inside
‘#ifdef PIC’ for example), you can turn off suppression by passing the
‘-no-suppress’ option to libtool's compile mode:

     burger$ libtool --mode=compile gcc -no-suppress -I. -g -O -c hello.c
     gcc -I. -g -O -c hello.c  -fPIC -DPIC -o .libs/hello.o
     gcc -I. -g -O -c hello.c -o hello.o
     burger$


File: libtool.info,  Node: Linking libraries,  Next: Linking executables,  Prev: Creating object files,  Up: Using libtool

3.2 Linking libraries
=====================

Without libtool, the programmer would invoke the ‘ar’ command to create
a static library:

     burger$ ar cr libhello.a hello.o foo.o
     burger$

   But of course, that would be too simple, so many systems require that
you run the ‘ranlib’ command on the resulting library in order to
generate a symbol table:

     burger$ ranlib libhello.a
     burger$

   It seems more natural to use the C compiler for this task, given
libtool's "libraries are programs" approach.  So, on platforms without
shared libraries, libtool simply acts as a wrapper for the system ‘ar’
(and possibly ‘ranlib’) commands.

   Again, the libtool control file name (‘.la’ suffix) differs from the
standard library name (‘.a’ suffix).  The arguments to libtool are the
same ones you would use to produce an executable named ‘libhello.la’
with your compiler (*note Link mode::):

     a23$ libtool --mode=link gcc -g -O -o libhello.la foo.o hello.o
     *** Warning: Linking the shared library libhello.la against the
     *** non-libtool objects foo.o hello.o is not portable!
     ar cr .libs/libhello.a
     ranlib .libs/libhello.a
     creating libhello.la
     (cd .libs && rm -f libhello.la && ln -s ../libhello.la libhello.la)
     a23$

   Aha!  Libtool caught a common error... trying to build a library from
standard objects instead of special ‘.lo’ object files.  This doesn't
matter so much for static libraries, but on shared library systems, it
is of great importance.  (Note that you may replace ‘libhello.la’ with
‘libhello.a’ in which case libtool won't issue the warning any more.
But although this method works, this is not intended to be used because
it makes you lose the benefits of using Libtool.)

   So, let's try again, this time with the library object files.
Remember also that we need to add ‘-lm’ to the link command line because
‘foo.c’ uses the ‘cos’ math library function (*note Using libtool::).

   Another complication in building shared libraries is that we need to
specify the path to the directory where they will (eventually) be
installed (in this case, ‘/usr/local/lib’)(1):

     a23$ libtool --mode=link gcc -g -O -o libhello.la foo.lo hello.lo \
                     -rpath /usr/local/lib -lm
     ar cr .libs/libhello.a foo.o hello.o
     ranlib .libs/libhello.a
     creating libhello.la
     (cd .libs && rm -f libhello.la && ln -s ../libhello.la libhello.la)
     a23$

   Now, let's try the same trick on the shared library platform:

     burger$ libtool --mode=link gcc -g -O -o libhello.la foo.lo hello.lo \
                     -rpath /usr/local/lib -lm
     rm -fr  .libs/libhello.a .libs/libhello.la
     ld -Bshareable -o .libs/libhello.so.0.0 .libs/foo.o .libs/hello.o -lm
     ar cr .libs/libhello.a foo.o hello.o
     ranlib .libs/libhello.a
     creating libhello.la
     (cd .libs && rm -f libhello.la && ln -s ../libhello.la libhello.la)
     burger$

   Now that's significantly cooler... Libtool just ran an obscure ‘ld’
command to create a shared library, as well as the static library.

   Note how libtool creates extra files in the ‘.libs’ subdirectory,
rather than the current directory.  This feature makes it easier to
clean up the build directory, and helps ensure other programs fail
horribly if you accidentally forget to use libtool when you should.

   Again, you should look at the ‘.la’ file to see what Libtool stores
in it.  You will see Libtool uses this file to remember the destination
directory for the library (the argument to ‘-rpath’) as well as the
dependency on the math library (‘-lm’).

   ---------- Footnotes ----------

   (1) If you don't specify an ‘rpath’, then libtool builds a libtool
convenience archive, not a shared library (*note Static libraries::).


File: libtool.info,  Node: Linking executables,  Next: Debugging executables,  Prev: Linking libraries,  Up: Using libtool

3.3 Linking executables
=======================

If you choose at this point to “install” the library (put it in a
permanent location) before linking executables against it, then you
don't need to use libtool to do the linking.  Simply use the appropriate
‘-L’ and ‘-l’ flags to specify the library's location.

   Some system linkers insist on encoding the full directory name of
each shared library in the resulting executable.  Libtool has to work
around this misfeature by special magic to ensure that only permanent
directory names are put into installed executables.

   The importance of this bug must not be overlooked: it won't cause
programs to crash in obvious ways.  It creates a security hole, and
possibly even worse, if you are modifying the library source code after
you have installed the package, you will change the behaviour of the
installed programs!

   So, if you want to link programs against the library before you
install it, you must use libtool to do the linking.

   Here's the old way of linking against an uninstalled library:

     burger$ gcc -g -O -o hell.old main.o libhello.a -lm
     burger$

   Libtool's way is almost the same(1) (*note Link mode::):

     a23$ libtool --mode=link gcc -g -O -o hell main.o libhello.la
     gcc -g -O -o hell main.o  ./.libs/libhello.a -lm
     a23$

   That looks too simple to be true.  All libtool did was transform
‘libhello.la’ to ‘./.libs/libhello.a’, but remember that ‘a23’ has no
shared libraries.  Notice Libtool also remembered ‘libhello.la’ depends
on ‘-lm’, so even though we didn't specify ‘-lm’ on the libtool command
line(2) Libtool has added it to the ‘gcc’ link line for us.

   On ‘burger’ Libtool links against the uninstalled shared library:

     burger$ libtool --mode=link gcc -g -O -o hell main.o libhello.la
     gcc -g -O -o .libs/hell main.o -L./.libs -R/usr/local/lib -lhello -lm
     creating hell
     burger$

   Now assume ‘libhello.la’ had already been installed, and you want to
link a new program with it.  You could figure out where it lives by
yourself, then run:

     burger$ gcc -g -O -o test test.o -L/usr/local/lib -lhello -lm

   However, unless ‘/usr/local/lib’ is in the standard library search
path, you won't be able to run ‘test’.  If instead you use libtool to
link the already-installed libtool library, it will do The Right Thing
(TM) for you:

     burger$ libtool --mode=link gcc -g -O -o test test.o \
                     /usr/local/lib/libhello.la
     gcc -g -O -o .libs/test test.o -Wl,--rpath \
             -Wl,/usr/local/lib /usr/local/lib/libhello.a -lm
     creating test
     burger$

   Note that libtool added the necessary run-time path flag, as well as
‘-lm’, the library libhello.la depended upon.  Nice, huh?

   Notice the executable, ‘hell’, was actually created in the ‘.libs’
subdirectory.  Then, a wrapper script (or, on certain platforms, a
wrapper executable *note Wrapper executables::) was created in the
current directory.

   Since libtool created a wrapper script, you should use libtool to
install it and debug it too.  However, since the program does not depend
on any uninstalled libtool library, it is probably usable even without
the wrapper script.

   On NetBSD 1.2, libtool encodes the installation directory of
‘libhello’, by using the ‘-R/usr/local/lib’ compiler flag.  Then, the
wrapper script guarantees the executable finds the correct shared
library (the one in ‘./.libs’) so it can be properly installed.

   Let's compare the two different programs:

     burger$ time ./hell.old
     Welcome to GNU Hell!
     ** This is not GNU Hello.  There is no built-in mail reader. **
             0.21 real         0.02 user         0.08 sys
     burger$ time ./hell
     Welcome to GNU Hell!
     ** This is not GNU Hello.  There is no built-in mail reader. **
             0.63 real         0.09 user         0.59 sys
     burger$

   The wrapper script takes significantly longer to execute, but at
least the results are correct, even though the shared library hasn't
been installed yet.

   So, what about all the space savings that shared libraries are
supposed to yield?

     burger$ ls -l hell.old libhello.a
     -rwxr-xr-x  1 gord  gord  15481 Nov 14 12:11 hell.old
     -rw-r--r--  1 gord  gord   4274 Nov 13 18:02 libhello.a
     burger$ ls -l .libs/hell .libs/libhello.*
     -rwxr-xr-x  1 gord  gord  11647 Nov 14 12:10 .libs/hell
     -rw-r--r--  1 gord  gord   4274 Nov 13 18:44 .libs/libhello.a
     -rwxr-xr-x  1 gord  gord  12205 Nov 13 18:44 .libs/libhello.so.0.0
     burger$

   Well, that sucks.  Maybe I should just scrap this project and take up
basket weaving.

   Actually, it just proves an important point: shared libraries incur
overhead because of their (relative) complexity.  In this situation, the
price of being dynamic is eight kilobytes, and the payoff is about four
kilobytes.  So, having a shared ‘libhello’ won't be an advantage until
we link it against at least a few more programs.

* Menu:

* Wrapper executables::         Wrapper executables for some platforms.

   ---------- Footnotes ----------

   (1) However, you should avoid using ‘-L’ or ‘-l’ flags to link
against an uninstalled libtool library.  Just specify the relative path
to the ‘.la’ file, such as ‘../intl/libintl.la’.  This is a design
decision to eliminate any ambiguity when linking against uninstalled
shared libraries.

   (2) And why should we?  ‘main.o’ doesn't directly depend on ‘-lm’
after all.


File: libtool.info,  Node: Wrapper executables,  Up: Linking executables

3.3.1 Wrapper executables for uninstalled programs
--------------------------------------------------

Some platforms, notably those hosted on Windows such as Cygwin and
MinGW, use a wrapper executable rather than a wrapper script to ensure
proper operation of uninstalled programs linked by libtool against
uninstalled shared libraries.  The wrapper executable thus performs the
same function as the wrapper script used on other platforms, but allows
to satisfy the ‘make’ rules for the program, whose name ends in
‘$(EXEEXT)’.  The actual program executable is created below ‘.libs’,
and its name will end in ‘$(EXEEXT)’ and may or may not contain an ‘lt-’
prefix.  This wrapper executable sets various environment values so that
the program executable may locate its (uninstalled) shared libraries,
and then launches the program executable.

   The wrapper executable provides a debug mode, enabled by passing the
command-line option ‘--lt-debug’ (see below).  When executing in debug
mode, diagnostic information will be printed to ‘stderr’ before the
program executable is launched.

   Finally, the wrapper executable supports a number of command line
options that may be useful when debugging the operation of the wrapper
system.  All of these options begin with ‘--lt-’, and if present they
and their arguments will be removed from the argument list passed on to
the program executable.  Therefore, the program executable may not
employ command line options that begin with ‘--lt-’.  (In fact, the
wrapper executable will detect any command line options that begin with
‘--lt-’ and abort with an error message if the option is not
recognized.)  If this presents a problem, please contact the Libtool
team at the Libtool bug reporting address <bug-libtool@gnu.org>.

   These command line options include:

‘--lt-dump-script’
     Causes the wrapper to print a copy of the wrapper _script_ to
     ‘stdout’, and exit.

‘--lt-debug’
     Causes the wrapper to print diagnostic information to ‘stdout’,
     before launching the program executable.

   For consistency, both the wrapper _script_ and the wrapper
_executable_ support these options.


File: libtool.info,  Node: Debugging executables,  Next: Installing libraries,  Prev: Linking executables,  Up: Using libtool

3.4 Debugging executables
=========================

If ‘hell’ was a complicated program, you would certainly want to test
and debug it before installing it on your system.  In the above section,
you saw how the libtool wrapper script makes it possible to run the
program directly, but unfortunately, this mechanism interferes with the
debugger:

     burger$ gdb hell
     GDB is free software and you are welcome to distribute copies of it
      under certain conditions; type "show copying" to see the conditions.
     There is no warranty for GDB; type "show warranty" for details.
     GDB 4.16 (i386-unknown-netbsd), (C) 1996 Free Software Foundation, Inc.

     "hell": not in executable format: File format not recognized

     (gdb) quit
     burger$

   Sad.  It doesn't work because GDB doesn't know where the executable
lives.  So, let's try again, by invoking GDB directly on the executable:

     burger$ gdb .libs/hell
     GNU gdb 5.3 (i386-unknown-netbsd)
     Copyright © 1996--2019, 2021--2026 Free Software Foundation,
     Inc.
     GDB is free software, covered by the GNU General Public License,
     and you are welcome to change it and/or distribute copies of it
     under certain conditions.  Type "show copying" to see the conditions.
     There is no warranty for GDB.  Type "show warranty" for details.
     (gdb) break main
     Breakpoint 1 at 0x8048547: file main.c, line 29.
     (gdb) run
     Starting program: /home/src/libtool/demo/.libs/hell
     /home/src/libtool/demo/.libs/hell: can't load library 'libhello.so.0'

     Program exited with code 020.
     (gdb) quit
     burger$

   Argh.  Now GDB complains because it cannot find the shared library
that ‘hell’ is linked against.  So, we must use libtool to properly set
the library path and run the debugger.  Fortunately, we can forget all
about the ‘.libs’ directory, and just run it on the executable wrapper
(*note Execute mode::):

     burger$ libtool --mode=execute gdb hell
     GNU gdb 5.3 (i386-unknown-netbsd)
     Copyright © 1996--2019, 2021--2026 Free Software Foundation,
     Inc.
     GDB is free software, covered by the GNU General Public License,
     and you are welcome to change it and/or distribute copies of it
     under certain conditions.  Type "show copying" to see the conditions.
     There is no warranty for GDB.  Type "show warranty" for details.
     (gdb) break main
     Breakpoint 1 at 0x8048547: file main.c, line 29.
     (gdb) run
     Starting program: /home/src/libtool/demo/.libs/hell

     Breakpoint 1, main (argc=1, argv=0xbffffc40) at main.c:29
     29        printf ("Welcome to GNU Hell!\n");
     (gdb) quit
     The program is running.  Quit anyway (and kill it)? (y or n) y
     burger$


File: libtool.info,  Node: Installing libraries,  Next: Installing executables,  Prev: Debugging executables,  Up: Using libtool

3.5 Installing libraries
========================

Installing libraries on a non-libtool system is quite straightforward...
just copy them into place:(1)

     burger$ su
     Password: ********
     burger# cp libhello.a /usr/local/lib/libhello.a
     burger#

   Oops, don't forget the ‘ranlib’ command:

     burger# ranlib /usr/local/lib/libhello.a
     burger#

   Libtool installation is quite simple, as well.  Just use the
‘install’ or ‘cp’ command that you normally would (*note Install
mode::):

     a23# libtool --mode=install cp libhello.la /usr/local/lib/libhello.la
     cp libhello.la /usr/local/lib/libhello.la
     cp .libs/libhello.a /usr/local/lib/libhello.a
     ranlib /usr/local/lib/libhello.a
     a23#

   Note that the libtool library ‘libhello.la’ is also installed, to
help libtool with uninstallation (*note Uninstall mode::) and linking
(*note Linking executables::) and to help programs with dlopening (*note
Dlopened modules::).

   Here is the shared library example:

     burger# libtool --mode=install install -c libhello.la \
                     /usr/local/lib/libhello.la
     install -c .libs/libhello.so.0.0 /usr/local/lib/libhello.so.0.0
     install -c libhello.la /usr/local/lib/libhello.la
     install -c .libs/libhello.a /usr/local/lib/libhello.a
     ranlib /usr/local/lib/libhello.a
     burger#

   It is safe to specify the ‘-s’ (strip symbols) flag if you use a
BSD-compatible install program when installing libraries.  Libtool will
either ignore the ‘-s’ flag, or will run a program that will strip only
debugging and compiler symbols from the library.

   Once the libraries have been put in place, there may be some
additional configuration that you need to do before using them.  First,
you must make sure that where the library is installed actually agrees
with the ‘-rpath’ flag you used to build it.

   Then, running ‘libtool -n finish LIBDIR’ can give you further hints
on what to do (*note Finish mode::):

     burger# libtool -n finish /usr/local/lib
     PATH="$PATH:/sbin" ldconfig -m /usr/local/lib
     -----------------------------------------------------------------
     Libraries have been installed in:
        /usr/local/lib

     To link against installed libraries in a given directory, LIBDIR,
     you must use the '-LLIBDIR' flag during linking.

      You will also need to do one of the following:
        - add LIBDIR to the 'LD_LIBRARY_PATH' environment variable
          during execution
        - add LIBDIR to the 'LD_RUN_PATH' environment variable
          during linking
        - use the '-RLIBDIR' linker flag

     See any operating system documentation about shared libraries for
     more information, such as the ld and ld.so manual pages.
     -----------------------------------------------------------------
     burger#

   After you have completed these steps, you can go on to begin using
the installed libraries.  You may also install any executables that
depend on libraries you created.

   ---------- Footnotes ----------

   (1) Don't strip static libraries though, or they will be unusable.


File: libtool.info,  Node: Installing executables,  Next: Static libraries,  Prev: Installing libraries,  Up: Using libtool

3.6 Installing executables
==========================

If you used libtool to link any executables against uninstalled libtool
libraries (*note Linking executables::), you need to use libtool to
install the executables after the libraries have been installed (*note
Installing libraries::).

   So, for our Ultrix example, we would run:

     a23# libtool --mode=install install -c hell /usr/local/bin/hell
     install -c hell /usr/local/bin/hell
     a23#

   On shared library systems that require wrapper scripts, libtool just
ignores the wrapper script and installs the correct binary:

     burger# libtool --mode=install install -c hell /usr/local/bin/hell
     install -c .libs/hell /usr/local/bin/hell
     burger#


File: libtool.info,  Node: Static libraries,  Prev: Installing executables,  Up: Using libtool

3.7 Linking static libraries
============================

Why return to ‘ar’ and ‘ranlib’ silliness when you've had a taste of
libtool?  Well, sometimes it is desirable to create a static archive
that can never be shared.  The most frequent case is when you have a set
of object files that you use to build several different libraries.  You
can create a "convenience library" out of those objects, and link
against that with the other libraries, instead of listing all the object
files every time.

   If you just want to link this convenience library into programs, then
you could just ignore libtool entirely, and use the old ‘ar’ and
‘ranlib’ commands (or the corresponding GNU Automake ‘_LIBRARIES’
rules).  You can even install a convenience library using GNU Libtool,
though you probably don't want to and hence GNU Automake doesn't allow
you to do so.

     burger$ libtool --mode=install ./install-sh -c libhello.a \
                     /local/lib/libhello.a
     ./install-sh -c libhello.a /local/lib/libhello.a
     ranlib /local/lib/libhello.a
     burger$

   Using libtool for static library installation protects your library
from being accidentally stripped (if the installer used the ‘-s’ flag),
as well as automatically running the correct ‘ranlib’ command.

   But libtool libraries are more than just collections of object files:
they can also carry library dependency information, which old archives
do not.  If you want to create a libtool static convenience library, you
can omit the ‘-rpath’ flag and use ‘-static’ to indicate that you're
only interested in a static library.  When you link a program with such
a library, libtool will actually link all object files and dependency
libraries into the program.

   If you omit both ‘-rpath’ and ‘-static’, libtool will create a
convenience library that can be used to create other libtool libraries,
even shared ones.  Just like in the static case, the library behaves as
an alias to a set of object files and dependency libraries, but in this
case the object files are suitable for inclusion in shared libraries.
But be careful not to link a single convenience library, directly or
indirectly, into a single program or library, otherwise you may get
errors about symbol redefinitions.

   The key is remembering that a convenience library contains PIC
objects, and can be linked where a list of PIC objects makes sense; i.e.
into a shared library.  A static convenience library contains non-PIC
objects, so can be linked into an old static library, or a program.

   When GNU Automake is used, you should use ‘noinst_LTLIBRARIES’
instead of ‘lib_LTLIBRARIES’ for convenience libraries, so that the
‘-rpath’ option is not passed when they are linked.

   As a rule of thumb, link a libtool convenience library into at most
one libtool library, and never into a program, and link libtool static
convenience libraries only into programs, and only if you need to carry
library dependency information to the user of the static convenience
library.

   Another common situation where static linking is desirable is in
creating a standalone binary.  Use libtool to do the linking and add the
‘-all-static’ flag.


File: libtool.info,  Node: Invoking libtool,  Next: Integrating libtool,  Prev: Using libtool,  Up: Top

4 Invoking ‘libtool’
********************

The ‘libtool’ program has the following synopsis:

     libtool [OPTION]... [MODE-ARG]...

and accepts the following options:

‘--config’
     Display libtool configuration variables and exit.

‘--debug’
     Dump a trace of shell script execution to standard output.  This
     produces a lot of output, so you may wish to pipe it to ‘less’ (or
     ‘more’) or redirect to a file.

‘-n’
‘--dry-run’
     Don't create, modify, or delete any files, just show what commands
     would be executed by libtool.

‘--features’
     Display basic configuration options.  This provides a way for
     packages to determine whether shared or static libraries will be
     built.

‘--finish’
     Same as ‘--mode=finish’.

‘-h’
     Display short help message.

‘--help’
     Display a help message and exit.  If ‘--mode=MODE’ is specified,
     then detailed help for MODE is displayed.

‘--help-all’
     Display help for the general options as well as detailed help for
     each operation mode, and exit.

‘--mode=MODE’
     Use MODE as the operation mode.  When using libtool from the
     command line, you can give just MODE (or a unique abbreviation of
     it) as the first argument as a shorthand for the full
     ‘--mode=MODE’.  For example, the following are equivalent:

          $ libtool --mode=execute --dry-run gdb prog.exe
          $ libtool        execute --dry-run gdb prog.exe
          $ libtool        exe     --dry-run gdb prog.exe
          $ libtool        e       --dry-run gdb prog.exe

     MODE must be set to one of the following:

     ‘compile’
          Compile a source file into a libtool object.

     ‘execute’
          Automatically set the library path so that another program can
          use uninstalled libtool-generated programs or libraries.

     ‘link’
          Create a library or an executable.

     ‘install’
          Install libraries or executables.

     ‘finish’
          Complete the installation of libtool libraries on the system.

     ‘uninstall’
          Delete installed libraries or executables.

     ‘clean’
          Delete uninstalled libraries or executables.

‘--tag=TAG’
     Use configuration variables from tag TAG (*note Tags::).

‘--preserve-dup-deps’
     Do not remove duplicate dependencies in libraries.  When building
     packages with static libraries, the libraries may depend circularly
     on each other (shared libs can too, but for those it doesn't
     matter), so there are situations, where -la -lb -la is required,
     and the second -la may not be stripped or the link will fail.  In
     cases where these duplications are required, this option will
     preserve them, only stripping the libraries that libtool knows it
     can safely.

‘--no-finish’
     Do not execute finish_cmds (disabled by default).  This option
     should be combined with the usage of ‘--mode=install’ and
     ‘--mode=finish’ to have any effect.  This option is relevant for
     OpenBSD 7.5, where ldconfig adds the shared libraries install
     directory to the shared library cache directories list.  Specifying
     this option is useful on that platform if you want to load local
     shared libraries but not shared libraries from the install
     directory.  If the shared library cache already contains the
     install directory, the option should not have any useful effect.
     In OpenBSD, the shared library cache can be reordered to prefer
     directories for testing shared libraries over the directories
     already listed in the shared library cache with
     ‘--reorder-cache=SHARED_LIB_DIRS’.

‘--reorder-cache=SHARED_LIB_DIRS’
     Reorder the shared library cache by providing the preferred
     directories (SHARED_LIB_DIRS) first to link shared libraries from.
     The directories previously present in the shared library cache are
     appended to the end (if not in the preferred directory list).
     Additionally, all directories that no longer exist are removed from
     the shared library cache.  This option is currently only available
     on OpenBSD.

     This option is essentially a wrapper for executing ‘ldconfig’, and
     it should be used as an independent option before and after testing
     changes to shared libraries.  Below are some examples of the
     effects of the option:

          $ libtool --reorder-cache=/tmp/testing
          Original: /usr/lib /usr/X11R6/lib /usr/local/lib
          Reordered: /tmp/testing /usr/lib /usr/X11R6/lib /usr/local/lib
          $ libtool --reorder-cache=/usr/lib:/usr/X11R6/lib:/usr/local/lib
          Original: /tmp/testing /usr/lib /usr/X11R6/lib /usr/local/lib
          Reordered: /usr/lib /usr/X11R6/lib /usr/local/lib /tmp/testing

          $ libtool --reorder-cache=/tmp/testing
          Original: /usr/lib /usr/X11R6/lib /usr/local/lib
          Reordered: /tmp/testing /usr/lib /usr/X11R6/lib /usr/local/lib
          $ rm -rf /tmp/testing
          $ libtool --reorder-cache=/usr/lib:/usr/X11R6/lib:/usr/local/lib
          Original: /tmp/testing /usr/lib /usr/X11R6/lib /usr/local/lib
          Reordered: /usr/lib /usr/X11R6/lib /usr/local/lib

          $ libtool --reorder-cache=/tmp/testing:/usr/local/lib:/home/user/dir
          Original: /usr/lib /usr/X11R6/lib /usr/local/lib
          Reordered: /tmp/testing /usr/local/lib /home/user/dir /usr/lib /usr/X11R6/lib
          $ libtool --reorder-cache=/usr/lib:/usr/X11R6/lib:/usr/local/lib
          Original: /tmp/testing /usr/local/lib /home/user/dir /usr/lib /usr/X11R6/lib
          Reordered: /usr/lib /usr/X11R6/lib /usr/local/lib /tmp/testing /home/user/dir

‘--quiet’
‘--silent’
     Do not print out any progress or informational messages.

‘-v’
‘--verbose’
     Print out progress and informational messages (enabled by default),
     as well as additional messages not ordinarily seen by default.

‘--no-quiet’
‘--no-silent’
     Print out the progress and informational messages that are seen by
     default.  This option has no effect on whether the additional
     messages seen in ‘--verbose’ mode are shown.

‘--no-verbose’
     Do not print out any additional informational messages beyond those
     ordinarily seen by default.  This option has no effect on whether
     the ordinary progress and informational messages enabled by
     ‘--no-quiet’ are shown.

     Thus, there are now three different message levels (not counting
     ‘--debug’), depending on whether the normal messages and/or the
     additional verbose messages are displayed.  Note that there is no
     mechanism to display verbose messages, without also displaying
     normal messages.

     *default*
          Normal messages are displayed, verbose messages are not
          displayed.  In addition to being the default mode, it can be
          forcibly achieved by using both option ‘--no-verbose’ and
          either option ‘--no-silent’ or option ‘--no-quiet’.

     *silent*
          Neither normal messages nor verbose messages are displayed.
          This mode can be achieved using either option ‘--silent’ or
          option ‘--quiet’.

     *verbose*
          Both normal messages and verbose messages are displayed.  This
          mode can be achieved using either option ‘-v’ or option
          ‘--verbose’.

‘--version’
     Print libtool version information and exit.

‘-W’
‘--warnings=CATEGORY’
     Report the warnings falling in category CATEGORY.  The default
     category is ‘all’.  To disable warnings, use the category ‘none’.

‘--no-warnings’
     Do not print out any warning messages.

   The current ‘libtool’ implementation is done with a shell script that
needs to be invoked by the shell that ‘configure’ chose for configuring
‘libtool’ (*note config.status Invocation: (autoconf).).  This shell is
set in the she-bang (‘#!’) line of the ‘libtool’ script.  Using a
different shell may cause undefined behavior.

   The MODE-ARGS are a variable number of arguments, depending on the
selected operation mode.  In general, each MODE-ARG is interpreted by
programs libtool invokes, rather than libtool itself.

* Menu:

* Compile mode::                Creating library object files.
* Link mode::                   Generating executables and libraries.
* Execute mode::                Debugging libtool-generated programs.
* Install mode::                Making libraries and executables public.
* Finish mode::                 Completing a library installation.
* Uninstall mode::              Removing installed executables and libraries.
* Clean mode::                  Removing uninstalled executables and libraries.


File: libtool.info,  Node: Compile mode,  Next: Link mode,  Up: Invoking libtool

4.1 Compile mode
================

For “compile” mode, MODE-ARGS is a compiler command to be used in
creating a "standard" object file.  These arguments should begin with
the name of the C compiler, and contain the ‘-c’ compiler flag so that
only an object file is created.

   Libtool determines the name of the output file by removing the
directory component from the source file name, then substituting the
source code suffix (e.g. ‘.c’ for C source code) with the library object
suffix, ‘.lo’.

   If shared libraries are being built, any necessary PIC generation
flags are substituted into the compilation command.

   The following components of MODE-ARGS are treated specially:

‘-o’
     Note that the ‘-o’ option is now fully supported.  It is emulated
     on the platforms that don't support it (by locking and moving the
     objects), so it is really easy to use libtool, just with minor
     modifications to your Makefiles.  Typing for example
          libtool --mode=compile gcc -c foo/x.c -o foo/x.lo
     will do what you expect.

     Note, however, that, if the compiler does not support ‘-c’ and
     ‘-o’, it is impossible to compile ‘foo/x.c’ without overwriting an
     existing ‘./x.o’.  Therefore, if you do have a source file ‘./x.c’,
     make sure you introduce dependencies in your ‘Makefile’ to make
     sure ‘./x.o’ (or ‘./x.lo’) is re-created after any sub-directory's
     ‘x.lo’:

          x.o x.lo: foo/x.lo bar/x.lo

     This will also ensure that make won't try to use a temporarily
     corrupted ‘x.o’ to create a program or library.  It may cause
     needless recompilation on platforms that support ‘-c’ and ‘-o’
     together, but it's the only way to make it safe for those that
     don't.

‘-no-suppress’
     If both PIC and non-PIC objects are being built, libtool will
     normally suppress the compiler output for the PIC object
     compilation to save showing very similar, if not identical
     duplicate output for each object.  If the ‘-no-suppress’ option is
     given in compile mode, libtool will show the compiler output for
     both objects.

‘-prefer-pic’
     Libtool will try to build only PIC objects.

‘-prefer-non-pic’
     Libtool will try to build only non-PIC objects.

‘-shared’
     Even if Libtool was configured with ‘--enable-static’, the object
     file Libtool builds will not be suitable for static linking.
     Libtool will signal an error if it was configured with
     ‘--disable-shared’, or if the host does not support shared
     libraries.

‘-static’
     Even if libtool was configured with ‘--disable-static’, the object
     file Libtool builds *will* be suitable for static linking.

‘-Wc,FLAG’
‘-Xcompiler FLAG’
     Pass a flag directly to the compiler.  With ‘-Wc,’, multiple flags
     may be separated by commas, whereas ‘-Xcompiler ’ passes through
     commas unchanged.


File: libtool.info,  Node: Link mode,  Next: Execute mode,  Prev: Compile mode,  Up: Invoking libtool

4.2 Link mode
=============

“Link” mode links together object files (including library objects) to
form another library or to create an executable program.

   MODE-ARGS consist of a command using the C compiler to create an
output file (with the ‘-o’ flag) from several object files.

   The following components of MODE-ARGS are treated specially:

‘-all-static’
     If OUTPUT-FILE is a program, then do not link it against any shared
     libraries at all.  If OUTPUT-FILE is a library, then only create a
     static library.  In general, this flag cannot be used together with
     ‘disable-static’ (*note LT_INIT::).

‘-avoid-version’
     Tries to avoid versioning (*note Versioning::) for libraries and
     modules, i.e. no version information is stored and no symbolic
     links are created.  If the platform requires versioning, this
     option has no effect.

‘-bindir DIRECTORY’
     Pass the absolute name of the directory for installing executable
     programs (*note Directory Variables: (standards)Directory
     Variables.).  ‘libtool’ may use this value to install shared
     libraries there on systems that do not provide for any library
     hardcoding and use the directory of a program and the ‘PATH’
     variable as library search path.  This is typically used for DLLs
     on Windows or other systems using the PE (Portable Executable)
     format.  On other systems, ‘-bindir’ is ignored.  The default value
     used is ‘LIBDIR/../bin’ for libraries installed to ‘LIBDIR’.  You
     should not use ‘-bindir’ for modules.

‘-dlopen FILE’
     Same as ‘-dlpreopen FILE’, if native dlopening is not supported on
     the host platform (*note Dlopened modules::) or if the program is
     linked with ‘-static’, ‘-static-libtool-libs’, or ‘-all-static’.
     Otherwise, no effect.  If FILE is ‘self’ Libtool will make sure
     that the program can ‘dlopen’ itself, either by enabling
     ‘-export-dynamic’ or by falling back to ‘-dlpreopen self’.

‘-dlpreopen FILE’
     Link FILE into the output program, and add its symbols to the list
     of preloaded symbols (*note Dlpreopening::).  If FILE is ‘self’,
     the symbols of the program itself will be added to preloaded symbol
     lists.  If FILE is ‘force’ Libtool will make sure that a preloaded
     symbol list is always _defined_, regardless of whether it's empty
     or not.

‘-export-dynamic’
     Allow symbols from OUTPUT-FILE to be resolved with ‘dlsym’ (*note
     Dlopened modules::).

‘-export-symbols SYMFILE’
     Tells the linker to export only the symbols listed in SYMFILE.  The
     symbol file should end in ‘.sym’ and must contain the name of one
     symbol per line.  This option has no effect:
        • on static libraries, and
        • on shared libraries on some platforms, such as AIX and Haiku.
     By default all symbols are exported.

‘-export-symbols-regex REGEX’
     Same as ‘-export-symbols’, except that only symbols matching the
     regular expression REGEX are exported.  By default all symbols are
     exported.

‘-LLIBDIR’
     Search LIBDIR for required libraries that have already been
     installed.

‘-lNAME’
     OUTPUT-FILE requires the installed library ‘libNAME’.  This option
     is required even when OUTPUT-FILE is not an executable.

‘-module’
     Creates a library that can be dlopened (*note Dlopened modules::).
     This option doesn't work for programs.  Module names don't need to
     be prefixed with ‘lib’.  In order to prevent name clashes, however,
     ‘libNAME’ and ‘NAME’ must not be used at the same time in your
     package.

‘-no-fast-install’
     Disable fast-install mode for the executable OUTPUT-FILE.  Useful
     if the program won't be necessarily installed.

‘-no-install’
     Link an executable OUTPUT-FILE that can't be installed and
     therefore doesn't need a wrapper script on systems that allow
     hardcoding of library paths.  Useful if the program is only used in
     the build tree, e.g., for testing or generating other files.  For
     Windows and Darwin hosts, ‘-no-install’ is ignored, and
     ‘-no-fast-install’ is assumed.  (1)

‘-no-undefined’
     Declare that OUTPUT-FILE does not depend on any libraries other
     than the ones listed on the command line, i.e., after linking, it
     will not have unresolved symbols.  Some platforms require all
     symbols in shared libraries to be resolved at library creation
     (*note Inter-library dependencies::), and using this parameter
     allows ‘libtool’ to assume that this will not happen.

‘-o OUTPUT-FILE’
     Create OUTPUT-FILE from the specified objects and libraries.

‘-objectlist FILE’
     Use a list of object files found in FILE to specify objects.

‘-os2dllname NAME’
     Use this to change the DLL base name on OS/2 to NAME, to keep
     within the 8 character base name limit on this system.

‘-precious-files-regex REGEX’
     Prevents removal of files from the temporary output directory whose
     names match this regular expression.  You might specify ‘\.bbg?$’
     to keep those files created with ‘gcc -ftest-coverage’ for example.

‘-release RELEASE’
     Specify that the library was generated by release RELEASE of your
     package, so that users can easily tell what versions are newer than
     others.  Be warned that no two releases of your package will be
     binary compatible if you use this flag.  If you want binary
     compatibility, use the ‘-version-info’ flag instead (*note
     Versioning::).

‘-rpath LIBDIR’
     If OUTPUT-FILE is a library, it will eventually be installed in
     LIBDIR.  If OUTPUT-FILE is a program, add LIBDIR to the run-time
     path of the program.  On platforms that don't support hardcoding
     library paths into executables and only search PATH for shared
     libraries, such as when OUTPUT-FILE is a Windows (or other PE
     platform) DLL, the ‘.la’ control file will be installed in LIBDIR,
     but see ‘-bindir’ above for the eventual destination of the ‘.dll’
     or other library file itself.

‘-R LIBDIR’
     If OUTPUT-FILE is a program, add LIBDIR to its run-time path.  If
     OUTPUT-FILE is a library, add ‘-RLIBDIR’ to its DEPENDENCY_LIBS, so
     that, whenever the library is linked into a program, LIBDIR will be
     added to its run-time path.

‘-shared’
     If OUTPUT-FILE is a program, then link it against any uninstalled
     shared libtool libraries (this is the default behavior).  If
     OUTPUT-FILE is a library, then only create a shared library.  In
     the later case, libtool will signal an error if it was configured
     with ‘--disable-shared’, or if the host does not support shared
     libraries.

‘-shrext SUFFIX’
     If OUTPUT-FILE is a libtool library, replace the system's standard
     file name extension for shared libraries with SUFFIX (most systems
     use ‘.so’ here).  This option is helpful in certain cases where an
     application requires that shared libraries (typically modules) have
     an extension other than the default one.  Please note you must
     supply the full file name extension including any leading dot.

‘-static’
     If OUTPUT-FILE is a program, then do not link it against any
     uninstalled shared libtool libraries.  If OUTPUT-FILE is a library,
     then only create a static library.

‘-static-libtool-libs’
     If OUTPUT-FILE is a program, then do not link it against any shared
     libtool libraries.  If OUTPUT-FILE is a library, then only create a
     static library.

‘-version-info CURRENT[:REVISION[:AGE]]’
     If OUTPUT-FILE is a libtool library, use interface version
     information CURRENT, REVISION, and AGE to build it (*note
     Versioning::).  Do *not* use this flag to specify package release
     information, rather see the ‘-release’ flag.

‘-version-number MAJOR[:MINOR[:REVISION]]’
     If OUTPUT-FILE is a libtool library, compute interface version
     information so that the resulting library uses the specified major,
     minor and revision numbers.  This is designed to permit libtool to
     be used with existing projects where identical version numbers are
     already used across operating systems.  New projects should use the
     ‘-version-info’ flag instead.

‘-weak LIBNAME’
     if OUTPUT-FILE is a libtool library, declare that it provides a
     weak LIBNAME interface.  This is a hint to libtool that there is no
     need to append LIBNAME to the list of dependency libraries of
     OUTPUT-FILE, because linking against OUTPUT-FILE already supplies
     the same interface (*note Linking with dlopened modules::).

‘-Wc,FLAG’
‘-Xcompiler FLAG’
     Pass a linker-specific flag directly to the compiler.  With ‘-Wc,’,
     multiple flags may be separated by commas, whereas ‘-Xcompiler ’
     passes through commas unchanged.

‘-Wa,FLAG’
‘-Xassembler FLAG’
     Pass a linker-specific flag directly to the assembler.  With
     ‘-Wa,’, multiple flags may be separated by commas, whereas
     ‘-Xassembler ’ passes through commas unchanged.

‘-Wl,FLAG’
‘-Xlinker FLAG’
     Pass a linker-specific flag directly to the linker.

‘-XCClinker FLAG’
     Pass a link-specific flag to the compiler driver (‘CC’) during
     linking.

   Note: When the output file shall depend on an installed library, you
cannot specify the library name as ‘DIR/libNAME.a’ or ‘DIR/libNAME.so’
as part of the MODE-ARGS.  Instead, pass the arguments ‘-LDIR’ and
‘-lNAME’.

   If the OUTPUT-FILE ends in ‘.la’, then a libtool library is created,
which must be built only from library objects (‘.lo’ files).  The
‘-rpath’ option is required.  In the current implementation, libtool
libraries may not depend on other uninstalled libtool libraries (*note
Inter-library dependencies::).

   If the OUTPUT-FILE ends in ‘.a’, then a standard library is created
using ‘ar’ and possibly ‘ranlib’.

   If OUTPUT-FILE ends in ‘.o’ or ‘.lo’, then a reloadable object file
is created from the input files (generally using ‘ld -r’).  This method
is often called “partial linking”.

   Otherwise, an executable program is created.

   ---------- Footnotes ----------

   (1) The PATH hackery in wrapper scripts is required on Windows and
Darwin in order for the loader to find any dlls it needs.


File: libtool.info,  Node: Execute mode,  Next: Install mode,  Prev: Link mode,  Up: Invoking libtool

4.3 Execute mode
================

For “execute” mode, the library path is automatically set, then a
program is executed.

   The first of the MODE-ARGS is treated as a program name, with the
rest as arguments to that program.

   The following components of MODE-ARGS are treated specially:

‘-dlopen FILE’
     Add the directory containing FILE to the library path.

   This mode sets the library path environment variable according to any
‘-dlopen’ flags.

   If any of the ARGS are libtool executable wrappers, then they are
translated into the name of their corresponding uninstalled binary, and
any of their required library directories are added to the library path.


File: libtool.info,  Node: Install mode,  Next: Finish mode,  Prev: Execute mode,  Up: Invoking libtool

4.4 Install mode
================

In “install” mode, libtool interprets most of the elements of MODE-ARGS
as an installation command beginning with ‘cp’, or a BSD-compatible
‘install’ program.

   The following components of MODE-ARGS are treated specially:

‘-inst-prefix-dir INST-PREFIX-DIR’
     When installing into a temporary staging area, rather than the
     final ‘prefix’, this argument is used to reflect the temporary
     path, in much the same way ‘automake’ uses ‘DESTDIR’.  For
     instance, if ‘prefix’ is ‘/usr/local’, but INST-PREFIX-DIR is
     ‘/tmp’, then the object will be installed under ‘/tmp/usr/local/’.
     If the installed object is a libtool library, then the internal
     fields of that library will reflect only ‘prefix’, not
     INST-PREFIX-DIR:

          # Directory that this library needs to be installed in:
          libdir='/usr/local/lib'

     not

          # Directory that this library needs to be installed in:
          libdir='/tmp/usr/local/lib'

     ‘inst-prefix’ is also used to ensure that if the installed object
     must be relinked upon installation, that it is relinked against the
     libraries in INST-PREFIX-DIR/‘prefix’, not ‘prefix’.

     In truth, this option is not really intended for use when calling
     libtool directly; it is automatically used when ‘libtool
     --mode=install’ calls ‘libtool --mode=relink’.  Libtool does this
     by analyzing the destination path given in the original ‘libtool
     --mode=install’ command and comparing it to the expected
     installation path established during ‘libtool --mode=link’.

     Thus, end-users need change nothing, and ‘automake’-style ‘make
     install DESTDIR=/tmp’ will Just Work(tm) most of the time.  For
     systems where fast installation cannot be turned on, relinking may
     be needed.  In this case, a ‘DESTDIR’ install will fail.

     Currently it is not generally possible to install into a temporary
     staging area that contains needed third-party libraries that are
     not yet visible at their final location.

   The rest of the MODE-ARGS are interpreted as arguments to the ‘cp’ or
‘install’ command.

   The command is run, and any necessary unprivileged post-installation
commands are also completed.


File: libtool.info,  Node: Finish mode,  Next: Uninstall mode,  Prev: Install mode,  Up: Invoking libtool

4.5 Finish mode
===============

“Finish” mode has two functions.  One is to help system administrators
install libtool libraries so that they can be located and linked into
user programs.  To invoke this functionality, pass the name of a library
directory as MODE-ARG.  Running this command may require superuser
privileges, and the ‘--dry-run’ option may be useful.

   The second is to facilitate transferring libtool libraries to a
native compilation environment after they were built in a
cross-compilation environment.  Cross-compilation environments may rely
on recent libtool features, and running libtool in finish mode will make
it easier to work with older versions of libtool.  This task is
performed whenever the MODE-ARG is a ‘.la’ file.


File: libtool.info,  Node: Uninstall mode,  Next: Clean mode,  Prev: Finish mode,  Up: Invoking libtool

4.6 Uninstall mode
==================

“Uninstall” mode deletes installed libraries, executables and objects.

   The first MODE-ARG is the name of the program to use to delete files
(typically ‘/bin/rm’).

   The remaining MODE-ARGS are either flags for the deletion program
(beginning with a ‘-’), or the names of files to delete.


File: libtool.info,  Node: Clean mode,  Prev: Uninstall mode,  Up: Invoking libtool

4.7 Clean mode
==============

“Clean” mode deletes uninstalled libraries, executables, objects and
libtool's temporary files associated with them.

   The first MODE-ARG is the name of the program to use to delete files
(typically ‘/bin/rm’).

   The remaining MODE-ARGS are either flags for the deletion program
(beginning with a ‘-’), or the names of files to delete.


File: libtool.info,  Node: Integrating libtool,  Next: Other languages,  Prev: Invoking libtool,  Up: Top

5 Integrating libtool with your package
***************************************

This chapter describes how to integrate libtool with your packages so
that your users can install hassle-free shared libraries.

   There are several ways that Libtool may be integrated in your
package, described in the following sections.  Typically, the Libtool
macro files as well as ‘ltmain.sh’ are copied into your package using
‘libtoolize’ and ‘aclocal’ after setting up the ‘configure.ac’ and
toplevel ‘Makefile.am’, then ‘autoconf’ adds the needed tests to the
‘configure’ script.  These individual steps are often automated with
‘autoreconf’.

   Here is a diagram showing how such a typical Libtool configuration
works when preparing a package for distribution, assuming that ‘m4’ has
been chosen as the location for additional Autoconf macros, and
‘build-aux’ as the location for auxiliary build tools (*note The
Autoconf Manual: (autoconf)Input.):

     libtool.m4 -----.                .--> aclocal.m4 -----.
     ltoptions.m4 ---+  .-> aclocal* -+                    +--> autoconf*
     ltversion.m4 ---+--+             `--> [copy in m4/] --+       |
     ltsugar.m4 -----+  |                    ^             |       \/
     lt~obsolete.m4 -+  +-> libtoolize* -----'             |    configure
     [ltdl.m4] ------+  |                                  |
                        `----------------------------------'

     ltmain.sh -----------> libtoolize* -> [copy in build-aux/]

   During configuration, the ‘libtool’ script is generated either
through ‘config.status’ or ‘config.lt’:

                  .--> config.status* --.
     configure* --+                     +--> libtool
                  `--> [config.lt*] ----'      ^
                                               |
     ltmain.sh --------------------------------'

   At ‘make’ run time, ‘libtool’ is then invoked as needed as a wrapper
around compilers, linkers, install and cleanup programs.

   There are alternatives choices to several parts of the setup; for
example, the Libtool macro files can either be copied or symlinked into
the package, or copied into ‘aclocal.m4’.  As another example, an
external, pre-configured ‘libtool’ script may be used, by-passing most
of the tests and package-specific setup for Libtool.

* Menu:

* Autoconf macros::             Autoconf macros exported by libtool.
* Makefile rules::              Writing ‘Makefile’ rules for libtool.
* Using Automake::              Automatically supporting libtool.
* Configuring::                 Configuring libtool for a host system.
* Distributing::                What files to distribute with your package.
* Static-only libraries::       Sometimes shared libraries are just a pain.


File: libtool.info,  Node: Autoconf macros,  Next: Makefile rules,  Up: Integrating libtool

5.1 Autoconf macros exported by libtool
=======================================

Libtool uses a number of macros to interrogate the host system when it
is being built, and you can use some of them yourself too.  Although
there are a great many other macros in the libtool installed m4 files,
these do not form part of the published interface, and are subject to
change between releases.

Macros in the ‘LT_CMD_’ namespace check for various shell commands:

 -- Macro: LT_CMD_MAX_LEN
     Finds the longest command line that can be safely passed to
     ‘$SHELL’ without being truncated, and store in the shell variable
     ‘$max_cmd_len’.  It is only an approximate value, but command lines
     of this length or shorter are guaranteed not to be truncated.

Macros in the ‘LT_FUNC_’ namespace check characteristics of library
functions:

 -- Macro: LT_FUNC_DLSYM_USCORE
     ‘AC_DEFINE’ the preprocessor symbol ‘DLSYM_USCORE’ if we have to
     add an underscore to symbol-names passed in to ‘dlsym’.

Macros in the ‘LT_LIB_’ namespace check characteristics of system
libraries:

 -- Macro: LT_LIB_M
     Set ‘LIBM’ to the math library or libraries required on this
     machine, if any.

 -- Macro: LT_LIB_DLLOAD
     This is the macro used by ‘libltdl’ to determine what dlloaders to
     use on this machine, if any.  Several shell variables are set (and
     ‘AC_SUBST’ed) depending on the dlload interfaces are available on
     this machine.  ‘LT_DLLOADERS’ contains a list of libtool libraries
     that can be used, and if necessary also sets ‘LIBADD_DLOPEN’ if
     additional system libraries are required by the ‘dlopen’ loader,
     and ‘LIBADD_SHL_LOAD’ if additional system libraries are required
     by the ‘shl_load’ loader, respectively.  Finally some symbols are
     set in ‘config.h’ depending on the loaders that are found to work:
     ‘HAVE_LIBDL’, ‘HAVE_SHL_LOAD’, ‘HAVE_DYLD’, ‘HAVE_DLD’.

Macros in the ‘LT_PATH_’ namespace search the system for the full path
to particular system commands:

 -- Macro: LT_PATH_LD
     Add a ‘--with-gnu-ld’ option to ‘configure’.  Try to find the path
     to the linker used by ‘$CC’, and whether it is the GNU linker.  The
     result is stored in the shell variable ‘$LD’, which is
     ‘AC_SUBST’ed.

 -- Macro: LT_PATH_NM
     Try to find a BSD-compatible ‘nm’ or a MS-compatible ‘dumpbin’
     command on this machine.  The result is stored in the shell
     variable ‘$NM’, which is ‘AC_SUBST’ed.

Macros in the ‘LT_SYS_’ namespace probe for system characteristics:

 -- Macro: LT_SYS_DLOPEN_SELF
     Tests whether a program can dlopen itself, and then also whether
     the same program can still dlopen itself when statically linked.
     Results are stored in the shell variables ‘$enable_dlopen_self’ and
     ‘enable_dlopen_self_static’ respectively.

 -- Macro: LT_SYS_DLOPEN_DEPLIBS
     Define the preprocessor symbol ‘LTDL_DLOPEN_DEPLIBS’ if the OS
     needs help to load dependent libraries for ‘dlopen’ (or
     equivalent).

 -- Macro: LT_SYS_DLSEARCH_PATH
     Define the preprocessor symbol ‘LT_DLSEARCH_PATH’ to the system
     default library search path.

 -- Macro: LT_SYS_MODULE_EXT
     Define the preprocessor symbol ‘LT_MODULE_EXT’ to the extension
     used for runtime loadable modules.  If you use libltdl to open
     modules, then you can simply use the libtool library extension,
     ‘.la’.

 -- Macro: LT_SYS_MODULE_PATH
     Define the preprocessor symbol ‘LT_MODULE_PATH_VAR’ to the name of
     the shell environment variable that determines the run-time module
     search path.

 -- Macro: LT_SYS_SYMBOL_USCORE
     Set the shell variable ‘sys_symbol_underscore’ to ‘no’ unless the
     compiler prefixes global symbols with an underscore.


File: libtool.info,  Node: Makefile rules,  Next: Using Automake,  Prev: Autoconf macros,  Up: Integrating libtool

5.2 Writing ‘Makefile’ rules for libtool
========================================

Libtool is fully integrated with Automake (*note Introduction:
(automake)Top.), starting with Automake version 1.2.

   If you want to use libtool in a regular ‘Makefile’ (or
‘Makefile.in’), you are on your own.  If you're not using Automake, and
you don't know how to incorporate libtool into your package you need to
do one of the following:

  1. Download the latest Automake distribution from your nearest GNU
     mirror, install it, and start using it.

  2. Learn how to write ‘Makefile’ rules by hand.  They're sometimes
     complex, but if you're clever enough to write rules for compiling
     your old libraries, then you should be able to figure out new rules
     for libtool libraries (hint: examine the ‘Makefile.in’ in the
     ‘tests/testsuite.dir/027’ subdirectory, generated from the Autotest
     labeled 'link against a preloaded static library' in
     ‘tests/demo.at’, of the libtool distribution; note especially that
     it was automatically generated from the ‘Makefile.am’ by Automake).


File: libtool.info,  Node: Using Automake,  Next: Configuring,  Prev: Makefile rules,  Up: Integrating libtool

5.3 Using Automake with libtool
===============================

Libtool library support is implemented under the ‘LTLIBRARIES’ primary.

   Here are some samples from the Automake ‘Makefile.am’ in the libtool
distribution's ‘tests/demo.at’.

   First, to link a program against a libtool library, just use the
‘program_LDADD’(1) variable:

     bin_PROGRAMS = hell hell_static

     # Build hell from main.c and libhello.la
     hell_SOURCES = main.c
     hell_LDADD = libhello.la

     # Create a statically linked version of hell.
     hell_static_SOURCES = main.c
     hell_static_LDADD = libhello.la
     hell_static_LDFLAGS = -static

   You may use the ‘program_LDFLAGS’ variable to stuff in any flags you
want to pass to libtool while linking ‘program’ (such as ‘-static’ to
avoid linking uninstalled shared libtool libraries).

   Building a libtool library is almost as trivial... note the use of
‘libhello_la_LDFLAGS’ to pass the ‘-version-info’ (*note Versioning::)
option to libtool:

     # Build a libtool library, libhello.la for installation in libdir.
     lib_LTLIBRARIES = libhello.la
     libhello_la_SOURCES = hello.c foo.c
     libhello_la_LDFLAGS = -version-info 3:12:1

   The ‘-rpath’ option is passed automatically by Automake (except for
libraries listed as ‘noinst_LTLIBRARIES’), so you should not specify it.

   *Note Building a Shared Library: (automake)A Shared Library, for more
information.

   When building libtool archives which depend on built sources (for
example a generated header file), you may find it necessary to manually
record these dependencies.  Because libtool archives generate object
file names manually recording these dependencies is not as
straightforward as the examples in Automake's manual describe.  This
affects header files in particular, because simply listing them as
‘nodist_libfoo_la_SOURCES’ will not cause Automake to establish a
dependent relationship for the object files of ‘libfoo.la’.  A useful
trick (although somewhat imprecise) is to manually record built sources
used by a libtool archive as dependencies of all the objects for that
library as shown below (as opposed to a particular object file):

     # Build a libtool library, libhello.la which depends on a generated header.
     hello.h:
     	echo '#define HELLO_MESSAGE  "Hello, World!"' > $@
     BUILT_SOURCES = hello.h
     CLEANFILES = hello.h
     nodist_libhello_la_SOURCES = hello.h
     libhello_la_SOURCES = hello.c foo.h foo.c bar.h bar.c
     # Manually record hello.h as a prerequisite for all objects in libhello.la
     $(libhello_la_OBJECTS): hello.h

   *Note Recording Dependencies manually: (automake)Built Sources
Example, for more information.

   ---------- Footnotes ----------

   (1) Since GNU Automake 1.5, the flags ‘-dlopen’ or ‘-dlpreopen’
(*note Link mode::) can be employed with the ‘program_LDADD’ variable.
Unfortunately, older releases didn't accept these flags, so if you are
stuck with an ancient Automake, we recommend quoting the flag itself,
and setting ‘program_DEPENDENCIES’ too:

     program_LDADD = "-dlopen" libfoo.la
     program_DEPENDENCIES = libfoo.la


File: libtool.info,  Node: Configuring,  Next: Distributing,  Prev: Using Automake,  Up: Integrating libtool

5.4 Configuring libtool
=======================

Libtool requires intimate knowledge of your compiler suite and operating
system to be able to create shared libraries and link against them
properly.  When you install the libtool distribution, a system-specific
libtool script is installed into your binary directory.

   However, when you distribute libtool with your own packages (*note
Distributing::), you do not always know the compiler suite and operating
system that are used to compile your package.

   For this reason, libtool must be “configured” before it can be used.
This idea should be familiar to anybody who has used a GNU ‘configure’
script.  ‘configure’ runs a number of tests for system features, then
generates the ‘Makefile’s (and possibly a ‘config.h’ header file), after
which you can run ‘make’ and build the package.

   Libtool adds its own tests to your ‘configure’ script to generate a
libtool script for the installer's host machine.

* Menu:

* LT_INIT::                     Configuring ‘libtool’ in ‘configure.ac’.
* Configure notes::             Platform-specific notes for configuration.


File: libtool.info,  Node: LT_INIT,  Next: Configure notes,  Up: Configuring

5.4.1 The ‘LT_INIT’ macro
-------------------------

If you are using GNU Autoconf (or Automake), you should add a call to
‘LT_INIT’ to your ‘configure.ac’ file.  This macro adds many new tests
to the ‘configure’ script so that the generated libtool script will
understand the characteristics of the host.  It's the most important of
a number of macros defined by Libtool:

 -- Macro: LT_PREREQ (VERSION)
     Ensure that a recent enough version of Libtool is being used.  If
     the version of Libtool used for ‘LT_INIT’ is earlier than VERSION,
     print an error message to the standard error output and exit with
     failure (exit status is 63).  For example:

          LT_PREREQ([2.6.1])

 -- Macro: LT_INIT (OPTIONS)
 -- Macro: AC_PROG_LIBTOOL
 -- Macro: AM_PROG_LIBTOOL
     Add support for the ‘--enable-shared’, ‘--disable-shared’,
     ‘--enable-static’, ‘--disable-static’, ‘--enable-pic’, and
     ‘--disable-pic’ ‘configure’ flags.(1)  ‘AC_PROG_LIBTOOL’ and
     ‘AM_PROG_LIBTOOL’ are deprecated names for older versions of this
     macro; ‘autoupdate’ will upgrade your ‘configure.ac’ files.

     By default, this macro turns on shared libraries if they are
     available, and also enables static libraries if they don't conflict
     with the shared libraries.  You can modify these defaults by
     passing either ‘disable-shared’ or ‘disable-static’ in the option
     list to ‘LT_INIT’, or using ‘AC_DISABLE_SHARED’ or
     ‘AC_DISABLE_STATIC’.

          # Turn off shared libraries during beta-testing, since they
          # make the build process take too long.
          LT_INIT([disable-shared])

     The user may specify modified forms of the configure flags
     ‘--enable-shared’ and ‘--enable-static’ to choose whether shared or
     static libraries are built based on the name of the package.  For
     example, to have shared ‘bfd’ and ‘gdb’ libraries built, but not
     shared ‘libg++’, you can run all three ‘configure’ scripts as
     follows:

          trick$ ./configure --enable-shared=bfd,gdb

     In general, specifying ‘--enable-shared=PKGS’ is the same as
     configuring with ‘--enable-shared’ every package named in the
     comma-separated PKGS list, and every other package with
     ‘--disable-shared’.  The ‘--enable-static=PKGS’ flag behaves
     similarly, but it uses ‘--enable-static’ and ‘--disable-static’.
     The same applies to the ‘--enable-fast-install=PKGS’ flag, which
     uses ‘--enable-fast-install’ and ‘--disable-fast-install’.

     The package name ‘default’ matches any packages that have not set
     their name in the ‘PACKAGE’ environment variable.

     The ‘--enable-pic’ and ‘--disable-pic’ configure flags can be used
     to specify whether or not ‘libtool’ uses PIC objects.  By default,
     ‘libtool’ uses PIC objects for shared libraries and non-PIC objects
     for static libraries.  The ‘--enable-pic’ option also accepts a
     comma-separated list of package names.  Specifying
     ‘--enable-pic=PKGS’ is the same as configuring every package in
     PKGS with ‘--enable-pic’ and every other package with the default
     configuration.  The package name ‘default’ is treated the same as
     for ‘--enable-shared’ and ‘--enable-static’.

     This macro also sets the shell variable ‘LIBTOOL_DEPS’, that you
     can use to automatically update the libtool script if it becomes
     out-of-date.  In order to do that, add to your ‘configure.ac’:

          LT_INIT
          AC_SUBST([LIBTOOL_DEPS])

     and, to ‘Makefile.in’ or ‘Makefile.am’:

          LIBTOOL_DEPS = @LIBTOOL_DEPS@
          libtool: $(LIBTOOL_DEPS)
                  $(SHELL) ./config.status libtool

     If you are using GNU Automake, you can omit the assignment, as
     Automake will take care of it.  You'll obviously have to create
     some dependency on ‘libtool’.

     Aside from ‘disable-static’ and ‘disable-shared’, there are other
     options that you can pass to ‘LT_INIT’ to modify its behaviour.
     Here is a full list:

     ‘dlopen’
          Enable checking for dlopen support.  This option should be
          used if the package makes use of the ‘-dlopen’ and
          ‘-dlpreopen’ libtool flags, otherwise libtool will assume that
          the system does not support dlopening.

     ‘win32-dll’
          This option should be used if the package has been ported to
          build clean dlls on win32 platforms.  Usually this means that
          any library data items are exported with
          ‘__declspec(dllexport)’ and imported with
          ‘__declspec(dllimport)’.  If this option is not used, libtool
          will assume that the package libraries are not dll clean and
          will build only static libraries on win32 hosts.

          Provision must be made to pass ‘-no-undefined’ to ‘libtool’ in
          link mode from the package ‘Makefile’.  Naturally, if you pass
          ‘-no-undefined’, you must ensure that all the library symbols
          *really are* defined at link time!

     ‘aix-soname=aix’
     ‘aix-soname=svr4’
     ‘aix-soname=both’
          Enable the ‘--enable-aix-soname’ to ‘configure’, which the
          user can pass to override the given default.

          By default (and *always* in releases prior to 2.4.4), Libtool
          always behaves as if ‘aix-soname=aix’ is given, with no
          ‘configure’ option for the user to override.  Specifically,
          when the ‘-brtl’ linker flag is seen in ‘LDFLAGS’ at
          build-time, static archives are built from static objects
          only, otherwise, traditional AIX shared library archives of
          shared objects using in-archive versioning are built (with the
          ‘.a’ file extension!).  Similarly, with ‘-brtl’ in ‘LDFLAGS’,
          libtool shared archives are built from shared objects, without
          any filename-based versioning; and without ‘-brtl’ no shared
          archives are built at all.

          When ‘aix-soname=svr4’ option is given, or the
          ‘--enable-aix-soname=svr4’ ‘configure’ option is passed,
          static archives are always created from static objects, even
          without ‘-brtl’ in ‘LDFLAGS’.  Shared archives are made from
          shared objects, and filename based versioning is enabled.

          When ‘aix-soname=both’ option is given, or the
          ‘--enable-aix-soname=svr4’ ‘configure’ option is passed,
          static archives are built traditionally (as ‘aix-soname=aix’),
          and both kinds of shared archives are built.  The ‘.la’
          pseudo-archive specifies one or the other depending on whether
          ‘-brtl’ is specified in ‘LDFLAGS’ when the library is built.

          When using ‘LT_INIT’, the ‘shared’ option must be set before
          the ‘aix-soname=(svr4|both)’ option to ensure shared libraries
          are built.

     ‘disable-fast-install’
          Change the default behaviour for ‘LT_INIT’ to disable
          optimization for fast installation.  The user may still
          override this default, depending on platform support, by
          specifying ‘--enable-fast-install’ to ‘configure’.

     ‘shared’
          Change the default behaviour for ‘LT_INIT’ to enable shared
          libraries.  This is the default on all systems where Libtool
          knows how to create shared libraries.  The user may still
          override this default by specifying ‘--disable-shared’ to
          ‘configure’.

     ‘disable-shared’
          Change the default behaviour for ‘LT_INIT’ to disable shared
          libraries.  The user may still override this default by
          specifying ‘--enable-shared’ to ‘configure’.

     ‘static’
          Change the default behaviour for ‘LT_INIT’ to enable static
          libraries.  This is the default on all systems where shared
          libraries have been disabled for some reason, and on most
          systems where shared libraries have been enabled.  If shared
          libraries are enabled, the user may still override this
          default by specifying ‘--disable-static’ to ‘configure’.

     ‘disable-static’
          Change the default behaviour for ‘LT_INIT’ to disable static
          libraries.  The user may still override this default by
          specifying ‘--enable-static’ to ‘configure’.

     ‘pic-only’
          Change the default behaviour for ‘libtool’ to try to use only
          PIC objects.  The user may still override this default by
          specifying ‘--disable-pic’ to ‘configure’.

     ‘no-pic’
          Change the default behaviour of ‘libtool’ to try to use only
          non-PIC objects.  The user may still override this default by
          specifying ‘--enable-pic’ to ‘configure’.

     ‘cxx-stdlib’
          Change the default behaviour for ‘libtool’ to let the compiler
          frontend decide what standard libraries to link when building
          C++ shared libraries and modules.  The user may still override
          this default by specifying ‘--disable-cxx-stdlib’ to
          ‘configure’.

     ‘no-cxx-stdlib’
          Change the default behaviour for ‘libtool’ to not link
          standard libraries when building C++ shared libraries and
          modules.  The user may still override this default by
          specifying ‘--enable-cxx-stdlib’ to ‘configure’.

 -- Macro: LT_LANG (LANGUAGE)
     Enable ‘libtool’ support for the language given if it has not yet
     already been enabled.  Languages accepted are "C++", "Objective-C",
     "Objective-C++", "ML64", "Fortran 77", "Java", "Go", and "Windows
     Resource".

     If Autoconf language support macros such as ‘AC_PROG_CXX’ are used
     in your ‘configure.ac’, Libtool language support will automatically
     be enabled.

     Conversely using ‘LT_LANG’ to enable language support for Libtool
     will automatically enable Autoconf language support as well.

     Both of the following examples are therefore valid ways of adding
     C++ language support to Libtool.

          LT_INIT
          LT_LANG([C++])

          LT_INIT
          AC_PROG_CXX

 -- Macro: AC_LIBTOOL_DLOPEN
     This macro is deprecated, the ‘dlopen’ option to ‘LT_INIT’ should
     be used instead.

 -- Macro: AC_LIBTOOL_WIN32_DLL
     This macro is deprecated, the ‘win32-dll’ option to ‘LT_INIT’
     should be used instead.

 -- Macro: AC_DISABLE_FAST_INSTALL
     This macro is deprecated, the ‘disable-fast-install’ option to
     ‘LT_INIT’ should be used instead.

 -- Macro: AC_DISABLE_SHARED
 -- Macro: AM_DISABLE_SHARED
     Change the default behaviour for ‘LT_INIT’ to disable shared
     libraries.  The user may still override this default by specifying
     ‘--enable-shared’.  The option ‘disable-shared’ to ‘LT_INIT’ is a
     shorthand for this.  ‘AM_DISABLE_SHARED’ is a deprecated alias for
     ‘AC_DISABLE_SHARED’.

 -- Macro: AC_ENABLE_SHARED
 -- Macro: AM_ENABLE_SHARED
     Change the default behaviour for ‘LT_INIT’ to enable shared
     libraries.  This is the default on all systems where Libtool knows
     how to create shared libraries.  The user may still override this
     default by specifying ‘--disable-shared’.  The option ‘shared’ to
     ‘LT_INIT’ is a shorthand for this.  ‘AM_ENABLE_SHARED’ is a
     deprecated alias for ‘AC_ENABLE_SHARED’.

 -- Macro: AC_DISABLE_STATIC
 -- Macro: AM_DISABLE_STATIC
     Change the default behaviour for ‘LT_INIT’ to disable static
     libraries.  The user may still override this default by specifying
     ‘--enable-static’.  The option ‘disable-static’ to ‘LT_INIT’ is a
     shorthand for this.  ‘AM_DISABLE_STATIC’ is a deprecated alias for
     ‘AC_DISABLE_STATIC’.

 -- Macro: AC_ENABLE_STATIC
 -- Macro: AM_ENABLE_STATIC
     Change the default behaviour for ‘LT_INIT’ to enable static
     libraries.  This is the default on all systems where shared
     libraries have been disabled for some reason, and on most systems
     where shared libraries have been enabled.  If shared libraries are
     enabled, the user may still override this default by specifying
     ‘--disable-static’.  The option ‘static’ to ‘LT_INIT’ is a
     shorthand for this.  ‘AM_ENABLE_STATIC’ is a deprecated alias for
     ‘AC_ENABLE_STATIC’.

   The tests in ‘LT_INIT’ also recognize the following environment
variables:

 -- Variable: CC
     The C compiler that will be used by the generated ‘libtool’.  If
     this is not set, ‘LT_INIT’ will look for ‘gcc’ or ‘cc’.

 -- Variable: CFLAGS
     Compiler flags used to generate standard object files.  If this is
     not set, ‘LT_INIT’ will not use any such flags.  It affects only
     the way ‘LT_INIT’ runs tests, not the produced ‘libtool’.

 -- Variable: CPPFLAGS
     C preprocessor flags.  If this is not set, ‘LT_INIT’ will not use
     any such flags.  It affects only the way ‘LT_INIT’ runs tests, not
     the produced ‘libtool’.

 -- Variable: LD
     The system linker to use (if the generated ‘libtool’ requires one).
     If this is not set, ‘LT_INIT’ will try to find out what is the
     linker used by ‘CC’.

 -- Variable: LDFLAGS
     The flags to be used by ‘libtool’ when it links a program.  If this
     is not set, ‘LT_INIT’ will not use any such flags.  It affects only
     the way ‘LT_INIT’ runs tests, not the produced ‘libtool’.

 -- Variable: LIBS
     The libraries to be used by ‘LT_INIT’ when it links a program.  If
     this is not set, ‘LT_INIT’ will not use any such flags.  It affects
     only the way ‘LT_INIT’ runs tests, not the produced ‘libtool’.

 -- Variable: NM
     Program to use rather than checking for ‘nm’.

 -- Variable: RANLIB
     Program to use rather than checking for ‘ranlib’.

 -- Variable: LN_S
     A command that creates a link of a program, a soft-link if
     possible, a hard-link otherwise.  ‘LT_INIT’ will check for a
     suitable program if this variable is not set.

 -- Variable: DLLTOOL
     Program to use rather than checking for ‘dlltool’.  Only meaningful
     for Cygwin/MS-Windows.

 -- Variable: OBJDUMP
     Program to use rather than checking for ‘objdump’.  Only meaningful
     for Cygwin/MS-Windows.

 -- Variable: AS
     Program to use rather than checking for ‘as’.  Only used on
     Cygwin/MS-Windows at the moment.

 -- Variable: MANIFEST_TOOL
     Program to use rather than checking for ‘mt’, the Manifest Tool.
     Only used on Cygwin/MS-Windows at the moment.

 -- Variable: LT_SYS_LIBRARY_PATH
     Libtool has heuristics for the system search path for
     runtime-loaded libraries.  If the guessed default does not match
     the setup of the host system, this variable can be used to modify
     that path list, as follows (‘LT_SYS_LIBRARY_PATH’ is a
     colon-delimited list like ‘PATH’):
        • ‘path:’ The heuristically determined paths will be appended
          after the trailing colon;
        • ‘:path’ The heuristically determined paths will be prepended
          before the leading colon;
        • ‘path::path’ The heuristically determined paths will be
          inserted between the double colons;
        • ‘path’ With no dangling colons, the heuristically determined
          paths will be ignored entirely.

   With 1.3 era libtool, if you wanted to know any details of what
libtool had discovered about your architecture and environment, you had
to run the script with ‘--config’ and grep through the results.  This
idiom was supported up to and including 1.5.x era libtool, where it was
possible to call the generated libtool script from ‘configure.ac’ as
soon as ‘LT_INIT’ had completed.  However, one of the features of
libtool 1.4 was that the libtool configuration was migrated out of a
separate ‘ltconfig’ file, and added to the ‘LT_INIT’ macro (nee
‘AC_PROG_LIBTOOL’), so the results of the configuration tests were
available directly to code in ‘configure.ac’, rendering the call out to
the generated libtool script obsolete.

   Starting with libtool 2.0, the multipass generation of the libtool
script has been consolidated into a single ‘config.status’ pass, which
happens after all the code in ‘configure.ac’ has completed.  The
implication of this is that the libtool script does not exist during
execution of code from ‘configure.ac’, and so obviously it cannot be
called for ‘--config’ details anymore.  If you are upgrading projects
that used this idiom to libtool 2.0 or newer, you should replace those
calls with direct references to the equivalent Autoconf shell variables
that are set by the configure time tests before being passed to
‘config.status’ for inclusion in the generated libtool script.

 -- Macro: LT_OUTPUT
     By default, the configured ‘libtool’ script is generated by the
     call to ‘AC_OUTPUT’ command, and there is rarely any need to use
     ‘libtool’ from ‘configure’.  However, sometimes it is necessary to
     run configure time compile and link tests using ‘libtool’.  You can
     add ‘LT_OUTPUT’ to your ‘configure.ac’ any time after ‘LT_INIT’ and
     any ‘LT_LANG’ calls; that done, ‘libtool’ will be created by a
     specially generated ‘config.lt’ file, and available for use in
     later tests.

     Also, when ‘LT_OUTPUT’ is used, for backwards compatibility with
     Automake regeneration rules, ‘config.status’ will call ‘config.lt’
     to regenerate ‘libtool’, rather than generating the file itself.

   When you invoke the ‘libtoolize’ program (*note Invoking
libtoolize::), it will tell you where to find a definition of ‘LT_INIT’.
If you use Automake, the ‘aclocal’ program will automatically add
‘LT_INIT’ support to your ‘configure’ script when it sees the invocation
of ‘LT_INIT’ in ‘configure.ac’.

   Because of these changes, and the runtime version compatibility
checks Libtool now executes, we now advise *against* including a copy of
‘libtool.m4’ (and brethren) in ‘acinclude.m4’.  Instead, you should set
your project macro directory with ‘AC_CONFIG_MACRO_DIRS’.  When you
‘libtoolize’ your project, a copy of the relevant macro definitions will
be placed in your ‘AC_CONFIG_MACRO_DIRS’, where ‘aclocal’ can reference
them directly from ‘aclocal.m4’.

   ---------- Footnotes ----------

   (1) ‘LT_INIT’ requires that you define the ‘Makefile’ variable
‘top_builddir’ in your ‘Makefile.in’.  Automake does this automatically,
but Autoconf users should set it to the relative path to the top of your
build directory (‘../..’, for example).


File: libtool.info,  Node: Configure notes,  Prev: LT_INIT,  Up: Configuring

5.4.2 Platform-specific configuration notes
-------------------------------------------

While Libtool tries to hide as many platform-specific features as
possible, some have to be taken into account when configuring either the
Libtool package or a libtoolized package.

   • You currently need GNU make to build the Libtool package itself.

   • On AIX there are two different styles of shared linking, one where
     symbols are bound at link-time and one where symbols are bound at
     runtime only, similar to ELF.  In case of doubt use
     ‘LDFLAGS=-Wl,-brtl’ for the latter style.

   • On AIX, native tools are to be preferred over binutils; especially
     for C++ code, if using the AIX Toolbox GCC 4.0 and binutils,
     configure with ‘AR=/usr/bin/ar LD=/usr/bin/ld NM='/usr/bin/nm -B'’.

   • On AIX, the ‘/bin/sh’ is very slow due to its inefficient handling
     of here-documents.  A modern shell is preferable:
          CONFIG_SHELL=/bin/bash; export $CONFIG_SHELL
          $CONFIG_SHELL ./configure [...]

   • For C++ code with templates, it may be necessary to specify the way
     the compiler will generate the instantiations.  For Portland pgCC
     version5, use ‘CXX='pgCC --one_instantiation_per_object'’ and avoid
     parallel ‘make’.

   • For C++ code, it may be necessary to specify a library if it is a
     dependency of a link/compile flag.  For example in GNU G++, if you
     want to use ‘-fsanitize=address’ you need to specify the ‘-lasan’
     library, like so: ‘g++ -o libx.la -fsanitize=address -lasan -rpath
     [...]’.

   • On Darwin, for C++ code with templates you need two level shared
     libraries.  Libtool builds these by default if
     ‘MACOSX_DEPLOYMENT_TARGET’ is set to 10.3 or later at ‘configure’
     time.  See <rdar://problem/4135857> for more information on this
     issue.

   • The default shell on UNICOS 9, a ksh 88e variant, is too buggy to
     correctly execute the libtool script.  Users are advised to install
     a modern shell such as GNU bash.

   • Some HP-UX ‘sed’ programs are horribly broken, and cannot handle
     libtool's requirements, so users may report unusual problems.
     There is no workaround except to install a working ‘sed’ (such as
     GNU sed) on these systems.

   • The vendor-distributed NCR MP-RAS ‘cc’ programs emits copyright on
     standard error that confuse tests on size of ‘conftest.err’.  The
     workaround is to specify ‘CC’ when run configure with ‘CC='cc
     -Hnocopyr'’.

   • Any earlier DG/UX system with ELF executables, such as R3.10 or
     R4.10, is also likely to work, but hasn't been explicitly tested.

   • On Reliant Unix libtool has only been tested with the Siemens
     C-compiler and an old version of ‘gcc’ provided by Marco Walther.

   • ‘libtool.m4’, ‘ltdl.m4’ and the ‘configure.ac’ files are marked to
     use autoconf-mode, which is distributed with GNU Emacs 21, Autoconf
     itself, and all recent releases of XEmacs.

   • When building on some GNU/Linux systems for multilib targets
     ‘libtool’ sometimes guesses the wrong paths that the linker and
     dynamic linker search by default.  If this occurs for the dynamic
     library path, you may use the ‘LT_SYS_LIBRARY_PATH’ environment
     variable to adjust.  Otherwise, at ‘configure’ time you may
     override libtool's guesses by setting the ‘autoconf’ cache
     variables ‘lt_cv_sys_lib_search_path_spec’ and
     ‘lt_cv_sys_lib_dlsearch_path_spec’ respectively.

   • For many GNU/Linux systems, the shared library cache is not updated
     after a ‘make install’.  Instead, the user will need to manually
     execute ‘ldconfig 'LIBDIR'’ to locate newly installed libraries.
     Users should consult a system administrator as this should be done
     with great care.  Here are some considerations:

        • Libtool is designed to be portable to many different operating
          systems, and these OSs behave in various ways.  Some OSs will
          find a new library automatically if it is installed in the
          existing configured library paths.

        • The library installed might not ever be intended to be used by
          the currently executing OS because it is part of new
          distribution builds.

        • Sometimes libraries are installed privately, using hard-coded
          run-paths in their dependent binaries.  This is very common
          when a different version of the software is to be run than the
          OS is designed to support.

   • When using the MSVC toolchain, users should set Fortran environment
     variables to the following F77="no" and FC="no".  This should avoid
     possible issues for symbols not being found when linking libltdl.


File: libtool.info,  Node: Distributing,  Next: Static-only libraries,  Prev: Configuring,  Up: Integrating libtool

5.5 Including libtool in your package
=====================================

In order to use libtool, you need to include the following files with
your package:

‘config.guess’
     Attempt to guess a canonical system name.

‘config.sub’
     Canonical system name validation subroutine script.

‘install-sh’
     BSD-compatible ‘install’ replacement script.

‘ltmain.sh’
     A generic script implementing basic libtool functionality.

   Note that the libtool script itself should _not_ be included with
your package.  *Note Configuring::.

   You should use the ‘libtoolize’ program, rather than manually copying
these files into your package.

* Menu:

* Invoking libtoolize::         ‘libtoolize’ command line options.
* Autoconf and LTLIBOBJS::      Autoconf automates LTLIBOBJS generation.


File: libtool.info,  Node: Invoking libtoolize,  Next: Autoconf and LTLIBOBJS,  Up: Distributing

5.5.1 Invoking ‘libtoolize’
---------------------------

The ‘libtoolize’ program provides a standard way to add libtool support
to your package.  In the future, it may implement better usage checking,
or other features to make libtool even easier to use.

   The ‘libtoolize’ program has the following synopsis:

     libtoolize [OPTION]...

and accepts the following options:

‘--copy’
‘-c’
     Copy files from the libtool data directory rather than creating
     symlinks.

‘--debug’
     Dump a trace of shell script execution to standard output.  This
     produces a lot of output, so you may wish to pipe it to ‘less’ (or
     ‘more’) or redirect to a file.

‘--dry-run’
‘-n’
     Don't run any commands that modify the file system, just print them
     out.

‘--force’
‘-f’
     Replace existing libtool files.  By default, ‘libtoolize’ won't
     overwrite existing files.

‘--help’
     Display a help message and exit.

‘--ltdl [TARGET-DIRECTORY-NAME]’
     Install libltdl in the TARGET-DIRECTORY-NAME subdirectory of your
     package.  Normally, the directory is extracted from the argument to
     ‘LT_CONFIG_LTDL_DIR’ in ‘configure.ac’, though you can also specify
     a subdirectory name here if you are not using Autoconf for example.
     If ‘libtoolize’ can't determine the target directory, ‘libltdl’ is
     used as the default.

‘--no-warn’
     Normally, Libtoolize tries to diagnose use of deprecated libtool
     macros and other stylistic issues.  If you are deliberately using
     outdated calling conventions, this option prevents Libtoolize from
     explaining how to update your project's Libtool conventions.

‘--nonrecursive’
     If passed in conjunction with ‘--ltdl’, this option will cause the
     ‘libltdl’ installed by ‘libtoolize’ to be set up for use with a
     non-recursive ‘automake’ build.  To make use of it, you will need
     to add the following to the ‘Makefile.am’ of the parent project:

          ## libltdl/ltdl.mk appends to the following variables
          ## so we set them here before including it:
          BUILT_SOURCES   =

          AM_CPPFLAGS        =
          AM_LDFLAGS         =

          include_HEADERS    =
          noinst_LTLIBRARIES =
          lib_LTLIBRARIES   =
          EXTRA_LTLIBRARIES  =

          EXTRA_DIST   =

          CLEANFILES   =
          MOSTLYCLEANFILES   =

          include libltdl/ltdl.mk

‘--quiet’
‘-q’
     Work silently.  ‘libtoolize --quiet’ is used by GNU Automake to add
     libtool files to your package if necessary.

‘--recursive’
     If passed in conjunction with ‘--ltdl’, this option will cause the
     ‘libtoolize’ installed ‘libltdl’ to be set up for use with a
     recursive ‘automake’ build.  To make use of it, you will need to
     adjust the parent project's ‘configure.ac’:

          AC_CONFIG_FILES([libltdl/Makefile])

     and ‘Makefile.am’:

          SUBDIRS += libltdl

‘--subproject’
     If passed in conjunction with ‘--ltdl’, this option will cause the
     ‘libtoolize’ installed ‘libltdl’ to be set up for independent
     configuration and compilation as a self-contained subproject.  To
     make use of it, you should arrange for your build to call
     ‘libltdl/configure’, and then run ‘make’ in the ‘libltdl’ directory
     (or the subdirectory you put libltdl into).  If your project uses
     Autoconf, you can use the supplied ‘LT_WITH_LTDL’ macro, or else
     call ‘AC_CONFIG_SUBDIRS’ directly.

     Previous releases of ‘libltdl’ built exclusively in this mode, but
     now it is the default mode both for backwards compatibility and
     because, for example, it is suitable for use in projects that wish
     to use ‘libltdl’, but not use the Autotools for their own build
     process.

‘--verbose’
‘-v’
     Work noisily!  Give a blow by blow account of what ‘libtoolize’ is
     doing.

‘--version’
     Print ‘libtoolize’ version information and exit.

   Sometimes it can be useful to pass options to ‘libtoolize’ even
though it is called by another program, such as ‘autoreconf’.  A limited
number of options are parsed from the environment variable
‘LIBTOOLIZE_OPTIONS’: currently ‘--debug’, ‘--no-warn’, ‘--quiet’ and
‘--verbose’.  Multiple options passed in ‘LIBTOOLIZE_OPTIONS’ must be
separated with a space, comma or a colon.

   By default, a warning is issued for unknown options found in
‘LIBTOOLIZE_OPTIONS’ unless the first such option is ‘--no-warn’.  Where
‘libtoolize’ has always quit on receipt of an unknown option at the
command line, this and all previous releases of ‘libtoolize’ will
continue unabated whatever the content of ‘LIBTOOLIZE_OPTIONS’ (modulo
some possible warning messages).

     trick$ LIBTOOLIZE_OPTIONS=--no-warn,--quiet autoreconf --install

   If ‘libtoolize’ detects an explicit call to ‘AC_CONFIG_MACRO_DIRS’
(*note The Autoconf Manual: (autoconf)Input.) in your ‘configure.ac’, it
will put the Libtool macros in the specified directory.

   In the future other Autotools will automatically check the contents
of ‘AC_CONFIG_MACRO_DIRS’, but at the moment it is more portable to add
the macro directory to ‘ACLOCAL_AMFLAGS’ in ‘Makefile.am’, which is
where the tools currently look.  If ‘libtoolize’ doesn't see
‘AC_CONFIG_MACRO_DIRS’, it too will honour the first ‘-I’ argument in
‘ACLOCAL_AMFLAGS’ when choosing a directory to store libtool
configuration macros in.  It is perfectly sensible to use both
‘AC_CONFIG_MACRO_DIRS’ and ‘ACLOCAL_AMFLAGS’, as long as they are kept
in synchronisation.

     ACLOCAL_AMFLAGS = -I m4

   When you bootstrap your project with ‘aclocal’, then you will need to
explicitly pass the same macro directory with ‘aclocal’'s ‘-I’ flag:

     trick$ aclocal -I m4

   If ‘libtoolize’ detects an explicit call to ‘AC_CONFIG_AUX_DIR’
(*note The Autoconf Manual: (autoconf)Input.) in your ‘configure.ac’, it
will put the other support files in the specified directory.  Otherwise
they too end up in the project root directory.

   Unless ‘--no-warn’ is passed, ‘libtoolize’ displays hints for adding
libtool support to your package, as well.


File: libtool.info,  Node: Autoconf and LTLIBOBJS,  Prev: Invoking libtoolize,  Up: Distributing

5.5.2 Autoconf and ‘LTLIBOBJS’
------------------------------

People used to add code like the following to their ‘configure.ac’:

     LTLIBOBJS=`echo "$LIBOBJS" | sed 's/\.[^.]* /.lo /g;s/\.[^.]*$/.lo/'`
     AC_SUBST([LTLIBOBJS])

This is no longer required (since Autoconf 2.54), and doesn't take
Automake's deansification support into account either, so doesn't work
correctly even with ancient Autoconfs!

   Provided you are using a recent (2.54 or better) incarnation of
Autoconf, the call to ‘AC_OUTPUT’ takes care of setting ‘LTLIBOBJS’ up
correctly, so you can simply delete such snippets from your
‘configure.ac’ if you had them.


File: libtool.info,  Node: Static-only libraries,  Prev: Distributing,  Up: Integrating libtool

5.6 Static-only libraries
=========================

When you are developing a package, it is often worthwhile to configure
your package with the ‘--disable-shared’ flag, or to override the
defaults for ‘LT_INIT’ by using the ‘disable-shared’ option (*note The
‘LT_INIT’ macro: LT_INIT.). This prevents libtool from building shared
libraries, which has several advantages:

   • compilation is twice as fast, which can speed up your development
     cycle,

   • debugging is easier because you don't need to deal with any
     complexities added by shared libraries, and

   • you can see how libtool behaves on static-only platforms.

   You may want to put a small note in your package ‘README’ to let
other developers know that ‘--disable-shared’ can save them time.  The
following example note is taken from the GIMP(1) distribution ‘README’:

     The GIMP uses GNU Libtool to build shared libraries on a
     variety of systems.  While this is very nice for making usable
     binaries, it can be a pain when trying to debug a program.  For that
     reason, compilation of shared libraries can be turned off by
     specifying the --disable-shared option to configure.

   ---------- Footnotes ----------

   (1) GNU Image Manipulation Program, for those who haven't taken the
plunge.  See <https://www.gimp.org/>.


File: libtool.info,  Node: Other languages,  Next: Versioning,  Prev: Integrating libtool,  Up: Top

6 Using libtool with other languages
************************************

Libtool was first implemented to add support for writing shared
libraries in the C language.  However, over time, libtool is being
integrated with other languages, so that programmers are free to reap
the benefits of shared libraries in their favorite programming language.

   This chapter describes how libtool interacts with other languages,
and what special considerations you need to make if you do not use C.

* Menu:

* C++ libraries::               Writing libraries for C++
* Tags::                        Tags


File: libtool.info,  Node: C++ libraries,  Next: Tags,  Up: Other languages

6.1 Writing libraries for C‘++’
===============================

Creating libraries of C++ code should be a fairly straightforward
process, because its object files differ from C ones in only three ways:

  1. Because of name mangling, C++ libraries are only usable by the C++
     compiler that created them.  This decision was made by the
     designers of C++ to protect users from conflicting implementations
     of features such as constructors, exception handling, and RTTI.

  2. On some systems, the C++ compiler must take special actions for the
     dynamic linker to run dynamic (i.e., run-time) initializers.  This
     means that we should not call ‘ld’ directly to link such libraries,
     and we should use the C++ compiler instead.

  3. C++ compilers will link some Standard C++ library in by default,
     but libtool does not know what these libraries are, so it cannot
     even run the inter-library dependence analyzer to check how to link
     it in.  Therefore, running ‘ld’ to link a C++ program or library is
     deemed to fail.

   Because of these three issues, Libtool has been designed to always
use the C++ compiler to compile and link C++ programs and libraries.  In
some instances the ‘main()’ function of a program must also be compiled
with the C++ compiler for static C++ objects to be properly initialized.


File: libtool.info,  Node: Tags,  Prev: C++ libraries,  Up: Other languages

6.2 Tags
========

Libtool supports multiple languages through the use of tags.
Technically a tag corresponds to a set of configuration variables
associated with a language.  These variables tell ‘libtool’ how it
should create objects and libraries for each language.

   Tags are defined at ‘configure’-time for each language activated in
the package (see ‘LT_LANG’ in *note LT_INIT::).  Here is the
correspondence between language names and tags names.

Language name      Tag name
C                  CC
C++                CXX
Objective-C        OBJC
Objective-C++      OBJCXX
Java               GCJ
Fortran 77         F77
Fortran            FC
Go                 GO
Windows Resource   RC

   ‘libtool’ tries to automatically infer what tag to use from the
compiler command being used to compile or link.  If it can't infer a
tag, then it exits immediately with a fatal error.

   The tag can also be specified using ‘libtool’'s ‘--tag=TAG’ option
(*note Invoking libtool::).  It is a good idea to do so in ‘Makefile’
rules, because that will allow users to substitute the compiler without
relying on ‘libtool’ inference heuristics.  When no tag is specified,
‘libtool’ will default to ‘CC’; this tag always exists.

   Finally, the set of tags available in a particular project can be
retrieved by tracing for the ‘LT_SUPPORTED_TAG’ macro (*note Trace
interface::).


File: libtool.info,  Node: Versioning,  Next: Library tips,  Prev: Other languages,  Up: Top

7 Library interface versions
****************************

The most difficult issue introduced by shared libraries is that of
creating and resolving runtime dependencies.  Dependencies on programs
and libraries are often described in terms of a single name, such as
‘sed’.  So, one may say "libtool depends on sed," and that is good
enough for most purposes.

   However, when an interface changes regularly, we need to be more
specific: "Gnus 5.1 requires Emacs 19.28 or above."  Here, the
description of an interface consists of a name, and a "version number."

   Even that sort of description is not accurate enough for some
purposes.  What if Emacs 20 changes enough to break Gnus 5.1?

   The same problem exists in shared libraries: we require a formal
version system to describe the sorts of dependencies that programs have
on shared libraries, so that the dynamic linker can guarantee that
programs are linked only against libraries that provide the interface
they require.

* Menu:

* Interfaces::                  What are library interfaces?
* Libtool versioning::          Libtool's versioning system.
* Updating version info::       Changing version information before releases.
* Release numbers::             Breaking binary compatibility for aesthetics.


File: libtool.info,  Node: Interfaces,  Next: Libtool versioning,  Up: Versioning

7.1 What are library interfaces?
================================

Interfaces for libraries may be any of the following (and more):

   • global variables: both names and types

   • global functions: argument types and number, return types, and
     function names

   • standard input, standard output, standard error, and file formats

   • sockets, pipes, and other inter-process communication protocol
     formats

   Note that static functions do not count as interfaces, because they
are not directly available to the user of the library.


File: libtool.info,  Node: Libtool versioning,  Next: Updating version info,  Prev: Interfaces,  Up: Versioning

7.2 Libtool's versioning system
===============================

Libtool has its own formal versioning system.  It is not as flexible as
some, but it is definitely the simplest of the more powerful versioning
systems.

   Think of a library as exporting several sets of interfaces,
arbitrarily represented by integers.  When a program is linked against a
library, it may use any subset of those interfaces.

   Libtool's description of the interfaces that a program uses is
simple: it encodes the least and the greatest interface numbers in the
resulting binary (FIRST-INTERFACE, LAST-INTERFACE).

   The dynamic linker is guaranteed that if a library supports _every_
interface number between FIRST-INTERFACE and LAST-INTERFACE, then the
program can be relinked against that library.

   Note that this can cause problems because libtool's compatibility
requirements are actually stricter than is necessary.

   Say ‘libhello’ supports interfaces 5, 16, 17, 18, and 19, and that
libtool is used to link ‘test’ against ‘libhello’.

   Libtool encodes the numbers 5 and 19 in ‘test’, and the dynamic
linker will only link ‘test’ against libraries that support _every_
interface between 5 and 19.  So, the dynamic linker refuses to link
‘test’ against ‘libhello’!

   In order to eliminate this problem, libtool only allows libraries to
declare consecutive interface numbers.  So, ‘libhello’ can declare at
most that it supports interfaces 16 through 19.  Then, the dynamic
linker will link ‘test’ against ‘libhello’.

   So, libtool library versions are described by three integers:

CURRENT
     The most recent interface number that this library implements.

REVISION
     The implementation number of the CURRENT interface.

AGE
     The difference between the newest and oldest interfaces that this
     library implements.  In other words, the library implements all the
     interface numbers in the range from number ‘CURRENT - AGE’ to
     ‘CURRENT’.

   If two libraries have identical CURRENT and AGE numbers, then the
dynamic linker chooses the library with the greater REVISION number.


File: libtool.info,  Node: Updating version info,  Next: Release numbers,  Prev: Libtool versioning,  Up: Versioning

7.3 Updating library version information
========================================

If you want to use libtool's versioning system, then you must specify
the version information to libtool using the ‘-version-info’ flag during
link mode (*note Link mode::).

   This flag accepts an argument of the form ‘CURRENT[:REVISION[:AGE]]’.
So, passing ‘-version-info 3:12:1’ sets CURRENT to 3, REVISION to 12,
and AGE to 1.

   If either REVISION or AGE are omitted, they default to 0.  Also note
that AGE must be less than or equal to the CURRENT interface number.

   Here are a set of rules to help you update your library version
information:

   • Start with version information of ‘0:0:0’ for each libtool library.

   • Update the version information only immediately before a public
     release of your software.  More frequent updates are unnecessary,
     and only guarantee that the current interface number gets larger
     faster.

   • Do the update either manually, or guided by the
     ‘libtool-next-version’ wizard.

   *_Never_* try to set the interface numbers so that they correspond to
the release number of your package.  This is an abuse that only fosters
misunderstanding of the purpose of library versions.  Instead, use the
‘-release’ flag (*note Release numbers::), but be warned that every
release of your package will not be binary compatible with any other
release.

* Menu:

* Manual version info update::    Updating version info manually.
* Guided version info update::    Using the libtool-next-version program.
* Invoking libtool-next-version:: ‘libtool-next-version’ command line options.


File: libtool.info,  Node: Manual version info update,  Next: Guided version info update,  Up: Updating version info

7.3.1 Updating the version info manually
----------------------------------------

Here are the steps that you need to do, to update your library version
information:

  1. If the library source code has changed at all since the last
     update, then increment REVISION (‘C:R:A’ becomes ‘C:r+1:A’).

  2. If any interfaces have been added, removed, or changed since the
     last update, increment CURRENT, and set REVISION to 0.

  3. If any interfaces have been added since the last public release,
     then increment AGE.

  4. If any interfaces have been removed or changed since the last
     public release, then set AGE to 0.

   The following explanation may help to understand the steps above a
bit better: consider that there are three possible kinds of reactions
from users of your library to changes in a shared library:

  1. Programs using the previous version may use the new version as
     drop-in replacement, and programs using the new version can also
     work with the previous one.  In other words, no recompiling nor
     relinking is needed.  In this case, bump REVISION only, don't touch
     CURRENT nor AGE.

  2. Programs using the previous version may use the new version as
     drop-in replacement, but programs using the new version may use
     APIs not present in the previous one.  In other words, a program
     linking against the new version may fail with "unresolved symbols"
     if linking against the old version at runtime: set REVISION to 0,
     bump CURRENT and AGE.

  3. Programs may need to be changed, recompiled, and relinked in order
     to use the new version.  Bump CURRENT, set REVISION and AGE to 0.

In the above description, _programs_ using the library in question may
also be replaced by other libraries using it.


File: libtool.info,  Node: Guided version info update,  Next: Invoking libtool-next-version,  Prev: Manual version info update,  Up: Updating version info

7.3.2 Updating the version info, guided by the libtool-next-version program
---------------------------------------------------------------------------

The ‘libtool-next-version’ program is a wizard-like interactive tool,
that is designed to avoid mistakes in the process of the manual update:

   • It asks you questions, one by one, so that you can focus on one
     thing at a time.
   • It prepares default answers, based on the set of symbols exported
     by library.
   • It gives a couple of additional explanations and hints.

   Before invoking the wizard, you need to build and install the
previous release and the current release candidate of your package into
different directories.  For example, assume the last release was
‘libfoo-1.4.tar.gz’, and before preparing ‘libfoo-1.5.tar.gz’, your
current release candidate is ‘libfoo-1.4.99.tar.gz’.  You build them
like this:
     $ rm -rf /tmp/prev /tmp/curr
     $ tar xfz libfoo-1.4.tar.gz
     $ (cd libfoo-1.4
        && ./configure --prefix=/tmp/prev
        && make
        && make install)
     $ tar xfz libfoo-1.4.99.tar.gz
     $ (cd libfoo-1.4.99
        && ./configure --prefix=/tmp/curr
        && make
        && make install)

   Then you are ready to invoke the wizard like this:
     $ libtool-next-version /tmp/prev/lib/libfoo.so /tmp/curr/lib/libfoo.so


File: libtool.info,  Node: Invoking libtool-next-version,  Prev: Guided version info update,  Up: Updating version info

7.3.3 Invoking ‘libtool-next-version’
-------------------------------------

The ‘libtool-next-version’ program determines the next version to use
for a libtool library.

   It is invoked as follows:
     libtool-next-version [OPTION]... PREVIOUS-LIBRARY CURRENT-LIBRARY

   PREVIOUS-LIBRARY is the installed library (in ‘.a’ or ‘.so’ format)
of the previous release.

   CURRENT-LIBRARY is the installed library (in ‘.a’ or ‘.so’ format) of
the current release candidate.

   It accepts the following options:

‘--help’
     Print a help message and exit.

‘--version’
     Print version information and exit.


File: libtool.info,  Node: Release numbers,  Prev: Updating version info,  Up: Versioning

7.4 Managing release information
================================

Often, people want to encode the name of the package release into the
shared library so that it is obvious to the user what package their
programs are linked against.  This convention is used especially on
GNU/Linux:

     trick$ ls /usr/lib/libbfd*
     /usr/lib/libbfd.a           /usr/lib/libbfd.so.2.7.0.2
     /usr/lib/libbfd.so
     trick$

   On ‘trick’, ‘/usr/lib/libbfd.so’ is a symbolic link to
‘libbfd.so.2.7.0.2’, which was distributed as a part of
‘binutils-2.7.0.2’.

   Unfortunately, this convention conflicts directly with libtool's idea
of library interface versions, because the library interface rarely
changes at the same time that the release number does, and the library
suffix is never the same across all platforms.

   So, to accommodate both views, you can use the ‘-release’ flag to set
release information for libraries for which you do not want to use
‘-version-info’.  For the ‘libbfd’ example, the next release that uses
libtool should be built with ‘-release 2.9.0’, which will produce the
following files on GNU/Linux:

     trick$ ls /usr/lib/libbfd*
     /usr/lib/libbfd-2.9.0.so     /usr/lib/libbfd.a
     /usr/lib/libbfd.so
     trick$

   In this case, ‘/usr/lib/libbfd.so’ is a symbolic link to
‘libbfd-2.9.0.so’.  This makes it obvious that the user is dealing with
‘binutils-2.9.0’, without compromising libtool's idea of interface
versions.

   Note that this option causes a modification of the library name, so
do not use it unless you want to break binary compatibility with any
past library releases.  In general, you should only use ‘-release’ for
package-internal libraries or for ones whose interfaces change very
frequently.


File: libtool.info,  Node: Library tips,  Next: Inter-library dependencies,  Prev: Versioning,  Up: Top

8 Tips for interface design
***************************

Writing a good library interface takes a lot of practice and thorough
understanding of the problem that the library is intended to solve.

   If you design a good interface, it won't have to change often, you
won't have to keep updating documentation, and users won't have to keep
relearning how to use the library.

   Here is a brief list of tips for library interface design that may
help you in your exploits:

Plan ahead
     Try to make every interface truly minimal, so that you won't need
     to delete entry points very often.

Avoid interface changes
     Some people love redesigning and changing entry points just for the
     heck of it (note: _renaming_ a function is considered changing an
     entry point).  Don't be one of those people.  If you must redesign
     an interface, then try to leave compatibility functions behind so
     that users don't need to rewrite their existing code.

Use opaque data types
     The fewer data type definitions a library user has access to, the
     better.  If possible, design your functions to accept a generic
     pointer (that you can cast to an internal data type), and provide
     access functions rather than allowing the library user to directly
     manipulate the data.  That way, you have the freedom to change the
     data structures without changing the interface.

     This is essentially the same thing as using abstract data types and
     inheritance in an object-oriented system.

Use header files
     If you are careful to document each of your library's global
     functions and variables in header files, and include them in your
     library source files, then the compiler will let you know if you
     make any interface changes by accident (*note C header files::).

Use the ‘static’ keyword (or equivalent) whenever possible
     The fewer global functions your library has, the more flexibility
     you'll have in changing them.  Static functions and variables may
     change forms as often as you like... your users cannot access them,
     so they aren't interface changes.

Be careful with array dimensions
     The number of elements in a global array is part of an interface,
     even if the header just declares ‘extern int foo[];’.  This is
     because on i386 and some other SVR4/ELF systems, when an
     application references data in a shared library the size of that
     data (whatever its type) is included in the application executable.
     If you might want to change the size of an array or string then
     provide a pointer not the actual array.

* Menu:

* C header files::              How to write portable include files.


File: libtool.info,  Node: C header files,  Up: Library tips

8.1 Writing C header files
==========================

Writing portable C header files can be difficult, since they may be read
by different types of compilers:

C++ compilers
     C++ compilers require that functions be declared with full
     prototypes, since C++ is more strongly typed than C.  C functions
     and variables also need to be declared with the ‘extern "C"’
     directive, so that the names aren't mangled.  *Note C++
     libraries::, for other issues relevant to using C++ with libtool.

ANSI C compilers
     ANSI C compilers are not as strict as C++ compilers, but functions
     should be prototyped to avoid unnecessary warnings when the header
     file is ‘#include’d.

non-ANSI C compilers
     Non-ANSI compilers will report errors if functions are prototyped.

   These complications mean that your library interface headers must use
some C preprocessor magic to be usable by each of the above compilers.

   ‘foo.h’, defined in the ‘tests/demo.at’ Autotest of the libtool
distribution, serves as an example for how to write a header file that
can be safely installed in a system directory.

   Here are the relevant portions of that file:

     #ifndef FOO_H
     #define FOO_H

     #ifdef __cplusplus
     extern "C" {
     #endif

     int foo (void);
     int hello (void);

     #ifdef __cplusplus
     }
     #endif

     #endif /* !FOO_H */

   This can also be achieved by utilizing macros:

     /* BEGIN_C_DECLS should be used at the beginning of your declarations,
        so that C++ compilers don't mangle their names.  Use END_C_DECLS at
        the end of C declarations. */
     #undef BEGIN_C_DECLS
     #undef END_C_DECLS
     #ifdef __cplusplus
     # define BEGIN_C_DECLS extern "C" {
     # define END_C_DECLS }
     #else
     # define BEGIN_C_DECLS /* empty */
     # define END_C_DECLS /* empty */
     #endif

     /* PARAMS is a macro used to wrap function prototypes, so that
        compilers that don't understand ANSI C prototypes still work,
        and ANSI C compilers can issue warnings about type mismatches. */
     #undef PARAMS
     #if defined __STDC__ || defined _AIX \
             || (defined __mips && defined _SYSTYPE_SVR4) \
             || defined WIN32 || defined __cplusplus
     # define PARAMS(protos) protos
     #else
     # define PARAMS(protos) ()
     #endif

   These macros can be used in ‘foo.h’ as follows:

     #ifndef FOO_H
     #define FOO_H

     /* The above macro definitions. */
     #include "..."

     BEGIN_C_DECLS

     int foo PARAMS((void));
     int hello PARAMS((void));

     END_C_DECLS

     #endif /* !FOO_H */

   Note that the ‘#ifndef FOO_H’ prevents the body of ‘foo.h’ from being
read more than once in a given compilation.

   Also the only thing that must go outside the
‘BEGIN_C_DECLS’/‘END_C_DECLS’ pair are ‘#include’ lines.  Strictly
speaking it is only C symbol names that need to be protected, but your
header files will be more maintainable if you have a single pair of
these macros around the majority of the header contents.

   You should use these definitions of ‘PARAMS’, ‘BEGIN_C_DECLS’, and
‘END_C_DECLS’ into your own headers.  Then, you may use them to create
header files that are valid for C++, ANSI, and non-ANSI compilers(1).

   Do not be naive about writing portable code.  Following the tips
given above will help you miss the most obvious problems, but there are
definitely other subtle portability issues.  You may need to cope with
some of the following issues:

   • Pre-ANSI compilers do not always support the ‘void *’ generic
     pointer type, and so need to use ‘char *’ in its place.

   • The ‘const’, ‘inline’ and ‘signed’ keywords are not supported by
     some compilers, especially pre-ANSI compilers.

   • The ‘long double’ type is not supported by many compilers.

   ---------- Footnotes ----------

   (1) We used to recommend ‘__P’, ‘__BEGIN_DECLS’ and ‘__END_DECLS’.
This was bad advice since symbols (even preprocessor macro names) that
begin with an underscore are reserved for the use of the compiler.


File: libtool.info,  Node: Inter-library dependencies,  Next: Dlopened modules,  Prev: Library tips,  Up: Top

9 Inter-library dependencies
****************************

By definition, every shared library system provides a way for
executables to depend on libraries, so that symbol resolution is
deferred until runtime.

   An “inter-library dependency” is where a library depends on other
libraries.  For example, if the libtool library ‘libhello’ uses the
‘cos’ function, then it has an inter-library dependency on ‘libm’, the
math library that implements ‘cos’.

   Some shared library systems provide this feature in an
internally-consistent way: these systems allow chains of dependencies of
potentially infinite length.

   However, most shared library systems are restricted in that they only
allow a single level of dependencies.  In these systems, programs may
depend on shared libraries, but shared libraries may not depend on other
shared libraries.

   In any event, libtool provides a simple mechanism for you to declare
inter-library dependencies: for every library ‘libNAME’ that your own
library depends on, simply add a corresponding ‘-lNAME’ option to the
link line when you create your library.  To make an example of our
‘libhello’ that depends on ‘libm’:

     burger$ libtool --mode=link gcc -g -O -o libhello.la foo.lo hello.lo \
                     -rpath /usr/local/lib -lm
     burger$

   When you link a program against ‘libhello’, you don't need to specify
the same ‘-l’ options again: libtool will do that for you, to guarantee
that all the required libraries are found.  This restriction is only
necessary to preserve compatibility with static library systems and
simple dynamic library systems.

   Some platforms, such as Windows, do not even allow you this
flexibility.  In order to build a shared library, it must be entirely
self-contained or it must have dependencies known at link time (that is,
have references only to symbols that are found in the ‘.lo’ files or the
specified ‘-l’ libraries), and you need to specify the ‘-no-undefined’
flag.  By default, libtool builds only static libraries on these kinds
of platforms.

   The simple-minded inter-library dependency tracking code of libtool
releases prior to 1.2 was disabled because it was not clear when it was
possible to link one library with another, and complex failures would
occur.  A more complex implementation of this concept was re-introduced
before release 1.3, but it has not been ported to all platforms that
libtool supports.  The default, conservative behavior is to avoid
linking one library with another, introducing their inter-dependencies
only when a program is linked with them.


File: libtool.info,  Node: Dlopened modules,  Next: Using libltdl,  Prev: Inter-library dependencies,  Up: Top

10 Dlopened modules
*******************

It can sometimes be confusing to discuss “dynamic linking”, because the
term is used to refer to two different concepts:

  1. Compiling and linking a program against a shared library, which is
     resolved automatically at run time by the dynamic linker.  In this
     process, dynamic linking is transparent to the application.

  2. The application calling functions such as ‘dlopen’ that load
     arbitrary, user-specified modules at runtime.  This type of dynamic
     linking is explicitly controlled by the application.

   To mitigate confusion, this manual refers to the second type of
dynamic linking as “dlopening” a module.

   The main benefit to dlopening object modules is the ability to access
compiled object code to extend your program, rather than using an
interpreted language.  In fact, dlopen calls are frequently used in
language interpreters to provide an efficient way to extend the
language.

   Libtool provides support for dlopened modules.  However, you should
indicate that your package is willing to use such support, by using the
‘LT_INIT’ option ‘dlopen’ in ‘configure.ac’.  If this option is not
given, libtool will assume no dlopening mechanism is available, and will
try to simulate it.

   This chapter discusses how you as a dlopen application developer
might use libtool to generate dlopen-accessible modules.

* Menu:

* Building modules::            Creating dlopenable objects and libraries.
* Dlpreopening::                Dlopening that works on static platforms.
* Linking with dlopened modules::  Using dlopenable modules in libraries.
* Finding the dlname::          Choosing the right file to ‘dlopen’.
* Dlopen issues::               Unresolved problems that need your attention.


File: libtool.info,  Node: Building modules,  Next: Dlpreopening,  Up: Dlopened modules

10.1 Building modules to dlopen
===============================

On some operating systems, a program symbol must be specially declared
in order to be dynamically resolved with the ‘dlsym’ (or equivalent)
function.  Libtool provides the ‘-export-dynamic’ and ‘-module’ link
flags (*note Link mode::), for you to make that declaration.  You need
to use these flags if you are linking an application program that
dlopens other modules or a libtool library that will also be dlopened.

   For example, if we wanted to build a shared library, ‘hello’, that
would later be dlopened by an application, we would add ‘-module’ to the
other link flags:

     burger$ libtool --mode=link gcc -module -o hello.la foo.lo \
                     hello.lo -rpath /usr/local/lib -lm
     burger$

   If symbols from your _executable_ are needed to satisfy unresolved
references in a library you want to dlopen you will have to use the flag
‘-export-dynamic’.  You should use ‘-export-dynamic’ while linking the
executable that calls dlopen:

     burger$ libtool --mode=link gcc -export-dynamic -o helldl main.o
     burger$


File: libtool.info,  Node: Dlpreopening,  Next: Linking with dlopened modules,  Prev: Building modules,  Up: Dlopened modules

10.2 Dlpreopening
=================

Libtool provides special support for dlopening libtool object and
libtool library files, so that their symbols can be resolved _even on
platforms without any ‘dlopen’ and ‘dlsym’ functions_.

   Consider the following alternative ways of loading code into your
program, in order of increasing "laziness":

  1. Linking against object files that become part of the program
     executable, whether or not they are referenced.  If an object file
     cannot be found, then the compile time linker refuses to create the
     executable.

  2. Declaring a static library to the linker, so that it is searched at
     link time to satisfy any undefined references in the above object
     files.  If the static library cannot be found, then the compile
     time linker refuses to create the executable.

  3. Declaring a shared library to the runtime linker, so that it is
     searched at runtime to satisfy any undefined references in the
     above files.  If the shared library cannot be found, then the
     dynamic linker aborts the program before it runs.

  4. Dlopening a module, so that the application can resolve its own,
     dynamically-computed references.  If there is an error opening the
     module, or the module is not found, then the application can
     recover without crashing.

   Libtool emulates ‘-dlopen’ on static platforms by linking objects
into the program at compile time, and creating data structures that
represent the program's symbol table.  In order to use this feature, you
must declare the objects you want your application to dlopen by using
the ‘-dlopen’ or ‘-dlpreopen’ flags when you link your program (*note
Link mode::).

 -- Data Type: lt_dlsymlist typedef struct { const char *name;
          void *address; } lt_dlsymlist
     The ‘name’ attribute is a null-terminated character string of the
     symbol name, such as ‘"fprintf"’.  The ‘address’ attribute is
     generic pointer to the appropriate object, such as ‘&fprintf’.

 -- Variable: const lt_dlsymlist lt_preloaded_symbols[]
     An array of ‘lt_dlsymlist’ structures, representing all the
     preloaded symbols linked into the program proper.  For each module
     ‘-dlpreopen’ed by the Libtool linked program there is an element
     with the ‘name’ of the module and an ‘address’ of ‘0’, followed by
     all symbols exported from this file.  For the executable itself the
     special name ‘@PROGRAM@’ is used.  The last element of all has a
     ‘name’ and ‘address’ of ‘0’.

     To facilitate inclusion of symbol lists into libraries,
     ‘lt_preloaded_symbols’ is ‘#define’d to a suitably unique name in
     ‘ltdl.h’.

     This variable may not be declared ‘const’ on some systems due to
     relocation issues.

   Some compilers may allow identifiers that are not valid in ANSI C,
such as dollar signs.  Libtool only recognizes valid ANSI C symbols (an
initial ASCII letter or underscore, followed by zero or more ASCII
letters, digits, and underscores), so non-ANSI symbols will not appear
in ‘lt_preloaded_symbols’.

 -- Function: int lt_dlpreload (const lt_dlsymlist *PRELOADED)
     Register the list of preloaded modules PRELOADED.  If PRELOADED is
     ‘NULL’, then all previously registered symbol lists, except the
     list set by ‘lt_dlpreload_default’, are deleted.  Return 0 on
     success.

 -- Function: int lt_dlpreload_default (const lt_dlsymlist *PRELOADED)
     Set the default list of preloaded modules to PRELOADED, which won't
     be deleted by ‘lt_dlpreload’.  Note that this function does _not_
     require libltdl to be initialized using ‘lt_dlinit’ and can be used
     in the program to register the default preloaded modules.  Instead
     of calling this function directly, most programs will use the macro
     ‘LTDL_SET_PRELOADED_SYMBOLS’.

     Return 0 on success.

 -- Macro: LTDL_SET_PRELOADED_SYMBOLS
     Set the default list of preloaded symbols.  Should be used in your
     program to initialize libltdl's list of preloaded modules.

          #include <ltdl.h>

          int main() {
            /* ... */
            LTDL_SET_PRELOADED_SYMBOLS();
            /* ... */
          }

 -- Function Type: int lt_dlpreload_callback_func (lt_dlhandle HANDLE)
     Functions of this type can be passed to ‘lt_dlpreload_open’, which
     in turn will call back into a function thus passed for each
     preloaded module that it opens.

 -- Function: int lt_dlpreload_open (const char *ORIGINATOR,
          lt_dlpreload_callback_func *FUNC)
     Load all of the preloaded modules for ORIGINATOR.  For every module
     opened in this way, call FUNC.

     To open all of the modules preloaded into ‘libhell.la’ (presumably
     from within the ‘libhell.a’ initialisation code):

          #define preloaded_symbols lt_libhell_LTX_preloaded_symbols

          static int hell_preload_callback (lt_dlhandle handle);

          int
          hell_init (void)
          {
            ...
            if (lt_dlpreload (&preloaded_symbols) == 0)
              {
                lt_dlpreload_open ("libhell", preload_callback);
              }
            ...
          }

     Note that to prevent clashes between multiple preloaded modules,
     the preloaded symbols are accessed via a mangled symbol name: to
     get the symbols preloaded into ‘libhell’, you must prefix
     ‘preloaded_symbols’ with ‘lt_’; the originator name, ‘libhell’ in
     this case; and ‘_LTX_’.  That is,
     ‘lt_libhell_LTX_preloaded_symbols’ here.


File: libtool.info,  Node: Linking with dlopened modules,  Next: Finding the dlname,  Prev: Dlpreopening,  Up: Dlopened modules

10.3 Linking with dlopened modules
==================================

When, say, an interpreter application uses dlopened modules to extend
the list of methods it provides, an obvious abstraction for the
maintainers of the interpreter is to have all methods (including the
built in ones supplied with the interpreter) accessed through dlopen.
For one thing, the dlopening functionality will be tested even during
routine invocations.  For another, only one subsystem has to be written
for getting methods into the interpreter.

   The downside of this abstraction is, of course, that environments
that provide only static linkage can't even load the intrinsic
interpreter methods.  Not so!  We can statically link those methods by
*dlpreopening* them.

   Unfortunately, since platforms such as AIX and cygwin require that
all library symbols must be resolved at compile time, the interpreter
maintainers will need to provide a library to both its own dlpreopened
modules, and third-party modules loaded by dlopen.  In itself, that is
not so bad, except that the interpreter too must provide those same
symbols otherwise it will be impossible to resolve all the symbols
required by the modules as they are loaded.  Things are even worse if
the code that loads the modules for the interpreter is itself in a
library - and that is usually the case for any non-trivial application.
Modern platforms take care of this by automatically loading all of a
module's dependency libraries as the module is loaded (libltdl can do
this even on platforms that can't do it by themselves).  In the end,
this leads to problems with duplicated symbols and prevents modules from
loading, and prevents the application from compiling when modules are
preloaded.

     ,-------------.    ,------------------.    ,-----------------.
     | Interpreter |---->     Module------------>   Third-party   |
     `-------------'    |     Loader       |    |Dlopened Modules |
                        |        |         |    `-----------------'
                        |,-------v--------.|             |
                        ||  Dlpreopened   ||             |
                        ||    Modules     ||             |
                        |`----------------'|             |
                        |        |         |             |
                        |,-------v--------.|    ,--------v--------.
                        ||Module Interface||    |Module Interface |
                        ||    Library     ||    |     Library     |
                        |`----------------'|    `-----------------'
                        `------------------'

   Libtool has the concept of “weak library interfaces” to circumvent
this problem.  Recall that the code that dlopens method-provider modules
for the interpreter application resides in a library: All of the modules
and the dlopener library itself should be linked against the common
library that resolves the module symbols at compile time.  To guard
against duplicate symbol definitions, and for dlpreopened modules to
work at all in this scenario, the dlopener library must declare that it
provides a weak library interface to the common symbols in the library
it shares with the modules.  That way, when ‘libtool’ links the *Module
Loader* library with some *Dlpreopened Modules* that were in turn linked
against the *Module Interface Library*, it knows that the *Module
Loader* provides an already loaded *Module Interface Library* to resolve
symbols for the *Dlpreopened Modules*, and doesn't ask the compiler
driver to link an identical *Module Interface Library* dependency
library too.

   In conjunction with Automake, the ‘Makefile.am’ for the *Module
Loader* might look like this:

     lib_LTLIBRARIES = libinterface.la libloader.la

     libinterface_la_SOURCES = interface.c interface.h
     libinterface_la_LDFLAGS = -version-info 3:2:1

     libloader_la_SOURCES    = loader.c
     libloader_la_LDFLAGS    = -weak libinterface.la \
                               -version-info 3:2:1 \
                               -dlpreopen ../modules/intrinsics.la
     libloader_la_LIBADD     = $(libinterface_la_OBJECTS)

   And the ‘Makefile.am’ for the ‘intrinsics.la’ module in a sibling
‘modules’ directory might look like this:

     AM_CPPFLAGS             = -I$(srcdir)/../libloader
     AM_LDFLAGS              = -no-undefined -module -avoid-version \
                               -export-dynamic

     noinst_LTLIBRARIES      = intrinsics.la

     intrinsics_la_LIBADD    = ../libloader/libinterface.la

     ../libloader/libinterface.la:
             cd ../libloader && $(MAKE) $(AM_MAKEFLAGS) libinterface.la

   For a more complex example, see the sources of ‘libltdl’ in the
Libtool distribution, which is built with the help of the ‘-weak’
option.


File: libtool.info,  Node: Finding the dlname,  Next: Dlopen issues,  Prev: Linking with dlopened modules,  Up: Dlopened modules

10.4 Finding the correct name to dlopen
=======================================

After a library has been linked with ‘-module’, it can be dlopened.
Unfortunately, because of the variation in library names, your package
needs to determine the correct file to dlopen.

   The most straightforward and flexible implementation is to determine
the name at runtime, by finding the installed ‘.la’ file, and searching
it for the following lines:

     # The name that we can dlopen.
     dlname='DLNAME'

   If DLNAME is empty, then the library cannot be dlopened.  Otherwise,
it gives the dlname of the library.  So, if the library was installed as
‘/usr/local/lib/libhello.la’, and the DLNAME was ‘libhello.so.3’, then
‘/usr/local/lib/libhello.so.3’ should be dlopened.

   If your program uses this approach, then it should search the
directories listed in the ‘LD_LIBRARY_PATH’(1) environment variable, as
well as the directory where libraries will eventually be installed.
Searching this variable (or equivalent) will guarantee that your program
can find its dlopened modules, even before installation, provided you
have linked them using libtool.

   ---------- Footnotes ----------

   (1) ‘LIBPATH’ on AIX, and ‘SHLIB_PATH’ on HP-UX.


File: libtool.info,  Node: Dlopen issues,  Prev: Finding the dlname,  Up: Dlopened modules

10.5 Unresolved dlopen issues
=============================

The following problems are not solved by using libtool's dlopen support:

   • Dlopen functions are generally only available on shared library
     platforms.  If you want your package to be portable to static
     platforms, you have to use either libltdl (*note Using libltdl::)
     or develop your own alternatives to dlopening dynamic code.  Most
     reasonable solutions involve writing wrapper functions for the
     ‘dlopen’ family, which do package-specific tricks when dlopening is
     unsupported or not available on a given platform.

   • There are major differences in implementations of the ‘dlopen’
     family of functions.  Some platforms do not even use the same
     function names (notably HP-UX, with its ‘shl_load’ family).

   • The application developer must write a custom search function to
     discover the correct module filename to supply to ‘dlopen’.


File: libtool.info,  Node: Using libltdl,  Next: Trace interface,  Prev: Dlopened modules,  Up: Top

11 Using libltdl
****************

Libtool provides a small library, called ‘libltdl’, that aims at hiding
the various difficulties of dlopening libraries from programmers.  It
consists of a few headers and small C source files that can be
distributed with applications that need dlopening functionality.  On
some platforms, whose dynamic linkers are too limited for a simple
implementation of ‘libltdl’ services, it requires GNU DLD, or it will
only emulate dynamic linking with libtool's dlpreopening mechanism.

libltdl supports currently the following dynamic linking mechanisms:

   • ‘dlopen’ (POSIX compliant systems, GNU/Linux, etc.)
   • ‘shl_load’ (HP-UX)
   • ‘LoadLibrary’ (Win16 and Win32)
   • ‘load_add_on’ (BeOS)
   • ‘NSAddImage’ or ‘NSLinkModule’ (Darwin and Mac OS X)
   • GNU DLD (emulates dynamic linking for static libraries)
   • libtool's dlpreopen (*note Dlpreopening::)

libltdl is licensed under the terms of the GNU Lesser General Public
License, with the following exception:

     As a special exception to the GNU Lesser General Public License, if
     you distribute this file as part of a program or library that is
     built using GNU Libtool, you may include it under the same
     distribution terms that you use for the rest of that program.

* Menu:

* Libltdl interface::           How to use libltdl in your programs.
* Modules for libltdl::         Creating modules that can be ‘dlopen’ed.
* Thread Safety in libltdl::    Registering callbacks for multi-thread safety.
* User defined module data::    Associating data with loaded modules.
* Module loaders for libltdl::  Creating user defined module loaders.
* Distributing libltdl::        How to distribute libltdl with your package.


File: libtool.info,  Node: Libltdl interface,  Next: Modules for libltdl,  Up: Using libltdl

11.1 How to use libltdl in your programs
========================================

The libltdl API is similar to the POSIX dlopen interface, which is very
simple but powerful.

To use libltdl in your program you have to include the header file
‘ltdl.h’:

     #include <ltdl.h>

The early releases of libltdl used some symbols that violated the POSIX
namespace conventions.  These symbols are now deprecated, and have been
replaced by those described here.  If you have code that relies on the
old deprecated symbol names, defining ‘LT_NON_POSIX_NAMESPACE’ before
you include ‘ltdl.h’ provides conversion macros.  Whichever set of
symbols you use, the new API is not binary compatible with the last, so
you will need to recompile your application to use this version of
libltdl.

Note that libltdl is not well tested in a multithreaded environment,
though the intention is that it should work (*note Using libltdl in a
multi threaded environment: Thread Safety in libltdl.).  If there are
any issues, working around them is left as an exercise for the reader;
contributions are certainly welcome.

The following macros are defined by including ‘ltdl.h’:

 -- Macro: LT_PATHSEP_CHAR
     ‘LT_PATHSEP_CHAR’ is the system-dependent path separator, that is,
     ‘;’ on Windows and ‘:’ everywhere else.

 -- Macro: LT_DIRSEP_CHAR
     If ‘LT_DIRSEP_CHAR’ is defined, it can be used as directory
     separator in addition to ‘/’.  On Windows, this contains ‘\’.

The following types are defined in ‘ltdl.h’:

 -- Type: lt_dlhandle
     ‘lt_dlhandle’ is a module "handle".  Every lt_dlopened module has a
     handle associated with it.

 -- Type: lt_dladvise
     ‘lt_dladvise’ is used to control optional module loading modes.  If
     it is not used, the default mode of the underlying system module
     loader is used.

 -- Type: lt_dlsymlist
     ‘lt_dlsymlist’ is a symbol list for dlpreopened modules (*note
     Dlpreopening::).

libltdl provides the following functions:

 -- Function: int lt_dlinit (void)
     Initialize libltdl.  This function must be called before using
     libltdl and may be called several times.  Return 0 on success,
     otherwise the number of errors.

 -- Function: int lt_dlexit (void)
     Shut down libltdl and close all modules.  This function will only
     then shut down libltdl when it was called as many times as
     ‘lt_dlinit’ has been successfully called.  Return 0 on success,
     otherwise the number of errors.

 -- Function: lt_dlhandle lt_dlopen (const char *FILENAME)
     Open the module with the file name FILENAME and return a handle for
     it.  ‘lt_dlopen’ is able to open libtool dynamic modules, preloaded
     static modules, the program itself and native dynamic modules(1).

     Unresolved symbols in the module are resolved using its dependency
     libraries and, on some platforms, previously dlopened modules.  If
     the executable using this module was linked with the
     ‘-export-dynamic’ flag, then the global symbols in the executable
     will also be used to resolve references in the module.

     If FILENAME is ‘NULL’ and the program was linked with
     ‘-export-dynamic’ or ‘-dlopen self’, ‘lt_dlopen’ will return a
     handle for the program itself, which can be used to access its
     symbols.

     If libltdl cannot find the library and the file name FILENAME does
     not have a directory component it will additionally look in the
     following search paths for the module (in the following order):

       1. user-defined search path: This search path can be changed by
          the program using the functions ‘lt_dlsetsearchpath’,
          ‘lt_dladdsearchdir’ and ‘lt_dlinsertsearchdir’.

       2. libltdl's search path: This search path is the value of the
          environment variable ‘LTDL_LIBRARY_PATH’.

       3. system library search path: The system dependent library
          search path (e.g. on GNU/Linux it is ‘LD_LIBRARY_PATH’).

     Each search path must be a list of absolute directories separated
     by ‘LT_PATHSEP_CHAR’, for example, ‘"/usr/lib/mypkg:/lib/foo"’.
     The directory names may not contain the path separator.

     If the same module is loaded several times, the same handle is
     returned.  If ‘lt_dlopen’ fails for any reason, it returns ‘NULL’.

 -- Function: lt_dlhandle lt_dlopenext (const char *FILENAME)
     The same as ‘lt_dlopen’, except that it tries to append different
     file name extensions to the file name.  If the file with the file
     name FILENAME cannot be found libltdl tries to append the following
     extensions:

       1. the libtool archive extension ‘.la’
       2. the extension used for native dynamically loadable modules on
          the host platform, e.g., ‘.so’, ‘.sl’, etc.

     This lookup strategy was designed to allow programs that don't have
     knowledge about native dynamic libraries naming conventions to be
     able to ‘dlopen’ such libraries as well as libtool modules
     transparently.

 -- Function: lt_dlhandle lt_dlopenadvise (const char *FILENAME,
          lt_dladvise ADVISE)
     The same as ‘lt_dlopen’, except that it also requires an additional
     argument that may contain additional hints to the underlying system
     module loader.  The ADVISE parameter is opaque and can only be
     accessed with the functions documented below.

     Note that this function does not change the content of ADVISE, so
     unlike the other calls in this API takes a direct ‘lt_dladvise’
     type, and not a pointer to the same.

 -- Function: int lt_dladvise_init (lt_dladvise *ADVISE)
     The ADVISE parameter can be used to pass hints to the module loader
     when using ‘lt_dlopenadvise’ to perform the loading.  The ADVISE
     parameter needs to be initialised by this function before it can be
     used.  Any memory used by ADVISE needs to be recycled with
     ‘lt_dladvise_destroy’ when it is no longer needed.

     On failure, ‘lt_dladvise_init’ returns non-zero and sets an error
     message that can be retrieved with ‘lt_dlerror’.

 -- Function: int lt_dladvise_destroy (lt_dladvise *ADVISE)
     Recycle the memory used by ADVISE.  For an example, see the
     documentation for ‘lt_dladvise_ext’.

     On failure, ‘lt_dladvise_destroy’ returns non-zero and sets an
     error message that can be retrieved with ‘lt_dlerror’.

 -- Function: int lt_dladvise_ext (lt_dladvise *ADVISE)
     Set the ‘ext’ hint on ADVISE.  Passing an ADVISE parameter to
     ‘lt_dlopenadvise’ with this hint set causes it to try to append
     different file name extensions like ‘lt_dlopenext’.

     The following example is equivalent to calling ‘lt_dlopenext
     (filename)’:

          lt_dlhandle
          my_dlopenext (const char *filename)
          {
            lt_dlhandle handle = 0;
            lt_dladvise advise;

            if (!lt_dladvise_init (&advise) && !lt_dladvise_ext (&advise))
              handle = lt_dlopenadvise (filename, advise);

            lt_dladvise_destroy (&advise);

            return handle;
          }

     On failure, ‘lt_dladvise_ext’ returns non-zero and sets an error
     message that can be retrieved with ‘lt_dlerror’.

 -- Function: int lt_dladvise_global (lt_dladvise *ADVISE)
     Set the ‘symglobal’ hint on ADVISE.  Passing an ADVISE parameter to
     ‘lt_dlopenadvise’ with this hint set causes it to try to make the
     loaded module's symbols globally available for resolving unresolved
     symbols in subsequently loaded modules.

     If neither the ‘symglobal’ nor the ‘symlocal’ hints are set, or if
     a module is loaded without using the ‘lt_dlopenadvise’ call in any
     case, then the visibility of the module's symbols will be as per
     the default for the underlying module loader and OS. Even if a
     suitable hint is passed, not all loaders are able to act upon it in
     which case ‘lt_dlgetinfo’ will reveal whether the hint was actually
     followed.

     On failure, ‘lt_dladvise_global’ returns non-zero and sets an error
     message that can be retrieved with ‘lt_dlerror’.

 -- Function: int lt_dladvise_local (lt_dladvise *ADVISE)
     Set the ‘symlocal’ hint on ADVISE.  Passing an ADVISE parameter to
     ‘lt_dlopenadvise’ with this hint set causes it to try to keep the
     loaded module's symbols hidden so that they are not visible to
     subsequently loaded modules.

     If neither the ‘symglobal’ nor the ‘symlocal’ hints are set, or if
     a module is loaded without using the ‘lt_dlopenadvise’ call in any
     case, then the visibility of the module's symbols will be as per
     the default for the underlying module loader and OS. Even if a
     suitable hint is passed, not all loaders are able to act upon it in
     which case ‘lt_dlgetinfo’ will reveal whether the hint was actually
     followed.

     On failure, ‘lt_dladvise_local’ returns non-zero and sets an error
     message that can be retrieved with ‘lt_dlerror’.

 -- Function: int lt_dladvise_resident (lt_dladvise *ADVISE)
     Set the ‘resident’ hint on ADVISE.  Passing an ADVISE parameter to
     ‘lt_dlopenadvise’ with this hint set causes it to try to make the
     loaded module resident in memory, so that it cannot be unloaded
     with a later call to ‘lt_dlclose’.

     On failure, ‘lt_dladvise_resident’ returns non-zero and sets an
     error message that can be retrieved with ‘lt_dlerror’.

 -- Function: int lt_dladvise_preload (lt_dladvise *ADVISE)
     Set the ‘preload’ hint on ADVISE.  Passing an ADVISE parameter to
     ‘lt_dlopenadvise’ with this hint set causes it to load only
     preloaded modules, so that if a suitable preloaded module is not
     found, ‘lt_dlopenadvise’ will return ‘NULL’.

 -- Function: int lt_dlclose (lt_dlhandle HANDLE)
     Decrement the reference count on the module HANDLE.  If it drops to
     zero and no other module depends on this module, then the module is
     unloaded.  Return 0 on success.

 -- Function: void * lt_dlsym (lt_dlhandle HANDLE, const char *NAME)
     Return the address in the module HANDLE, where the symbol given by
     the null-terminated string NAME is loaded.  If the symbol cannot be
     found, ‘NULL’ is returned.

 -- Function: const char * lt_dlerror (void)
     Return a human readable string describing the most recent error
     that occurred from any of libltdl's functions.  Return ‘NULL’ if no
     errors have occurred since initialization or since it was last
     called.

 -- Function: int lt_dladdsearchdir (const char *SEARCH_DIR)
     Append the search directory SEARCH_DIR to the current user-defined
     library search path.  Return 0 on success.

 -- Function: int lt_dlinsertsearchdir (const char *BEFORE,
          const char *SEARCH_DIR)
     Insert the search directory SEARCH_DIR into the user-defined
     library search path, immediately before the element starting at
     address BEFORE.  If BEFORE is ‘NULL’, then SEARCH_DIR is appending
     as if ‘lt_dladdsearchdir’ had been called.  Return 0 on success.

 -- Function: int lt_dlsetsearchpath (const char *SEARCH_PATH)
     Replace the current user-defined library search path with
     SEARCH_PATH, which must be a list of absolute directories separated
     by ‘LT_PATHSEP_CHAR’.  Return 0 on success.

 -- Function: const char * lt_dlgetsearchpath (void)
     Return the current user-defined library search path.

 -- Function: int lt_dlforeachfile (const char *SEARCH_PATH,
          int (*FUNC) (const char *FILENAME, void * DATA), void * DATA)
     In some applications you may not want to load individual modules
     with known names, but rather find all of the modules in a set of
     directories and load them all during initialisation.  With this
     function you can have libltdl scan the ‘LT_PATHSEP_CHAR’-delimited
     directory list in SEARCH_PATH for candidates, and pass them, along
     with DATA to your own callback function, FUNC.  If SEARCH_PATH is
     ‘NULL’, then search all of the standard locations that ‘lt_dlopen’
     would examine.  This function will continue to make calls to FUNC
     for each file that it discovers in SEARCH_PATH until one of these
     calls returns non-zero, or until the files are exhausted.
     ‘lt_dlforeachfile’ returns the value returned by the last call made
     to FUNC.

     For example you could define FUNC to build an ordered “argv”-like
     vector of files using DATA to hold the address of the start of the
     vector.

 -- Function: int lt_dlmakeresident (lt_dlhandle HANDLE)
     Mark a module so that it cannot be ‘lt_dlclose’d.  This can be
     useful if a module implements some core functionality in your
     project that would cause your code to crash if removed.  Return 0
     on success.

     If you use ‘lt_dlopen (NULL)’ to get a HANDLE for the running
     binary, that handle will always be marked as resident, and
     consequently cannot be successfully ‘lt_dlclose’d.

 -- Function: int lt_dlisresident (lt_dlhandle HANDLE)
     Check whether a particular module has been marked as resident,
     returning 1 if it has or 0 otherwise.  If there is an error while
     executing this function, return -1 and set an error message for
     retrieval with ‘lt_dlerror’.

   ---------- Footnotes ----------

   (1) Some platforms, notably Mac OS X, differentiate between a runtime
library that cannot be opened by ‘lt_dlopen’ and a dynamic module that
can.  For maximum portability you should try to ensure that you only
pass ‘lt_dlopen’ objects that have been compiled with libtool's
‘-module’ flag.


File: libtool.info,  Node: Modules for libltdl,  Next: Thread Safety in libltdl,  Prev: Libltdl interface,  Up: Using libltdl

11.2 Creating modules that can be ‘dlopen’ed
============================================

Libtool modules are created like normal libtool libraries with a few
exceptions:

   You have to link the module with libtool's ‘-module’ switch, and you
should link any program that is intended to dlopen the module with
‘-dlopen MODULENAME.LA’ where possible, so that libtool can dlpreopen
the module on platforms that do not support dlopening.  If the module
depends on any other libraries, make sure you specify them either when
you link the module or when you link programs that dlopen it.  If you
want to disable versioning (*note Versioning::) for a specific module
you should link it with the ‘-avoid-version’ switch.  Note that libtool
modules don't need to have a "lib" prefix.  However, Automake 1.4 or
higher is required to build such modules.

   Usually a set of modules provide the same interface, i.e. exports the
same symbols, so that a program can dlopen them without having to know
more about their internals: In order to avoid symbol conflicts all
exported symbols must be prefixed with "modulename_LTX_" (MODULENAME is
the name of the module).  Internal symbols must be named in such a way
that they won't conflict with other modules, for example, by prefixing
them with "_modulename_".  Although some platforms support having the
same symbols defined more than once it is generally not portable and it
makes it impossible to dlpreopen such modules.

   libltdl will automatically cut the prefix off to get the real name of
the symbol.  Additionally, it supports modules that do not use a prefix
so that you can also dlopen non-libtool modules.

   ‘foo1.c’ gives an example of a portable libtool module.  Exported
symbols are prefixed with "foo1_LTX_", internal symbols with "_foo1_".
Aliases are defined at the beginning so that the code is more readable.

     /* aliases for the exported symbols */
     #define foo  foo1_LTX_foo
     #define bar  foo1_LTX_bar

     /* a global variable definition */
     int bar = 1;

     /* a private function */
     int _foo1_helper() {
       return bar;
     }

     /* an exported function */
     int foo() {
       return _foo1_helper();
     }

The ‘Makefile.am’ contains the necessary rules to build the module
‘foo1.la’:

     ...
     lib_LTLIBRARIES = foo1.la

     foo1_la_SOURCES = foo1.c
     foo1_la_LDFLAGS = -module
     ...


File: libtool.info,  Node: Thread Safety in libltdl,  Next: User defined module data,  Prev: Modules for libltdl,  Up: Using libltdl

11.3 Using libltdl in a multi threaded environment
==================================================

Libltdl provides a wrapper around whatever dynamic run-time object
loading mechanisms are provided by the host system, many of which are
themselves not thread safe.  Consequently libltdl cannot itself be
consistently thread safe.

   If you wish to use libltdl in a multithreaded environment, then you
must mutex lock around libltdl calls, since they may in turn be calling
non-thread-safe system calls on some target hosts.

   Some old releases of libtool provided a mutex locking API that was
unusable with POSIX threads, so callers were forced to lock around all
libltdl API calls anyway.  That mutex locking API was next to useless,
and is not present in current releases.

   Some future release of libtool may provide a new POSIX thread
compliant mutex locking API.


File: libtool.info,  Node: User defined module data,  Next: Module loaders for libltdl,  Prev: Thread Safety in libltdl,  Up: Using libltdl

11.4 Data associated with loaded modules
========================================

Some of the internal information about each loaded module that is
maintained by libltdl is available to the user, in the form of this
structure:

 -- Type: struct lt_dlinfo { char *filename; char *name; int ref_count;
          int is_resident; int is_symglobal; int is_symlocal;}
     ‘lt_dlinfo’ is used to store information about a module.  The
     ‘filename’ attribute is a null-terminated character string of the
     real module file name.  If the module is a libtool module then
     ‘name’ is its module name (e.g. ‘"libfoo"’ for ‘"dir/libfoo.la"’),
     otherwise it is set to ‘NULL’.  The ‘ref_count’ attribute is a
     reference counter that describes how often the same module is
     currently loaded.  The remaining fields can be compared to any
     hints that were passed to ‘lt_dlopenadvise’ to determine whether
     the underlying loader was able to follow them.

   The following function will return a pointer to libltdl's internal
copy of this structure for the given HANDLE:

 -- Function: const lt_dlinfo * lt_dlgetinfo (lt_dlhandle HANDLE)
     Return a pointer to a struct that contains some information about
     the module HANDLE.  The contents of the struct must not be
     modified.  Return ‘NULL’ on failure.

   Furthermore, to save you from having to keep a list of the handles of
all the modules you have loaded, these functions allow you to iterate
over libltdl's list of loaded modules:

 -- Type: lt_dlinterface_id
     The opaque type used to hold the module interface details for each
     registered libltdl client.

 -- Type: int lt_dlhandle_interface (lt_dlhandle HANDLE,
          const char *ID_STRING)
     Functions of this type are called to check that a handle conforms
     to a library's expected module interface when iterating over the
     global handle list.  You should be careful to write a callback
     function of this type that can correctly identify modules that
     belong to this client, both to prevent other clients from
     accidentally finding your loaded modules with the iterator
     functions below, and vice versa.  The best way to do this is to
     check that module HANDLE conforms to the interface specification of
     your loader using ‘lt_dlsym’.

     The callback may be given *every* module loaded by all the libltdl
     module clients in the current address space, including any modules
     loaded by other libraries such as libltdl itself, and should return
     non-zero if that module does not fulfill the interface requirements
     of your loader.

          int
          my_interface_cb (lt_dlhandle handle, const char *id_string)
          {
            char *(*module_id) (void) = NULL;

            /* A valid my_module must provide all of these symbols.  */
            if (!((module_id = (char*(*)(void)) lt_dlsym ("module_version"))
                  && lt_dlsym ("my_module_entrypoint")))
                return 1;

            if (strcmp (id_string, module_id()) != 0)
                return 1;

            return 0;
          }

 -- Function: lt_dlinterface_id lt_dlinterface_register
          (const char *ID_STRING, lt_dlhandle_interface *IFACE)
     Use this function to register your interface validator with
     libltdl, and in return obtain a unique key to store and retrieve
     per-module data.  You supply an ID_STRING and IFACE so that the
     resulting ‘lt_dlinterface_id’ can be used to filter the module
     handles returned by the iteration functions below.  If IFACE is
     ‘NULL’, all modules will be matched.

 -- Function: void lt_dlinterface_free (lt_dlinterface_id IFACE)
     Release the data associated with IFACE.

 -- Function: int lt_dlhandle_map (lt_dlinterface_id IFACE,
          int (*FUNC) (lt_dlhandle HANDLE, void * DATA), void * DATA)
     For each module that matches IFACE, call the function FUNC.  When
     writing the FUNC callback function, the argument HANDLE is the
     handle of a loaded module, and DATA is the last argument passed to
     ‘lt_dlhandle_map’.  As soon as FUNC returns a non-zero value for
     one of the handles, ‘lt_dlhandle_map’ will stop calling FUNC and
     immediately return that non-zero value.  Otherwise 0 is eventually
     returned when FUNC has been successfully called for all matching
     modules.

 -- Function: lt_dlhandle lt_dlhandle_iterate (lt_dlinterface_id IFACE,
          lt_dlhandle PLACE)
     Iterate over the module handles loaded by IFACE, returning the
     first matching handle in the list if PLACE is ‘NULL’, and the next
     one on subsequent calls.  If PLACE is the last element in the list
     of eligible modules, this function returns ‘NULL’.

          lt_dlhandle handle = 0;
          lt_dlinterface_id iface = my_interface_id;

          while ((handle = lt_dlhandle_iterate (iface, handle)))
            {
              ...
            }

 -- Function: lt_dlhandle lt_dlhandle_fetch (lt_dlinterface_id IFACE,
          const char *MODULE_NAME)
     Search through the module handles loaded by IFACE for a module
     named MODULE_NAME, returning its handle if found or else ‘NULL’ if
     no such named module has been loaded by IFACE.

   However, you might still need to maintain your own list of loaded
module handles (in parallel with the list maintained inside libltdl) if
there were any other data that your application wanted to associate with
each open module.  Instead, you can use the following API calls to do
that for you.  You must first obtain a unique interface id from libltdl
as described above, and subsequently always use it to retrieve the data
you stored earlier.  This allows different libraries to each store their
own data against loaded modules, without interfering with one another.

 -- Function: void * lt_dlcaller_set_data (lt_dlinterface_id KEY,
          lt_dlhandle HANDLE, void * DATA)
     Set DATA as the set of data uniquely associated with KEY and HANDLE
     for later retrieval.  This function returns the DATA previously
     associated with KEY and HANDLE if any.  A result of 0, may indicate
     that a diagnostic for the last error (if any) is available from
     ‘lt_dlerror()’.

     For example, to correctly remove some associated data:

          void *stale = lt_dlcaller_set_data (key, handle, 0);
          if (stale != NULL)
            {
              free (stale);
            }
          else
            {
              char *error_msg = lt_dlerror ();

              if (error_msg != NULL)
                {
                  my_error_handler (error_msg);
                  return STATUS_FAILED;
                }
            }

 -- Function: void * lt_dlcaller_get_data (lt_dlinterface_id KEY,
          lt_dlhandle HANDLE)
     Return the address of the data associated with KEY and HANDLE, or
     else ‘NULL’ if there is none.

   Old versions of libltdl also provided a simpler, but similar, API
based around ‘lt_dlcaller_id’.  Unfortunately, it had no provision for
detecting whether a module belonged to a particular interface as libltdl
didn't support multiple loaders in the same address space at that time.
Those APIs are no longer supported as there would be no way to stop
clients of the old APIs from seeing (and accidentally altering) modules
loaded by other libraries.


File: libtool.info,  Node: Module loaders for libltdl,  Next: Distributing libltdl,  Prev: User defined module data,  Up: Using libltdl

11.5 How to create and register new module loaders
==================================================

Sometimes libltdl's many ways of gaining access to modules are not
sufficient for the purposes of a project.  You can write your own
loader, and register it with libltdl so that ‘lt_dlopen’ will be able to
use it.

   Writing a loader involves writing at least three functions that can
be called by ‘lt_dlopen’, ‘lt_dlsym’ and ‘lt_dlclose’.  Optionally, you
can provide a finalisation function to perform any cleanup operations
when ‘lt_dlexit’ executes, and a symbol prefix string that will be
prepended to any symbols passed to ‘lt_dlsym’.  These functions must
match the function pointer types below, after which they can be
allocated to an instance of ‘lt_user_dlloader’ and registered.

   Registering the loader requires that you choose a name for it, so
that it can be recognised by ‘lt_dlloader_find’ and removed with
‘lt_dlloader_remove’.  The name you choose must be unique, and not
already in use by libltdl's builtin loaders:

"dlopen"
     The system dynamic library loader, if one exists.
"dld"
     The GNU dld loader, if ‘libdld’ was installed when libltdl was
     built.
"dlpreload"
     The loader for ‘lt_dlopen’ing of preloaded static modules.

   The prefix "dl" is reserved for loaders supplied with future versions
of libltdl, so you should not use that for your own loader names.

The following types are defined in ‘ltdl.h’:

 -- Type: lt_module
     ‘lt_module’ is a dlloader dependent module.  The dynamic module
     loader extensions communicate using these low level types.

 -- Type: lt_dlloader
     ‘lt_dlloader’ is a handle for module loader types.

 -- Type: lt_user_data
     ‘lt_user_data’ is used for specifying loader instance data.

 -- Type: struct lt_user_dlloader {const char *sym_prefix;
          lt_module_open *module_open; lt_module_close *module_close;
          lt_find_sym *find_sym; lt_dlloader_exit *dlloader_exit; }
     If you want to define a new way to open dynamic modules, and have
     the ‘lt_dlopen’ API use it, you need to instantiate one of these
     structures and pass it to ‘lt_dlloader_add’.  You can pass whatever
     you like in the DLLOADER_DATA field, and it will be passed back as
     the value of the first parameter to each of the functions specified
     in the function pointer fields.

 -- Type: lt_module lt_module_open (const char *FILENAME)
     The type of the loader function for an ‘lt_dlloader’ module loader.
     The value set in the dlloader_data field of the ‘struct
     lt_user_dlloader’ structure will be passed into this function in
     the LOADER_DATA parameter.  Implementation of such a function
     should attempt to load the named module, and return an ‘lt_module’
     suitable for passing in to the associated ‘lt_module_close’ and
     ‘lt_sym_find’ function pointers.  If the function fails it should
     return ‘NULL’, and set the error message with ‘lt_dlseterror’.

 -- Type: int lt_module_close (lt_user_data LOADER_DATA,
          lt_module MODULE)
     The type of the unloader function for a user defined module loader.
     Implementation of such a function should attempt to release any
     resources tied up by the MODULE module, and then unload it from
     memory.  If the function fails for some reason, set the error
     message with ‘lt_dlseterror’ and return non-zero.

 -- Type: void * lt_find_sym (lt_module MODULE, const char *SYMBOL)
     The type of the symbol lookup function for a user defined module
     loader.  Implementation of such a function should return the
     address of the named SYMBOL in the module MODULE, or else set the
     error message with ‘lt_dlseterror’ and return ‘NULL’ if lookup
     fails.

 -- Type: int lt_dlloader_exit (lt_user_data LOADER_DATA)
     The type of the finalisation function for a user defined module
     loader.  Implementation of such a function should free any
     resources associated with the loader, including any user specified
     data in the ‘dlloader_data’ field of the ‘lt_user_dlloader’.  If
     non-‘NULL’, the function will be called by ‘lt_dlexit’, and
     ‘lt_dlloader_remove’.

   For example:

     int
     register_myloader (void)
     {
       lt_user_dlloader dlloader;

       /* User modules are responsible for their own initialisation. */
       if (myloader_init () != 0)
         return MYLOADER_INIT_ERROR;

       dlloader.sym_prefix    = NULL;
       dlloader.module_open   = myloader_open;
       dlloader.module_close  = myloader_close;
       dlloader.find_sym      = myloader_find_sym;
       dlloader.dlloader_exit = myloader_exit;
       dlloader.dlloader_data = (lt_user_data)myloader_function;

       /* Add my loader as the default module loader. */
       if (lt_dlloader_add (lt_dlloader_next (NULL), &dlloader,
                            "myloader") != 0)
         return ERROR;

       return OK;
     }

   Note that if there is any initialisation required for the loader, it
must be performed manually before the loader is registered - libltdl
doesn't handle user loader initialisation.

   Finalisation _is_ handled by libltdl however, and it is important to
ensure the ‘dlloader_exit’ callback releases any resources claimed
during the initialisation phase.

libltdl provides the following functions for writing your own module
loaders:

 -- Function: int lt_dlloader_add (lt_dlloader *PLACE,
          lt_user_dlloader *DLLOADER, const char *LOADER_NAME)
     Add a new module loader to the list of all loaders, either as the
     last loader (if PLACE is ‘NULL’), else immediately before the
     loader passed as PLACE.  LOADER_NAME will be returned by
     ‘lt_dlloader_name’ if it is subsequently passed a newly registered
     loader.  These LOADER_NAMEs must be unique, or ‘lt_dlloader_remove’
     and ‘lt_dlloader_find’ cannot work.  Returns 0 for success.

          /* Make myloader be the last one. */
          if (lt_dlloader_add (NULL, myloader) != 0)
            perror (lt_dlerror ());

 -- Function: int lt_dlloader_remove (const char *LOADER_NAME)
     Remove the loader identified by the unique name, LOADER_NAME.
     Before this can succeed, all modules opened by the named loader
     must have been closed.  Returns 0 for success, otherwise an error
     message can be obtained from ‘lt_dlerror’.

          /* Remove myloader. */
          if (lt_dlloader_remove ("myloader") != 0)
            perror (lt_dlerror ());

 -- Function: lt_dlloader * lt_dlloader_next (lt_dlloader *PLACE)
     Iterate over the module loaders, returning the first loader if
     PLACE is ‘NULL’, and the next one on subsequent calls.  The handle
     is for use with ‘lt_dlloader_add’.

          /* Make myloader be the first one. */
          if (lt_dlloader_add (lt_dlloader_next (NULL), myloader) != 0)
            return ERROR;

 -- Function: lt_dlloader * lt_dlloader_find (const char *LOADER_NAME)
     Return the first loader with a matching LOADER_NAME identifier, or
     else ‘NULL’, if the identifier is not found.

     The identifiers that may be used by libltdl itself, if the host
     architecture supports them are “dlopen”(1), “dld” and “dlpreload”.

          /* Add a user loader as the next module loader to be tried if
             the standard dlopen loader were to fail when lt_dlopening. */
          if (lt_dlloader_add (lt_dlloader_find ("dlopen"), myloader) != 0)
            return ERROR;

 -- Function: const char * lt_dlloader_name (lt_dlloader *PLACE)
     Return the identifying name of PLACE, as obtained from
     ‘lt_dlloader_next’ or ‘lt_dlloader_find’.  If this function fails,
     it will return ‘NULL’ and set an error for retrieval with
     ‘lt_dlerror’.

 -- Function: lt_user_data * lt_dlloader_data (lt_dlloader *PLACE)
     Return the address of the ‘dlloader_data’ of PLACE, as obtained
     from ‘lt_dlloader_next’ or ‘lt_dlloader_find’.  If this function
     fails, it will return ‘NULL’ and set an error for retrieval with
     ‘lt_dlerror’.

11.5.1 Error handling within user module loaders
------------------------------------------------

 -- Function: int lt_dladderror (const char *DIAGNOSTIC)
     This function allows you to integrate your own error messages into
     ‘lt_dlerror’.  Pass in a suitable diagnostic message for return by
     ‘lt_dlerror’, and an error identifier for use with ‘lt_dlseterror’
     is returned.

     If the allocation of an identifier fails, this function returns -1.

          int myerror = lt_dladderror ("doh!");
          if (myerror < 0)
            perror (lt_dlerror ());

 -- Function: int lt_dlseterror (int ERRORCODE)
     When writing your own module loaders, you should use this function
     to raise errors so that they are propagated through the
     ‘lt_dlerror’ interface.  All of the standard errors used by libltdl
     are declared in ‘ltdl.h’, or you can add more of your own with
     ‘lt_dladderror’.  This function returns 0 on success.

          if (lt_dlseterror (LTDL_ERROR_NO_MEMORY) != 0)
            perror (lt_dlerror ());

   ---------- Footnotes ----------

   (1) This is used for the host dependent module loading API -
‘shl_load’ and ‘LoadLibrary’ for example


File: libtool.info,  Node: Distributing libltdl,  Prev: Module loaders for libltdl,  Up: Using libltdl

11.6 How to distribute libltdl with your package
================================================

Even though libltdl is installed together with libtool, you may wish to
include libltdl in the distribution of your package, for the convenience
of users of your package that don't have libtool or libltdl installed,
or if you are using features of a very new version of libltdl that you
don't expect your users to have yet.  In such cases, you must decide
what flavor of libltdl you want to use: a convenience library or an
installable libtool library.

   The most simplistic way to add ‘libltdl’ to your package is to copy
all the ‘libltdl’ source files to a subdirectory within your package and
to build and link them along with the rest of your sources.  To help you
do this, the m4 macros for Autoconf are available in ‘ltdl.m4’.  You
must ensure that they are available in ‘aclocal.m4’ before you run
Autoconf(1).  Having made the macros available, you must add a call to
the ‘LTDL_INIT’ macro (after the call to ‘LT_INIT’) to your package's
‘configure.ac’ to perform the configure time checks required to build
the library correctly.  Unfortunately, this method has problems if you
then try to link the package binaries with an installed libltdl, or a
library that depends on libltdl, because of the duplicate symbol
definitions.  For example, ultimately linking against two different
versions of libltdl, or against both a local convenience library and an
installed libltdl is bad.  Ensuring that only one copy of the libltdl
sources are linked into any program is left as an exercise for the
reader.

 -- Macro: LT_CONFIG_LTDL_DIR (DIRECTORY)
     Declare DIRECTORY to be the location of the ‘libltdl’ source files,
     for ‘libtoolize --ltdl’ to place them.  *Note Invoking
     libtoolize::, for more details.  Provided that you add an
     appropriate ‘LT_CONFIG_LTDL_DIR’ call in your ‘configure.ac’ before
     calling ‘libtoolize’, the appropriate ‘libltdl’ files will be
     installed automatically.

 -- Macro: LTDL_INIT (OPTIONS)
 -- Macro: LT_WITH_LTDL
 -- Macro: AC_WITH_LTDL
     ‘AC_WITH_LTDL’ and ‘LT_WITH_LTDL’ are deprecated names for older
     versions of this macro; ‘autoupdate’ will update your
     ‘configure.ac’ file.

     This macro adds the following options to the ‘configure’ script:

     ‘--with-ltdl-include INSTALLED-LTDL-HEADER-DIR’
          The ‘LTDL_INIT’ macro will look in the standard header file
          locations to find the installed ‘libltdl’ headers.  If
          ‘LTDL_INIT’ can't find them by itself, the person who builds
          your package can use this option to tell ‘configure’ where the
          installed ‘libltdl’ headers are.

     ‘--with-ltdl-lib INSTALLED-LTDL-LIBRARY-DIR’
          Similarly, the person building your package can use this
          option to help ‘configure’ find the installed ‘libltdl.la’.

     ‘--with-included-ltdl’
          If there is no installed ‘libltdl’, or in any case if the
          person building your package would rather use the ‘libltdl’
          sources shipped with the package in the subdirectory named by
          ‘LT_CONFIG_LTDL_DIR’, they should pass this option to
          ‘configure’.

     If the ‘--with-included-ltdl’ is not passed at configure time, and
     an installed ‘libltdl’ is not found(2), then ‘configure’ will exit
     immediately with an error that asks the user to either specify the
     location of an installed ‘libltdl’ using the ‘--with-ltdl-include’
     and ‘--with-ltdl-lib’ options, or to build with the ‘libltdl’
     sources shipped with the package by passing ‘--with-included-ltdl’.

     If an installed ‘libltdl’ is found, then ‘LIBLTDL’ is set to the
     link flags needed to use it, and ‘LTDLINCL’ to the preprocessor
     flags needed to find the installed headers, and ‘LTDLDEPS’ will be
     empty.  Note, however, that no version checking is performed.  You
     should manually check for the ‘libltdl’ features you need in
     ‘configure.ac’:

          LT_INIT([dlopen])
          LTDL_INIT

          # The lt_dladvise_init symbol was added with libtool-2.2
          if test yes != "$with_included_ltdl"; then
            save_CFLAGS=$CFLAGS
            save_LDFLAGS=$LDFLAGS
            CFLAGS="$CFLAGS $LTDLINCL"
            LDFLAGS="$LDFLAGS $LIBLTDL"
            AC_CHECK_LIB([ltdl], [lt_dladvise_init],
                          [],
                  [AC_MSG_ERROR([installed libltdl is too old])])
            LDFLAGS=$save_LDFLAGS
            CFLAGS=$save_CFLAGS
          fi

     OPTIONS may include no more than one of the following build modes
     depending on how you want your project to build ‘libltdl’:
     ‘nonrecursive’, ‘recursive’, or ‘subproject’.  In order for
     ‘libtoolize’ to detect this option correctly, if you supply one of
     these arguments, they must be given literally (i.e., macros or
     shell variables that expand to the correct ltdl mode will not
     work).

     ‘nonrecursive’
          This is how the Libtool project distribution builds the
          ‘libltdl’ we ship and install.  If you wish to use Automake to
          build ‘libltdl’ without invoking a recursive make to descend
          into the ‘libltdl’ subdirectory, then use this option.  You
          will need to set your configuration up carefully to make this
          work properly, and you will need releases of Autoconf and
          Automake that support ‘subdir-objects’ and ‘LIBOBJDIR’
          properly.  In your ‘configure.ac’, add:

               AM_INIT_AUTOMAKE([subdir-objects])
               AC_CONFIG_HEADERS([config.h])
               LT_CONFIG_LTDL_DIR([libltdl])
               LT_INIT([dlopen])
               LTDL_INIT([nonrecursive])

          You _have to_ use a config header, but it may have a name
          different than ‘config.h’.

          Also, add the following near the top of your ‘Makefile.am’:

               AM_CPPFLAGS =
               AM_LDFLAGS =

               BUILT_SOURCES =
               EXTRA_DIST =
               CLEANFILES =
               MOSTLYCLEANFILES =

               include_HEADERS =
               noinst_LTLIBRARIES =
               lib_LTLIBRARIES =
               EXTRA_LTLIBRARIES =

               include libltdl/ltdl.mk

          Unless you build no other libraries from this ‘Makefile.am’,
          you will also need to change ‘lib_LTLIBRARIES’ to assign with
          ‘+=’ so that the ‘libltdl’ targets declared in ‘ltdl.mk’ are
          not overwritten.

     ‘recursive’
          This build mode still requires that you use Automake, but (in
          contrast with ‘nonrecursive’) uses the more usual device of
          starting another ‘make’ process in the ‘libltdl’ subdirectory.
          To use this mode, you should add to your ‘configure.ac’:

               AM_INIT_AUTOMAKE
               AC_CONFIG_HEADERS([config.h])
               LT_CONFIG_LTDL_DIR([libltdl])
               LT_INIT([dlopen])
               LTDL_INIT([recursive])
               AC_CONFIG_FILES([libltdl/Makefile])

          Again, you _have to_ use a config header, but it may have a
          name different than ‘config.h’ if you like.

          Also, add this to your ‘Makefile.am’:

               SUBDIRS = libltdl

     ‘subproject’
          This mode is the default unless you explicitly add ‘recursive’
          or ‘nonrecursive’ to your ‘LTDL_INIT’ options; ‘subproject’ is
          the only mode supported by previous releases of libltdl.  Even
          if you do not use Autoconf in the parent project, then, in
          ‘subproject’ mode, still ‘libltdl’ contains all the necessary
          files to configure and build itself - you just need to arrange
          for your build system to call ‘libltdl/configure’ with
          appropriate options, and then run ‘make’ in the ‘libltdl’
          subdirectory.

          If you _are_ using Autoconf and Automake, then you will need
          to add the following to your ‘configure.ac’:

               LT_CONFIG_LTDL_DIR([libltdl])
               LTDL_INIT

          and to ‘Makefile.am’:

               SUBDIRS = libltdl

     Aside from setting the libltdl build mode, there are other keywords
     that you can pass to ‘LTDL_INIT’ to modify its behavior when
     ‘--with-included-ltdl’ has been given:

     ‘convenience’
          This is the default unless you explicitly add ‘installable’ to
          your ‘LTDL_INIT’ options.

          This keyword will cause options to be passed to the
          ‘configure’ script in the subdirectory named by
          ‘LT_CONFIG_LTDL_DIR’ to cause it to be built as a convenience
          library.  If you're not using automake, you will need to
          define ‘top_build_prefix’, ‘top_builddir’, and ‘top_srcdir’ in
          your makefile so that ‘LIBLTDL’, ‘LTDLDEPS’, and ‘LTDLINCL’
          expand correctly.

          One advantage of the convenience library is that it is not
          installed, so the fact that you use ‘libltdl’ will not be
          apparent to the user, and it won't overwrite a pre-installed
          version of ‘libltdl’ the system might already have in the
          installation directory.  On the other hand, if you want to
          upgrade ‘libltdl’ for any reason (e.g. a bugfix) you'll have
          to recompile your package instead of just replacing the shared
          installed version of ‘libltdl’.  However, if your programs or
          libraries are linked with other libraries that use such a
          pre-installed version of ‘libltdl’, you may get linker errors
          or run-time crashes.  Another problem is that you cannot link
          the convenience library into more than one libtool library,
          then link a single program with those libraries, because you
          may get duplicate symbols.  In general you can safely use the
          convenience library in programs that don't depend on other
          libraries that might use ‘libltdl’ too.

     ‘installable’
          This keyword will pass options to the ‘configure’ script in
          the subdirectory named by ‘LT_CONFIG_LTDL_DIR’ to cause it to
          be built as an installable library.  If you're not using
          automake, you will need to define ‘top_build_prefix’,
          ‘top_builddir’ and ‘top_srcdir’ in your makefile so that
          ‘LIBLTDL’, ‘LTDLDEPS’, and ‘LTDLINCL’ are expanded properly.

          Be aware that you could overwrite another ‘libltdl’ already
          installed to the same directory if you use this option.

   Whatever method you use, ‘LTDL_INIT’ will define the shell variable
‘LIBLTDL’ to the link flag that you should use to link with ‘libltdl’,
the shell variable ‘LTDLDEPS’ to the files that can be used as a
dependency in ‘Makefile’ rules, and the shell variable ‘LTDLINCL’ to the
preprocessor flag that you should use to compile programs that include
‘ltdl.h’.  So, when you want to link a program with libltdl, be it a
convenience, installed or installable library, just use ‘$(LTDLINCL)’
for preprocessing and compilation, and ‘$(LIBLTDL)’ for linking.

   • If your package is built using an installed version of ‘libltdl’,
     ‘LIBLTDL’ will be set to the compiler flags needed to link against
     the installed library, ‘LTDLDEPS’ will be empty, and ‘LTDLINCL’
     will be set to the compiler flags needed to find the ‘libltdl’
     header files.

   • If your package is built using the convenience libltdl, ‘LIBLTDL’
     and ‘LTDLDEPS’ will be the pathname for the convenience version of
     libltdl (starting with ‘${top_builddir}/’ or ‘${top_build_prefix}’)
     and ‘LTDLINCL’ will be ‘-I’ followed by the directory that contains
     ‘ltdl.h’ (starting with ‘${top_srcdir}/’).

   • If an installable version of the included ‘libltdl’ is being built,
     its pathname starting with ‘${top_builddir}/’ or
     ‘${top_build_prefix}’, will be stored in ‘LIBLTDL’ and ‘LTDLDEPS’,
     and ‘LTDLINCL’ will be set just like in the case of convenience
     library.

   You should probably also use the ‘dlopen’ option to ‘LT_INIT’ in your
‘configure.ac’, otherwise libtool will assume no dlopening mechanism is
supported, and revert to dlpreopening, which is probably not what you
want.  Avoid using the ‘-static’, ‘-static-libtool-libs’, or
‘-all-static’ switches when linking programs with libltdl.  This will
not work on all platforms, because the dlopening functions may not be
available for static linking.

   The following example shows you how to embed an installable libltdl
in your package.  In order to use the convenience variant, just replace
the ‘LTDL_INIT’ option ‘installable’ with ‘convenience’.  We assume that
libltdl was embedded using ‘libtoolize --ltdl’.

   configure.ac:
     ...
     # Name the subdirectory that contains libltdl sources
     LT_CONFIG_LTDL_DIR([libltdl])

     # Configure libtool with dlopen support if possible
     LT_INIT([dlopen])

     # Enable building of the installable libltdl library
     LTDL_INIT([installable])
     ...

   Makefile.am:
     ...
     SUBDIRS = libltdl

     AM_CPPFLAGS = $(LTDLINCL)

     myprog_LDFLAGS = -export-dynamic
     myprog_LDADD = $(LIBLTDL) -dlopen self -dlopen foo1.la
     myprog_DEPENDENCIES = $(LTDLDEPS) foo1.la
     ...

 -- Macro: LTDL_INSTALLABLE
 -- Macro: AC_LIBLTDL_INSTALLABLE
     These macros are deprecated, the ‘installable’ option to
     ‘LTDL_INIT’ should be used instead.

 -- Macro: LTDL_CONVENIENCE
 -- Macro: AC_LIBLTDL_CONVENIENCE
     These macros are deprecated, the ‘convenience’ option to
     ‘LTDL_INIT’ should be used instead.

   ---------- Footnotes ----------

   (1) We used to recommend adding the contents of ‘ltdl.m4’ to
‘acinclude.m4’, but with ‘aclocal’ from a modern Automake (1.8 or newer)
and this release of libltdl that is not only unnecessary but makes it
easy to forget to upgrade ‘acinclude.m4’ if you move to a different
release of libltdl.

   (2) Even if libltdl is installed, ‘LTDL_INIT’ may fail to detect it
if libltdl depends on symbols provided by libraries other than the C
library.


File: libtool.info,  Node: Trace interface,  Next: FAQ,  Prev: Using libltdl,  Up: Top

12 Libtool's trace interface
****************************

This section describes macros whose sole purpose is to be traced using
Autoconf's ‘--trace’ option (*note The Autoconf Manual:
(autoconf)autoconf Invocation.) to query the Libtool configuration of a
project.  These macros are called by Libtool internals and should never
be called by user code; they should only be traced.

 -- Macro: LT_SUPPORTED_TAG (TAG)
     This macro is called once for each language enabled in the package.
     Its only argument, TAG, is the tag-name corresponding to the
     language (*note Tags::).

     You can therefore retrieve the list of all tags enabled in a
     project using the following command:
          autoconf --trace 'LT_SUPPORTED_TAG:$1'


File: libtool.info,  Node: FAQ,  Next: Troubleshooting,  Prev: Trace interface,  Up: Top

13 Frequently Asked Questions about libtool
*******************************************

This chapter covers some questions that often come up on the mailing
lists.

* Menu:

* Stripped link flags::         Dropped flags when creating a library


File: libtool.info,  Node: Stripped link flags,  Up: FAQ

13.1 Why does libtool strip link flags when creating a library?
===============================================================

When creating a shared library, but not when compiling or creating a
program, ‘libtool’ drops some flags from the command line provided by
the user.  This is done because flags unknown to ‘libtool’ may interfere
with library creation or require additional support from ‘libtool’, and
because omitting flags is usually the conservative choice for a
successful build.

   If you encounter flags that you think are useful to pass, as a
work-around you can prepend flags with ‘-Wc,’ or ‘-Xcompiler ’ to allow
them to be passed through to the compiler driver (*note Link mode::).
Another possibility is to add flags already to the compiler command at
‘configure’ run time:

     ./configure CC='gcc -m64'

   If you think ‘libtool’ should let some flag through by default,
here's how you can test such an inclusion: grab the Libtool development
tree, edit the ‘ltmain.in’ file in the ‘libltdl/config’ subdirectory to
pass through the flag (search for ‘Flags to be passed through’),
re-bootstrap and build with the flags in question added to ‘LDFLAGS’,
‘CFLAGS’, ‘CXXFLAGS’, etc.  on the ‘configure’ command line as
appropriate.  Run the testsuite as described in the ‘README’ file and
report results to the Libtool bug reporting address
<bug-libtool@gnu.org>.


File: libtool.info,  Node: Troubleshooting,  Next: Maintaining,  Prev: FAQ,  Up: Top

14 Troubleshooting
******************

Libtool is under constant development, changing to remain up-to-date
with modern operating systems.  If libtool doesn't work the way you
think it should on your platform, you should read this chapter to help
determine what the problem is, and how to resolve it.

* Menu:

* Libtool test suite::          Libtool's self-tests.
* Reporting bugs::              How to report problems with libtool.


File: libtool.info,  Node: Libtool test suite,  Next: Reporting bugs,  Up: Troubleshooting

14.1 The libtool test suite
===========================

Libtool comes with an integrated set of tests to check that your build
is sane, that test its capabilities, and report obvious bugs in the
libtool program.  The test suite is based on Autotest from Autoconf
(*note Generating Test Suites with Autotest: (autoconf)testsuite
Invocation.).  These tests, too, are constantly evolving, based on past
problems with libtool, and known deficiencies in other operating
systems.

   As described in the ‘README’ file, you may run ‘make -k check’ after
you have built libtool (possibly before you install it) to make sure
that it meets basic functional requirements.

* Menu:

* Test descriptions::           The contents of the test suite.
* When tests fail::             What to do when a test fails.


File: libtool.info,  Node: Test descriptions,  Next: When tests fail,  Up: Libtool test suite

14.1.1 Description of test suite
--------------------------------

The test suite uses keywords to classify certain test groups:

‘CXX’
‘F77’
‘FC’
‘GCJ’
     The test group exercises one of these ‘libtool’ language tags.

‘autoconf’
‘automake’
     These keywords denote that the respective external program is
     needed by the test group.  The tests are typically skipped if the
     program is not installed.  The ‘automake’ keyword may also denote
     use of the ‘aclocal’ program.

‘interactive’
     This test group may require user interaction on some systems.
     Typically, this means closing a popup window about a DLL load error
     on Windows.

‘libltdl’
     Denote that the ‘libltdl’ library is exercised by the test group.

‘libtool’
‘libtoolize’
     Denote that the ‘libtool’ or ‘libtoolize’ scripts are exercised by
     the test group, respectively.

‘recursive’
     Denote that this test group may recursively re-invoke the test
     suite itself, with changed settings and maybe a changed ‘libtool’
     script.  You may use the ‘INNER_TESTSUITEFLAGS’ variable to pass
     additional settings to this recursive invocation.  Typically,
     recursive invocations delimit the set of tests with another
     keyword, for example by passing ‘-k libtool’ right before the
     expansion of the ‘INNER_TESTSUITEFLAGS’ variable (without an
     intervening space, so you get the chance for further delimitation).

     Test groups with the keyword ‘recursive’ should not be denoted with
     keywords, in order to avoid infinite recursion.  As a consequence,
     recursive test groups themselves should never require user
     interaction, while the test groups they invoke may do so.

   There is a convenience target ‘check-noninteractive’ that runs all
tests from both test suites that do not cause user interaction on
Windows.  Conversely, the target ‘check-interactive’ runs the complement
of tests and might require closing popup windows about DLL load errors
on Windows.

   Here is a list of the some of the current files in the test suite,
and what they test for:

‘tests/am-subdirs.at’
     Tests that a binary can be built and ran from outside of the
     sub-directory that it is built and ran in.

‘tests/archive-in-archive.at’
     Tests convenience archive within another convenience archive.
     Compiles ‘foo()’ in ‘libfoo’, then compiles ‘libfoo’ (and ‘bar()’
     function) into ‘libbar’.

‘tests/bindir.at’
     Tests include a demonstration of various scenarios related to using
     the ‘-bindir’ option with ‘libtool’, verifying that installed files
     are correctly linked and executed, and demonstrating flexibility in
     handling different installation paths and adapting to changing
     directory structures.

‘tests/bug_42313.at’
     Tests that there are no conflicting warnings about AC_PROG_RANLIB
     by verifying no autoscan AC_PROG_RANLIB warning and checking that
     AC_PROG_RANLIB declaration has a warning.

‘tests/bug_62343.at’
     Tests that the ‘-no-canonical-prefixes’ flag is not removed from
     the linking command, but it is instead passed through to the
     linker.

‘tests/bug_71489.at’
     Tests that the local version of an installed program is used both
     with and without an external library.

‘tests/cdemo.at’
     Tests include a demonstration of ‘libtool’ convenience libraries, a
     mechanism that allows build-time static libraries to be created, in
     a way that their components can be later linked into programs or
     other libraries, even shared ones.

‘tests/cmdline_wrap.at’
     Tests include a verification that ‘libtool’ operates properly when
     the maximum command line length is very small.

‘tests/configure-funcs.at’
     Tests creation of shell functions shared with ‘configure’ and
     ‘libtool’.

‘tests/configure-iface.at’
     Tests that exercise the configure interface to ‘libltdl’.

‘tests/convenience.at’
     Tests demonstrate ‘libtool’'s convenience archive capabilities for
     C, C++, Fortran, and Java languages.

‘tests/ctor.at’
     Test that ‘libtool’ can handle code with C++ constructors.

‘tests/cwrapper.at’
     Test cwrapper compliance with standards for uninstalled
     executables, string length, and installed shared libraries.

‘tests/darwin.at’
     Tests for macOS X including compilation, concurrent library
     extraction, ‘GDB’ debug information, ‘ld’ warnings, and verifying
     ‘.dylib’ and ‘.so’ files can be used with ‘lt_dlopen’.

‘tests/demo.at’
     Tests include a demonstration of a trivial package that uses
     ‘libtool’.  The tests include scenarios to build both static and
     shared libraries, only static libraries, only shared libraries,
     disabling fast-install mode, building PIC code, and building
     non-PIC code.

‘tests/depdemo.at’
     Tests include a demonstration of inter-library dependencies with
     ‘libtool’.  The test programs link some interdependent libraries
     under different scenarios.

‘tests/deplib-in-subdir.at’
     Tests building and linking various libraries within various
     directories and sub-directories while changing directories as well.
     It should be possible to use a nontrivial relative path to the
     output file name when creating libraries and programs.  The
     ‘deplibs’ of these might have relative paths as well.  When
     executing uninstalled programs, the paths relative to ‘$PWD’ at
     build time needs to be translated to a path valid at execution
     time.  Also test installing these libraries and programs; however,
     use consistent relative paths between ‘libtool’ ‘--mode=link’ and
     ‘libtool’ ‘--mode=install’ in this test.

‘tests/deplib-path.at’
     Tests linking shared deplibs by ‘libN.so’ name and ensures linking
     ‘libN.so’, ‘-l:libN.so’ and ‘-lN’ is equivalent.

‘tests/deplibs-ident.at’
     Tests the correct detection and handling of identical dependency
     libraries when using ‘libtool’.

‘tests/deplibs-mingw.at’
     Tests include various scenarios related to detecting deplibs
     correctly, including cases where there is no ‘file’ command
     installed as well as the host OS being MinGW.

‘tests/destdir.at’
     Installs some libs in ‘$DESTDIR’, moves them to a different dir,
     then installs some false libraries in ‘$DESTDIR’ that should not be
     linked against.  If the program refers to these false libraries,
     there is a bug.

‘tests/dlloader-api.at’
     Tests that ‘lt_dlopen’ can open a shared library and call a
     function from the library, verifies that ‘lt_dlsym’ can find a
     symbol in the opened library, and make sure that ‘lt_dlclose’ can
     properly close the library.

‘tests/dumpbin-symbols.at’
     Tests whether on Windows a convenience symbol in a section of a
     ‘.lib’ file is present even if that section is hidden in the ‘.obj’
     file.

‘tests/duplicate_conv.at’
     Tests two convenience archives with the same name, and also
     containing an object with the same name.

‘tests/duplicate_deps.at’
     Tests circular call of dependencies between two libraries, ‘liba’
     and ‘libb’.  Function ‘a1()’ from ‘liba’ calls ‘b1()’ from ‘libb’
     and function ‘b1()’ from ‘libb’ calls ‘a2()’ from ‘liba’.

‘tests/duplicate_members.at’
     Tests a library with multiple files of the same name (from
     different directories), such as ‘1/a.c’, ‘2/a.c’, ‘3/a.c’, etc.

‘tests/early-libtool.at’
     Tests building binaries using ‘libtool’ using two configure
     approaches.

‘tests/exceptions.at’
     Tests C++ exception handling with ‘libtool’.

‘tests/execute-mode.at’
     Tests the ‘--mode=execute’ feature of ‘libtool’.

‘tests/exeext.at’
     Tests ensure that ‘$EXEEXT’ handling works by linking and
     installing an executable.

‘tests/export.at’
     Tests symbol exports if shared libraries are enabled.

‘tests/export-def.at’
     Test exporting from a DLL with module definition (‘.def’ files).
     This test only runs if shared libraries are enabled and building a
     DLL is supported.

‘tests/f77demo.at’
     Tests test Fortran 77 support in ‘libtool’ by creating libraries
     from Fortran 77 sources, and mixed Fortran and C sources, and a
     Fortran 77 program to use the former library, and a C program to
     use the latter library.

‘tests/fail.at’
     Tests to ensure that ‘libtool’ really fails when it should,
     including after compile failure, program creation failure, and
     shared library creation failure.

‘tests/fcdemo.at’
     Tests are similar to the ‘tests/f77demo.at’ tests, except that
     Fortran 90 is used in combination with the ‘FC’ interface provided
     by Autoconf and Automake.

‘tests/flags.at’
     Tests include checks that compile and linker flags get passed
     through ‘libtool’.  Tests flags for C, C++, Objective-C,
     Objective-C++, ML64, Fortran 77, Fortran 90, and Java.

‘tests/help.at’
     Tests a variety of mode commands, including mode short-hands, to
     ensure basic command line functionality.  Also verify that the
     ‘--debug’ flag is handled correctly in each mode.

‘tests/indirect_deps.at’
     Tests indirect dependencies (or nested dependencies).  ‘libd’
     depends on ‘libconv’, which depends on ‘libb’, which depends on
     ‘liba’.

‘tests/infer-tag.at’
     Tests that ‘func_infer_tag’ works by compiling various code
     snippets in various languages (C, C++, Objective-C, Objective-C++,
     ML64, Fortran, Java) without a ‘--tag’ flag.

‘tests/inherited_flags.at’
     Tests the functionality of the ‘inherited_linker_flags’ variable in
     ‘libtool’ library files.

‘tests/install.at’
     Tests install mode and ensures that ‘install_override_mode’
     overrides the mode of the shared library (and only the shared
     library).

‘tests/lalib-syntax.at’
     Tests parsing of ‘.la’ files including both correctly formed ‘.la’
     files and malformed or bogus ‘.la’ files.

‘tests/libtool.at’
     Tests basic ‘libtool’ functionality including shell meta-character
     removal, lack of supplied mode option, file extension handling,
     simple linking, ‘objectlist’ flag usage, and ‘LT_SUPPORTED_TAG’
     usage.

‘tests/libtoolize.at’
     Tests that errors and warnings are reported for invalid and unknown
     ‘LIBTOOLIZE_OPTIONS’ as well as checking the ‘--no-warn’ flag
     suppresses ‘libtoolize’ warnings.

‘tests/link-order2.at’
     Tests to make sure that ‘depdepls’ are added right after the
     libraries that pull them in which is necessary at least for static
     linking and on systems where libraries do not link against other
     libraries.

‘tests/link-order.at’
     Tests for problems with linking libraries in different order.

‘tests/loadlibrary.at’
     Tests ‘libltdl’ (a ‘libdl’ API for ‘dlopen’) support of
     ‘LoadLibrary’ for dynamically linking DLLs in Windows environments.

‘tests/localization.at’
     Tests to verify that invoking C compiler language localization
     options do not cause problems with ‘libtool’.

‘tests/lt_dladvise.at’
     Tests ‘libltdl’ (a ‘libdl’ API for ‘dlopen’) module loading advisor
     functions.

‘tests/lt_dlexit.at’
     Tests ‘libltdl’ (a ‘libdl’ API for ‘dlopen’) for a memory use after
     free bug in ‘lt_dlexit’.

‘tests/lt_dlopen_a.at’
     Tests ‘lt_dlopen’ with an archive file.  Verifies that ‘lt_dlopen’
     can load an archive file and successfully return a handle to it.

‘tests/lt_dlopen.at’
     Tests ‘libltdl’ (a ‘libdl’ API for ‘dlopen’) with a basic C
     example.

‘tests/lt_dlopenext.at’
     Tests ‘libltdl’ (a ‘libdl’ API for ‘dlopen’) with an extern C++
     function.

‘tests/ltdl-api.at’
     Tests that ‘libtool’ doesn't mangle ‘gnulib’ ‘argv’ names.

‘tests/ltdl-libdir.at’
     Tests if ‘ltdl’ can find an installed module using the ‘libdir’
     variable in the ‘.la’ file.  Tests also include a MinGW test which
     uses a Windows-style ‘libdir’ name.

‘tests/mdemo.at’
     Tests include a demonstration of a package that uses ‘libtool’ and
     the system independent ‘dlopen’ wrapper ‘libltdl’ to load modules.
     The library ‘libltdl’ provides a ‘dlopen’ wrapper for various
     platforms (POSIX) including support for ‘dlpreopened’ modules
     (*note Dlpreopening::).

‘tests/need_lib_prefix.at’
     Tests to check for failures on systems that require libraries to be
     prefixed with ‘lib’.

‘tests/nocase.at’
     Tests the ‘want_nocaseglob’ configuration option to search for
     libraries regardless of case.

‘tests/no-executables.at’
     Tests ‘AC_NO_EXECUTABLES’ macro with ‘gcc’.

‘tests/nonrecursive.at’
     Tests non-recursive DLL libraries with nonrecursive Automake
     ‘libltdl’ (a ‘libdl’ API for ‘dlopen’) build.

‘tests/old-m4-iface.at’
     Tests various aspects of ‘libtool’'s old ‘m4’ interface.

‘tests/pic_flag.at’
     Tests the ‘-fpic’ flag with ‘gcc’ and ‘g++’.

‘tests/recursive.at’
     Tests recursive DLL libraries with recursive Automake ‘libltdl’ (a
     ‘libdl’ API for ‘dlopen’) build.

‘tests/resident.at’
     Tests that resident modules are not unloaded at program exit, as
     they need to be able to invoke ‘atexit’ handlers.  A module being a
     resident module means it is prevented from being ‘lt_dlclosed’.

‘tests/runpath-in-lalib.at’
     Tests the runpath configuration options in ‘libtool’ (‘-R’ and
     ‘-rpath’) and that the resulting ‘.la’ files are installed in the
     directory appended to their respective library's ‘dependency_libs’
     by ‘-R’.

‘tests/search-path.at’
     Tests ‘sys_lib_search_path_spec’, which is an expression to get the
     compile-time system library search path.  Tests include determining
     if it is possible to link an executable to system libraries and if
     ‘sys_lib_search_path_spec’ also works on MSVC (Microsoft Visual
     C++).

‘tests/shlibpath.at’
     Tests include verifying proper behaviour of the ‘shlibpath_var’,
     which is the variable responsible for telling the linker where to
     find shared libraries.  Additionally, this tests the behaviour of
     the ‘shlibpath_overrides_runpath’ variable, which determines if it
     possible to override an executable's hardcoded library search path
     with an environment variable.

‘tests/slimcc.at’
     Tests for issues linking libraries in different order with the
     slimcc compiler.

‘tests/slist.at’
     Tests the functionality of the SList datastructure, which is an
     implementation of singly linked lists.  Tests include verifying the
     usual linked list functions such as finding, removing, deleting,
     reversing, and sorting the element(s) in SLists.

‘tests/standalone.at’
     Tests ‘libltdl’ functionality as a standalone tool.  Tests include
     compiling a symlinked version of ‘libltdl’ and a copied version of
     ‘libltdl’.  Next ‘libltdl’ is installed locally and linked in a
     project without the use of Autoconf or Automake.

‘tests/static.at’
     Tests various flags related to static and dynamic linking including
     ‘-Bstatic’ and ‘-Bdynamic’.

‘tests/stresstest.at’
     Tests various ‘libtool’ flag and option combinations, tests linking
     various types of objects in different sections, and tests regular
     expressions to cover edge cases and unusual configurations over
     multiple iterations.

‘tests/subproject.at’
     Tests the ‘libltdl’ flag for building subprojects in individual
     directories.  Tests cover soft-linked libltdl trees, copied
     ‘libltdl’ trees, and linking ‘libltdl’ without Autotools.

‘tests/sysroot.at’
     Tests that ‘libtool’ runs properly in sandboxed ‘sysroot’
     directories.

‘tests/tagdemo.at’
     Tests include a demonstration of a package that uses ‘libtool’'s
     multi-language support through configuration tags.  It generates a
     library from C++ sources, which is then linked to a C++ program.

‘tests/template.at’
     Tests that ‘libtool’ can handle C++ code utilizing templates.

‘tests/testsuite.at’
     Main testsuite framework file processed by ‘autom4te’ processes
     enable running the ‘libtool’ test cases.

‘tests/versioning_script.at’
     Tests ‘libtool-next-version’ for verifying the helper script
     determines the next version to use for a ‘libtool’ library
     correctly.

‘tests/versioning.at’
     Tests ‘libtool’'s versioning system.  Tests begin with verifying
     the behaviour of ‘libtool’ versioning flags ‘-version-info’ and
     ‘-version-number’.  Next, this tests installing a library, then
     updating the library with a new revision, a compatible update, and
     an incompatible update.  In each case, the tests verify that the
     original library will link and install as expected.

‘tests/with_pic.at’
     Tests the function of the ‘--enable-pic’ flag.  The ‘--enable-pic’
     flag is used to specify whether or not ‘libtool’ uses PIC objects.
     This includes tests for setting ‘--enable-pic’ to no, yes, or a
     comma delimited list of package names.


File: libtool.info,  Node: When tests fail,  Prev: Test descriptions,  Up: Libtool test suite

14.1.2 When tests fail
----------------------

The Autotest-based test suite produces as output a file
‘tests/testsuite.log’ that contains information about failed tests.

   You can pass options to the test suite through the ‘make’ variable
‘TESTSUITEFLAGS’ (*note Making testsuite Scripts: (autoconf)testsuite
Invocation.).


File: libtool.info,  Node: Reporting bugs,  Prev: Libtool test suite,  Up: Troubleshooting

14.2 Reporting bugs
===================

If you think you have discovered a bug in libtool, you should think
twice: the libtool maintainer is notorious for passing the buck (or
maybe that should be "passing the bug").  Libtool was invented to fix
known deficiencies in shared library implementations, so, in a way, most
of the bugs in libtool are actually bugs in other operating systems.
However, the libtool maintainer would definitely be happy to add support
for somebody else's buggy operating system.  [I wish there was a good
way to do winking smiley-faces in Texinfo.]

   Genuine bugs in libtool include problems with shell script
portability, documentation errors, and failures in the test suite (*note
Libtool test suite::).

   First, check the documentation and help screens to make sure that the
behaviour you think is a problem is not already mentioned as a feature.

   Then, you should read the Emacs guide to reporting bugs (*note
Reporting Bugs: (emacs)Bugs.).  Some of the details listed there are
specific to Emacs, but the principle behind them is a general one.

   Finally, send a bug report to the Libtool bug reporting address
<bug-libtool@gnu.org> with any appropriate _facts_, such as test suite
output (*note When tests fail::), all the details needed to reproduce
the bug, and a brief description of why you think the behaviour is a
bug.  Be sure to include the word "libtool" in the subject line, as well
as the version number you are using (which can be found by typing
‘libtool --version’).


File: libtool.info,  Node: Maintaining,  Next: GNU Free Documentation License,  Prev: Troubleshooting,  Up: Top

15 Maintenance notes for libtool
********************************

This chapter contains information that the libtool maintainer finds
important.  It will be of no use to you unless you are considering
porting libtool to new systems, or writing your own libtool.

* Menu:

* New ports::                   How to port libtool to new systems.
* Tested platforms::            When libtool was last tested.
* Platform quirks::             Information about different library systems.
* libtool script contents::     Configuration information that libtool uses.
* Cheap tricks::                Making libtool maintainership easier.


File: libtool.info,  Node: New ports,  Next: Tested platforms,  Up: Maintaining

15.1 Porting libtool to new systems
===================================

Before you embark on porting libtool to an unsupported system, it is
worthwhile to send e-mail to the Libtool mailing list <libtool@gnu.org>,
to make sure that you are not duplicating existing work.

   If you find that any porting documentation is missing, please
complain!  Complaints with patches and improvements to the
documentation, or to libtool itself, are more than welcome.

* Menu:

* Information sources::         Where to find relevant documentation
* Porting inter-library dependencies::  Implementation details explained


File: libtool.info,  Node: Information sources,  Next: Porting inter-library dependencies,  Up: New ports

15.1.1 Information sources
--------------------------

Once it is clear that a new port is necessary, you'll generally need the
following information:

canonical system name
     You need the output of ‘config.guess’ for this system, so that you
     can make changes to the libtool configuration process without
     affecting other systems.

man pages for ‘ld’ and ‘cc’
     These generally describe what flags are used to generate PIC, to
     create shared libraries, and to link against only static libraries.
     You may need to follow some cross references to find the
     information that is required.

man pages for ‘ld.so’, ‘rtld’, or equivalent
     These are a valuable resource for understanding how shared
     libraries are loaded on the system.

man page for ‘ldconfig’, or equivalent
     This page usually describes how to install shared libraries.

output from ‘ls -l /lib /usr/lib’
     This shows the naming convention for shared libraries on the
     system, including what names should be symbolic links.

any additional documentation
     Some systems have special documentation on how to build and install
     shared libraries.

   If you know how to program the Bourne shell, then you can complete
the port yourself; otherwise, you'll have to find somebody with the
relevant skills who will do the work.  People on the libtool mailing
list are usually willing to volunteer to help you with new ports, so you
can send the information to them.

   To do the port yourself, you'll definitely need to modify the
‘libtool.m4’ macros to make platform-specific changes to the
configuration process.  You should search that file for the ‘PORTME’
keyword, which will give you some hints on what you'll need to change.
In general, all that is involved is modifying the appropriate
configuration variables (*note libtool script contents::).

   Your best bet is to find an already-supported system that is similar
to yours, and make your changes based on that.  In some cases, however,
your system will differ significantly from every other supported system,
and it may be necessary to add new configuration variables, and modify
the ‘ltmain.in’ script accordingly.  Be sure to write to the mailing
list before you make changes to ‘ltmain.in’, since they may have advice
on the most effective way of accomplishing what you want.


File: libtool.info,  Node: Porting inter-library dependencies,  Prev: Information sources,  Up: New ports

15.1.2 Porting inter-library dependencies support
-------------------------------------------------

Since version 1.2c, libtool has re-introduced the ability to do
inter-library dependency on some platforms, thanks to a patch by Toshio
Kuratomi <badger@prtr-13.ucsc.edu>.  Here's a shortened version of the
message that contained his patch:

   The basic architecture is this: in ‘libtool.m4’, the person who
writes libtool makes sure ‘$deplibs’ is included in ‘$archive_cmds’
somewhere and also sets the variable ‘$deplibs_check_method’, and maybe
‘$file_magic_cmd’ when ‘deplibs_check_method’ is file_magic.

   ‘deplibs_check_method’ can be one of five things:
‘file_magic [REGEX]’
     looks in the library link path for libraries that have the right
     libname.  Then it runs ‘$file_magic_cmd’ on the library and checks
     for a match against the extended regular expression REGEX.  When
     ‘file_magic_test_file’ is set by ‘libtool.m4’, it is used as an
     argument to ‘$file_magic_cmd’ to verify whether the regular
     expression matches its output, and warn the user otherwise.

‘pass_all’
     will pass everything without any checking.  This may work on
     platforms where code is position-independent by default and
     inter-library dependencies are properly supported by the dynamic
     linker, for example, on DEC OSF/1 3 and 4.

‘none’
     It causes deplibs to be reassigned ‘deplibs=""’.  That way
     ‘archive_cmds’ can contain deplibs on all platforms, but not have
     deplibs used unless needed.

‘unknown’
     is the default for all systems unless overridden in ‘libtool.m4’.
     It is the same as ‘none’, but it documents that we really don't
     know what the correct value should be, and we welcome patches that
     improve it.

   Then in ‘ltmain.in’ we have the real workhorse: a little
initialization and postprocessing (to setup/release variables for use
with eval echo libname_spec etc.)  and a case statement that decides the
method that is being used.  This is the real code... I wish I could
condense it a little more, but I don't think I can without function
calls.  I've mostly optimized it (moved things out of loops, etc.)  but
there is probably some fat left.  I thought I should stop while I was
ahead, work on whatever bugs you discover, etc. before thinking about
more than obvious optimizations.


File: libtool.info,  Node: Tested platforms,  Next: Platform quirks,  Prev: New ports,  Up: Maintaining

15.2 Tested platforms
=====================

This table describes when libtool was last known to be tested on
platforms where it claims to support shared libraries:

     -------------------------------------------------------
     canonical host name          compiler  libtool results
       (tools versions)                     release
     -------------------------------------------------------
     alpha-dec-osf5.1		cc	 1.3e	  ok (1.910)
     alpha-dec-osf4.0f               gcc      1.3e     ok (1.910)
     alpha-dec-osf4.0f               cc       1.3e     ok (1.910)
     alpha-dec-osf3.2                gcc      0.8      ok
     alpha-dec-osf3.2                cc       0.8      ok
     alpha-dec-osf2.1                gcc      1.2f     NS
     alpha*-unknown-linux-gnu        gcc      1.3b     ok
       (egcs-1.1.2, GNU ld 2.9.1.0.23)
     hppa2.0w-hp-hpux11.00           cc       1.2f     ok
     hppa2.0-hp-hpux10.20            cc       1.3.2    ok
     hppa1.1-hp-hpux10.20            gcc      1.2f     ok
     hppa1.1-hp-hpux10.20            cc       1.3c     ok (1.821)
     hppa1.1-hp-hpux10.10            gcc      1.2f     ok
     hppa1.1-hp-hpux10.10            cc       1.2f     ok
     hppa1.1-hp-hpux9.07             gcc      1.2f     ok
     hppa1.1-hp-hpux9.07             cc       1.2f     ok
     hppa1.1-hp-hpux9.05             gcc      1.2f     ok
     hppa1.1-hp-hpux9.05             cc       1.2f     ok
     hppa1.1-hp-hpux9.01             gcc      1.2f     ok
     hppa1.1-hp-hpux9.01             cc       1.2f     ok
     i*86-*-beos                     gcc      1.2f     ok
     i*86-*-bsdi4.0.1                gcc      1.3c     ok
       (gcc-2.7.2.1)
     i*86-*-bsdi4.0                  gcc      1.2f     ok
     i*86-*-bsdi3.1                  gcc      1.2e     NS
     i*86-*-bsdi3.0                  gcc      1.2e     NS
     i*86-*-bsdi2.1                  gcc      1.2e     NS
     i*86-pc-cygwin                  gcc      1.3b     NS
       (egcs-1.1 stock b20.1 compiler)
     i*86-*-dguxR4.20MU01            gcc      1.2      ok
     i*86-*-freebsd4.3		gcc      1.3e     ok (1.912)
     i*86-*-freebsdelf4.0            gcc      1.3c     ok
       (egcs-1.1.2)
     i*86-*-freebsdelf3.2            gcc      1.3c     ok
       (gcc-2.7.2.1)
     i*86-*-freebsdelf3.1            gcc      1.3c     ok
       (gcc-2.7.2.1)
     i*86-*-freebsdelf3.0            gcc      1.3c     ok
     i*86-*-freebsd3.0               gcc      1.2e     ok
     i*86-*-freebsd2.2.8             gcc      1.3c     ok
       (gcc-2.7.2.1)
     i*86-*-freebsd2.2.6             gcc      1.3b     ok
       (egcs-1.1 & gcc-2.7.2.1, native ld)
     i*86-*-freebsd2.1.5             gcc      0.5      ok
     i*86-*-netbsd1.5                gcc      1.3e     ok (1.901)
       (egcs-1.1.2)
     i*86-*-netbsd1.4                gcc      1.3c     ok
       (egcs-1.1.1)
     i*86-*-netbsd1.4.3A             gcc      1.3e     ok (1.901)
     i*86-*-netbsd1.3.3              gcc      1.3c     ok
       (gcc-2.7.2.2+myc2)
     i*86-*-netbsd1.3.2              gcc      1.2e     ok
     i*86-*-netbsd1.3I               gcc      1.2e     ok
       (egcs 1.1?)
     i*86-*-netbsd1.2                gcc      0.9g     ok
     i*86-*-linux-gnu		gcc	 1.3e	  ok (1.901)
       (Red Hat 7.0, gcc "2.96")
     i*86-*-linux-gnu		gcc	 1.3e	  ok (1.911)
       (SuSE 7.0, gcc 2.95.2)
     i*86-*-linux-gnulibc1           gcc      1.2f     ok
     i*86-*-openbsd2.5               gcc      1.3c     ok
       (gcc-2.8.1)
     i*86-*-openbsd2.4               gcc      1.3c     ok
       (gcc-2.8.1)
     i*86-*-solaris2.7               gcc      1.3b     ok
       (egcs-1.1.2, native ld)
     i*86-*-solaris2.6               gcc      1.2f     ok
     i*86-*-solaris2.5.1             gcc      1.2f     ok
     i*86-ncr-sysv4.3.03             gcc      1.2f     ok
     i*86-ncr-sysv4.3.03             cc       1.2e     ok
       (cc -Hnocopyr)
     i*86-pc-sco3.2v5.0.5		cc	 1.3c	  ok
     i*86-pc-sco3.2v5.0.5		gcc	 1.3c	  ok
       (gcc 95q4c)
     i*86-pc-sco3.2v5.0.5		gcc	 1.3c	  ok
       (egcs-1.1.2)
     i*86-sco-sysv5uw7.1.1		gcc	 1.3e	  ok (1.901)
       (gcc-2.95.2, SCO linker)
     i*86-UnixWare7.1.0-sysv5	cc	 1.3c	  ok
     i*86-UnixWare7.1.0-sysv5	gcc	 1.3c	  ok
       (egcs-1.1.1)
     m68k-next-nextstep3             gcc      1.2f     NS
     m68k-sun-sunos4.1.1             gcc      1.2f     NS
       (gcc-2.5.7)
     m88k-dg-dguxR4.12TMU01          gcc      1.2      ok
     m88k-motorola-sysv4             gcc      1.3      ok
       (egcs-1.1.2)
     mips-sgi-irix6.5                gcc      1.2f     ok
       (gcc-2.8.1)
     mips-sgi-irix6.4                gcc      1.2f     ok
     mips-sgi-irix6.3                gcc      1.3b     ok
       (egcs-1.1.2, native ld)
     mips-sgi-irix6.3                cc       1.3b     ok
       (cc 7.0)
     mips-sgi-irix6.2                gcc      1.2f     ok
     mips-sgi-irix6.2                cc       0.9      ok
     mips-sgi-irix5.3                gcc      1.2f     ok
       (egcs-1.1.1)
     mips-sgi-irix5.3                gcc      1.2f     NS
       (gcc-2.6.3)
     mips-sgi-irix5.3                cc       0.8      ok
     mips-sgi-irix5.2                gcc      1.3b     ok
       (egcs-1.1.2, native ld)
     mips-sgi-irix5.2                cc       1.3b     ok
       (cc 3.18)
     mips-sni-sysv4			cc       1.3.5    ok
       (Siemens C-compiler)
     mips-sni-sysv4			gcc      1.3.5    ok
       (gcc-2.7.2.3, GNU assembler 2.8.1, native ld)
     mipsel-unknown-openbsd2.1       gcc      1.0      ok
     powerpc-apple-darwin6.4         gcc      1.5      ok
     (apple dev tools released 12/2002)
     powerpc-ibm-aix4.3.1.0          gcc      1.2f     ok
       (egcs-1.1.1)
     powerpc-ibm-aix4.2.1.0          gcc      1.2f     ok
       (egcs-1.1.1)
     powerpc-ibm-aix4.1.5.0          gcc      1.2f     ok
       (egcs-1.1.1)
     powerpc-ibm-aix4.1.5.0          gcc      1.2f     NS
       (gcc-2.8.1)
     powerpc-ibm-aix4.1.4.0          gcc      1.0      ok
     powerpc-ibm-aix4.1.4.0          xlc      1.0i     ok
     rs6000-ibm-aix4.1.5.0           gcc      1.2f     ok
       (gcc-2.7.2)
     rs6000-ibm-aix4.1.4.0           gcc      1.2f     ok
       (gcc-2.7.2)
     rs6000-ibm-aix3.2.5             gcc      1.0i     ok
     rs6000-ibm-aix3.2.5             xlc      1.0i     ok
     sparc-sun-solaris2.8		gcc	 1.3e	  ok (1.913)
       (gcc-2.95.3 & native ld)
     sparc-sun-solaris2.7            gcc      1.3e     ok (1.913)
       (gcc-2.95.3 & native ld)
     sparc-sun-solaris2.6            gcc      1.3e     ok (1.913)
       (gcc-2.95.3 & native ld)
     sparc-sun-solaris2.5.1          gcc      1.3e     ok (1.911)
     sparc-sun-solaris2.5            gcc      1.3b     ok
       (egcs-1.1.2, GNU ld 2.9.1 & native ld)
     sparc-sun-solaris2.5            cc       1.3b     ok
       (SC 3.0.1)
     sparc-sun-solaris2.4            gcc      1.0a     ok
     sparc-sun-solaris2.4            cc       1.0a     ok
     sparc-sun-solaris2.3            gcc      1.2f     ok
     sparc-sun-sunos4.1.4            gcc      1.2f     ok
     sparc-sun-sunos4.1.4            cc       1.0f     ok
     sparc-sun-sunos4.1.3_U1         gcc      1.2f     ok
     sparc-sun-sunos4.1.3C           gcc      1.2f     ok
     sparc-sun-sunos4.1.3            gcc      1.3b     ok
       (egcs-1.1.2, GNU ld 2.9.1 & native ld)
     sparc-sun-sunos4.1.3            cc       1.3b     ok
     sparc-unknown-bsdi4.0           gcc      1.2c     ok
     sparc-unknown-linux-gnulibc1    gcc      1.2f     ok
     sparc-unknown-linux-gnu         gcc      1.3b     ok
       (egcs-1.1.2, GNU ld 2.9.1.0.23)
     sparc64-unknown-linux-gnu       gcc      1.2f     ok

     Notes:
     - "ok" means "all tests passed".
     - "NS" means "Not Shared", but OK for static libraries

   Note: The vendor-distributed HP-UX ‘sed’(1) programs are horribly
broken, and cannot handle libtool's requirements, so users may report
unusual problems.  There is no workaround except to install a working
‘sed’ (such as GNU ‘sed’) on these systems.

   Note: The vendor-distributed NCR MP-RAS ‘cc’ programs emits copyright
on standard error that confuse tests on size of ‘conftest.err’.  The
workaround is to specify ‘CC’ when run ‘configure’ with ‘CC='cc
-Hnocopyr'’.


File: libtool.info,  Node: Platform quirks,  Next: libtool script contents,  Prev: Tested platforms,  Up: Maintaining

15.3 Platform quirks
====================

This section is dedicated to the sanity of the libtool maintainers.  It
describes the programs that libtool uses, how they vary from system to
system, and how to test for them.

   Because libtool is a shell script, it can be difficult to understand
just by reading it from top to bottom.  This section helps show why
libtool does things a certain way.  Combined with the scripts
themselves, you should have a better sense of how to improve libtool, or
write your own.

* Menu:

* Compilers::                   Creating object files from source files.
* Reloadable objects::          Binding object files together.
* Multiple dependencies::       Removing duplicate dependent libraries.
* Archivers::                   Programs that create static archives.
* Cross compiling::             Issues that arise when cross compiling.
* File name conversion::        Converting file names between platforms.
* Windows DLLs::                Windows header defines.


File: libtool.info,  Node: Compilers,  Next: Reloadable objects,  Up: Platform quirks

15.3.1 Compilers
----------------

The only compiler characteristics that affect libtool are the flags
needed (if any) to generate PIC objects.  In general, if a C compiler
supports certain PIC flags, then any derivative compilers support the
same flags.  Until there are some noteworthy exceptions to this rule,
this section will document only C compilers.

   The following C compilers have standard command line options,
regardless of the platform:

‘gcc’

     This is the GNU C compiler, which is also the system compiler for
     many free operating systems (FreeBSD, GNU/Hurd, GNU/Linux, Lites,
     NetBSD, and OpenBSD, to name a few).

     The ‘-fpic’ or ‘-fPIC’ flags can be used to generate
     position-independent code.  ‘-fPIC’ is guaranteed to generate
     working code, but the code is slower on m68k, m88k, and SPARC
     chips.  However, using ‘-fpic’ on those chips imposes arbitrary
     size limits on the shared libraries.

   The rest of this subsection lists compilers by the operating system
that they are bundled with:

‘aix3*’
‘aix4*’
     Most AIX compilers have no PIC flags, since AIX (with the exception
     of AIX for IA-64) runs on PowerPC and RS/6000 chips.  (1)

‘hpux10*’
     Use ‘+Z’ to generate PIC.

‘osf3*’
     Digital/UNIX 3.x does not have PIC flags, at least not on the
     PowerPC platform.

‘solaris2*’
     Use ‘-KPIC’ to generate PIC.

‘sunos4*’
     Use ‘-PIC’ to generate PIC.

   ---------- Footnotes ----------

   (1) All code compiled for the PowerPC and RS/6000 chips
(‘powerpc-*-*’, ‘powerpcle-*-*’, and ‘rs6000-*-*’) is
position-independent, regardless of the operating system or compiler
suite.  So, "regular objects" can be used to build shared libraries on
these systems and no special PIC compiler flags are required.


File: libtool.info,  Node: Reloadable objects,  Next: Multiple dependencies,  Prev: Compilers,  Up: Platform quirks

15.3.2 Reloadable objects
-------------------------

On all known systems, a reloadable object can be created by running ‘ld
-r -o OUTPUT.o INPUT1.o INPUT2.o’.  This reloadable object may be
treated as exactly equivalent to other objects.


File: libtool.info,  Node: Multiple dependencies,  Next: Archivers,  Prev: Reloadable objects,  Up: Platform quirks

15.3.3 Multiple dependencies
----------------------------

On most modern platforms the order where dependent libraries are listed
has no effect on object generation.  In theory, there are platforms that
require libraries that provide missing symbols to other libraries to be
listed after those libraries whose symbols they provide.

   Particularly, if a pair of static archives each resolve some of the
other's symbols, it might be necessary to list one of those archives
both before and after the other one.  Libtool does not currently cope
with this situation well, since duplicate libraries are removed from the
link line by default.  Libtool provides the command line option
‘--preserve-dup-deps’ to preserve all duplicate dependencies in cases
where it is necessary.


File: libtool.info,  Node: Archivers,  Next: Cross compiling,  Prev: Multiple dependencies,  Up: Platform quirks

15.3.4 Archivers
----------------

On all known systems, building a static library can be accomplished by
running ‘ar cr libNAME.a OBJ1.o OBJ2.o ...’, where the ‘.a’ file is the
output library, and each ‘.o’ file is an object file.

   On all known systems, if there is a program named ‘ranlib’, then it
must be used to "bless" the created library before linking against it,
with the ‘ranlib libNAME.a’ command.  Some systems, like Irix, use the
‘ar ts’ command, instead.


File: libtool.info,  Node: Cross compiling,  Next: File name conversion,  Prev: Archivers,  Up: Platform quirks

15.3.5 Cross compiling
----------------------

Most build systems support the ability to compile libraries and
applications on one platform for use on a different platform, provided a
compiler capable of generating the appropriate output is available.  In
such cross compiling scenarios, the platform where the libraries or
applications are compiled is called the “build platform”, while the
platform where the libraries or applications are intended to be used or
executed is called the “host platform”.  *note The GNU Build System:
(automake)GNU Build System, of which libtool is a part, supports cross
compiling via arguments passed to the configure script: ‘--build=...’
and ‘--host=...’.  However, when the build platform and host platform
are very different, libtool is required to make certain accommodations
to support these scenarios.

   In most cases, because the build platform and host platform differ,
the cross-compiled libraries and executables can't be executed or tested
on the build platform where they were compiled.  The testsuites of most
build systems will often skip any tests that involve executing such
foreign executables when cross-compiling.  However, if the build
platform and host platform are sufficiently similar, it is often
possible to run cross-compiled applications.  Libtool's own testsuite
often attempts to execute cross-compiled tests, but will mark any
failures as _skipped_ since the failure might simply be due to the
differences between the two platforms.

   In addition to cases where the host platform and build platform are
extremely similar (e.g.  ‘i586-pc-linux-gnu’ and ‘i686-pc-linux-gnu’),
there is another case where cross-compiled host applications may be
executed on the build platform.  This is possible when the build
platform supports an emulation or API-enhanced environment for the host
platform.  One example of this situation would be if the build platform
were MinGW, and the host platform were Cygwin (or vice versa).  Both of
these platforms can actually operate within a single Windows instance,
so Cygwin applications can be launched from a MinGW context, and vice
versa--provided certain care is taken.  Another example would be if the
build platform were GNU/Linux on an x86 32bit processor, and the host
platform were MinGW. In this situation, the Wine
(https://www.winehq.org/) environment can be used to launch Windows
applications from the GNU/Linux operating system; again, provided
certain care is taken.

   One particular issue occurs when a Windows platform such as MinGW,
Cygwin, or MSYS is the host or build platform, while the other platform
is a Unix-style system.  In these cases, there are often conflicts
between the format of the file names and paths expected within host
platform libraries and executables, and those employed on the build
platform.

   This situation is best described using a concrete example: suppose
the build platform is GNU/Linux with canonical triplet
‘i686-pc-linux-gnu’.  Suppose further that the host platform is MinGW
with canonical triplet ‘i586-pc-mingw32’.  On the GNU/Linux platform
there is a cross compiler following the usual naming conventions of such
compilers, where the compiler name is prefixed by the host canonical
triplet (or suitable alias).  (For more information concerning canonical
triplets and platform aliases, see *note Specifying Target Triplets:
(autoconf)Specifying Target Triplets. and *note Canonicalizing:
(autoconf)Canonicalizing.) In this case, the C compiler is named
‘i586-pc-mingw32-gcc’.

   As described in *note Wrapper executables::, for the MinGW host
platform libtool uses a wrapper executable to set various environment
variables before launching the actual program executable.  Like the
program executable, the wrapper executable is cross-compiled for the
host platform (that is, for MinGW). As described above, ordinarily a
host platform executable cannot be executed on the build platform, but
in this case the Wine environment could be used to launch the MinGW
application from GNU/Linux.  However, the wrapper executable, as a host
platform (MinGW) application, must set the ‘PATH’ variable so that the
true application's dependent libraries can be located--but the contents
of the ‘PATH’ variable must be structured for MinGW. Libtool must use
the Wine file name mapping facilities to determine the correct value so
that the wrapper executable can set the ‘PATH’ variable to point to the
correct location.

   For example, suppose we are compiling an application in ‘/var/tmp’ on
GNU/Linux, using separate source code and build directories:

     /var/tmp/foo-1.2.3/app/              (application source code)
     /var/tmp/foo-1.2.3/lib/              (library source code)
     /var/tmp/BUILD/app/                  (application build objects here)
     /var/tmp/BUILD/lib/                  (library build objects here)

   Since the library will be built in ‘/var/tmp/BUILD/lib’, the wrapper
executable (which will be in ‘/var/tmp/BUILD/app’) must add that
directory to ‘PATH’ (actually, it must add the directory named OBJDIR
under ‘/var/tmp/BUILD/lib’, but we'll ignore that detail for now).
However, Windows does not have a concept of Unix-style file or directory
names such as ‘/var/tmp/BUILD/lib’.  Therefore, Wine provides a mapping
from Windows file names such as ‘C:\Program Files’ to specific
Unix-style file names.  Wine also provides a utility that can be used to
map Unix-style file names to Windows file names.

   In this case, the wrapper executable should actually add the value

     Z:\var\tmp\BUILD\lib

to the ‘PATH’.  libtool contains support for path conversions of this
type, for a certain limited set of build and host platform combinations.
In this case, libtool will invoke Wine's ‘winepath’ utility to ensure
that the correct ‘PATH’ value is used.  *Note File name conversion::.


File: libtool.info,  Node: File name conversion,  Next: Windows DLLs,  Prev: Cross compiling,  Up: Platform quirks

15.3.6 File name conversion
---------------------------

In certain situations, libtool must convert file names and paths between
formats appropriate to different platforms.  Usually this occurs when
cross-compiling, and affects only the ability to launch host platform
executables on the build platform using an emulation or API-enhancement
environment such as Wine.  Failure to convert paths (*note File Name
Conversion Failure::) will cause a warning to be issued, but rarely
causes the build to fail--and should have no effect on the compiled
products, once installed properly on the host platform.  For more
information, *note Cross compiling::.

   However, file name conversion may also occur in another scenario:
when using a Unix emulation system on Windows (such as Cygwin or MSYS),
combined with a native Windows compiler such as MinGW or MSVC. Only a
limited set of such scenarios are currently supported; in other cases
file name conversion is skipped.  The lack of file name conversion
usually means that uninstalled executables can't be launched, but only
rarely causes the build to fail (*note File Name Conversion Failure::).

   libtool supports file name conversion in the following scenarios:

build platform     host platform      Notes
---------------------------------------------------------------------------
MinGW (MSYS)       MinGW (Windows)    *note Native MinGW File Name Conversion::
                                      
Cygwin             MinGW (Windows)    *note Cygwin/Windows File Name Conversion::
                                      
Unix + Wine        MinGW (Windows)    Requires Wine.
                                      *Note Unix/Windows File Name Conversion::.
                                      
MinGW (MSYS)       Cygwin             Requires ‘LT_CYGPATH’.
                                      *Note LT_CYGPATH::.  Provided for
                                      testing purposes only.
                                      
Unix + Wine        Cygwin             Requires both Wine and
                                      ‘LT_CYGPATH’, but does not yet
                                      work with Cygwin 1.7.7 and
                                      Wine-1.2.
                                      *Note Unix/Windows File Name Conversion::,
                                      and *note LT_CYGPATH::.

* Menu:

* File Name Conversion Failure::  What happens when file name conversion fails
* Native MinGW File Name Conversion::  MSYS file name conversion idiosyncrasies
* Cygwin/Windows File Name Conversion::  Using ‘cygpath’ to convert Cygwin file names
* Unix/Windows File Name Conversion::  Using Wine to convert Unix paths
* LT_CYGPATH::                  Invoking ‘cygpath’ from other environments
* Cygwin to MinGW Cross::       Other notes concerning MinGW cross


File: libtool.info,  Node: File Name Conversion Failure,  Next: Native MinGW File Name Conversion,  Up: File name conversion

15.3.6.1 File Name Conversion Failure
.....................................

In most cases, file name conversion is not needed or attempted.
However, when libtool detects that a specific combination of build and
host platform does require file name conversion, it is possible that the
conversion may fail.  In these cases, you may see a warning such as the
following:

     Could not determine the host file name corresponding to
       `... a file name ...'
     Continuing, but uninstalled executables may not work.

or

     Could not determine the host path corresponding to
       `... a path ...'
     Continuing, but uninstalled executables may not work.

This should not cause the build to fail.  At worst, it means that the
wrapper executable will specify file names or paths appropriate for the
build platform.  Since those are not appropriate for the host platform,
the uninstalled executables would not operate correctly, even when the
wrapper executable is launched via the appropriate emulation or
API-enhancement (e.g.  Wine).  Simply install the executables on the
host platform, and execute them there.


File: libtool.info,  Node: Native MinGW File Name Conversion,  Next: Cygwin/Windows File Name Conversion,  Prev: File Name Conversion Failure,  Up: File name conversion

15.3.6.2 Native MinGW File Name Conversion
..........................................

MSYS is a Unix emulation environment for Windows, and is specifically
designed such that in normal usage it _pretends_ to be MinGW or native
Windows, but understands Unix-style file names and paths, and supports
standard Unix tools and shells.  Thus, "native" MinGW builds are
actually an odd sort of cross-compile, from an MSYS Unix emulation
environment "pretending" to be MinGW, to actual native Windows.

   When an MSYS shell launches a native Windows executable (as opposed
to other _MSYS_ executables), it uses a system of heuristics to detect
any command-line arguments that contain file names or paths.  It
automatically converts these file names from the MSYS (Unix-like)
format, to the corresponding Windows file name, before launching the
executable.  However, this auto-conversion facility is only available
when using the MSYS runtime library.  The wrapper executable itself is a
MinGW application (that is, it does not use the MSYS runtime library).
The wrapper executable must set ‘PATH’ to, and call ‘_spawnv’ with,
values that have already been converted from MSYS format to Windows.
Thus, when libtool writes the source code for the wrapper executable, it
must manually convert MSYS paths to Windows format, so that the Windows
values can be hard-coded into the wrapper executable.


File: libtool.info,  Node: Cygwin/Windows File Name Conversion,  Next: Unix/Windows File Name Conversion,  Prev: Native MinGW File Name Conversion,  Up: File name conversion

15.3.6.3 Cygwin/Windows File Name Conversion
............................................

Cygwin provides a Unix emulation environment for Windows.  As part of
that emulation, it provides a file system mapping that presents the
Windows file system in a Unix-compatible manner.  Cygwin also provides a
utility ‘cygpath’ that can be used to convert file names and paths
between the two representations.  In a correctly configured Cygwin
installation, ‘cygpath’ is always present, and is in the ‘PATH’.

   Libtool uses ‘cygpath’ to convert from Cygwin (Unix-style) file names
and paths to Windows format when the build platform is Cygwin and the
host platform is MinGW.

   When the host platform is Cygwin, but the build platform is MSYS or
some Unix system, libtool also uses ‘cygpath’ to convert from Windows to
Cygwin format (after first converting from the build platform format to
Windows format; *Note Native MinGW File Name Conversion::, and *note
Unix/Windows File Name Conversion::.)  Because the build platform is not
Cygwin, ‘cygpath’ is not (and should not be) in the ‘PATH’.  Therefore,
in this configuration the environment variable ‘LT_CYGPATH’ is required.
*Note LT_CYGPATH::.


File: libtool.info,  Node: Unix/Windows File Name Conversion,  Next: LT_CYGPATH,  Prev: Cygwin/Windows File Name Conversion,  Up: File name conversion

15.3.6.4 Unix/Windows File Name Conversion
..........................................

Wine (https://www.winehq.org/) provides an interpretation environment
for some Unix platforms where Windows applications can be executed.  It
provides a mapping between the Unix file system and a virtual Windows
file system used by the Windows programs.  For the file name conversion
to work, Wine must be installed and properly configured on the build
platform, and the ‘winepath’ application must be in the build platform's
‘PATH’.  In addition, on 32bit GNU/Linux it is usually helpful if the
binfmt extension is enabled.


File: libtool.info,  Node: LT_CYGPATH,  Next: Cygwin to MinGW Cross,  Prev: Unix/Windows File Name Conversion,  Up: File name conversion

15.3.6.5 LT_CYGPATH
...................

For some cross-compile configurations (where the host platform is
Cygwin), the ‘cygpath’ program is used to convert file names from the
build platform notation to the Cygwin form (technically, this conversion
is from Windows notation to Cygwin notation; the conversion from the
build platform format to Windows notation is performed via other means).
However, because the ‘cygpath’ program is not (and should not be) in the
‘PATH’ on the build platform, ‘LT_CYGPATH’ must specify the full build
platform file name (that is, the full Unix or MSYS file name) of the
‘cygpath’ program.

   The reason ‘cygpath’ should not be in the build platform ‘PATH’ is
twofold: first, ‘cygpath’ is usually installed in the same directory as
many other Cygwin executables, such as ‘sed’, ‘cp’, etc.  If the build
platform environment had this directory in its ‘PATH’, then these Cygwin
versions of common Unix utilities might be used in preference to the
ones provided by the build platform itself, with deleterious effects.
Second, especially when Cygwin-1.7 or later is used, multiple Cygwin
installations can coexist within the same Windows instance.  Each
installation will have separate "mount tables" specified in
‘CYGROOT-N/etc/fstab’.  These “mount tables” control how that instance
of Cygwin will map Windows file names and paths to Cygwin form.  Each
installation's ‘cygpath’ utility automatically deduces the appropriate
‘/etc/fstab’ file.  Since each ‘CYGROOT-N/etc/fstab’ mount table may
specify different mappings, it matters what ‘cygpath’ is used.

   Note that ‘cygpath’ is a Cygwin application; to execute this tool
from Unix requires a working and properly configured Wine installation,
as well as enabling the GNU/Linux ‘binfmt’ extension.  Furthermore, the
Cygwin ‘setup.exe’ tool should have been used, via Wine, to properly
install Cygwin into the Wine file system (and registry).

   Unfortunately, Wine support for Cygwin is intermittent.  Recent
releases of Cygwin (1.7 and above) appear to require more Windows API
support than Wine provides (as of Wine version 1.2); most Cygwin
applications fail to execute.  This includes ‘cygpath’ itself.  Hence,
it is best _not_ to use the LT_CYGPATH machinery in libtool when
performing Unix to Cygwin cross-compiles.  Similarly, it is best _not_
to enable the GNU/Linux binfmt support in this configuration, because
while Wine will fail to execute the compiled Cygwin applications, it
will still exit with status zero.  This tends to confuse build systems
and test suites (including libtool's own testsuite, resulting in
spurious reported failures).  Wine support for the older Cygwin-1.5
series appears satisfactory, but the Cygwin team no longer supports
Cygwin-1.5.  It is hoped that Wine will eventually be improved such that
Cygwin-1.7 will again operate correctly under Wine.  Until then, libtool
will report warnings as described in *note File Name Conversion
Failure:: in these scenarios.

   However, ‘LT_CYGPATH’ is also used for the MSYS to Cygwin cross
compile scenario, and operates as expected.


File: libtool.info,  Node: Cygwin to MinGW Cross,  Prev: LT_CYGPATH,  Up: File name conversion

15.3.6.6 Cygwin to MinGW Cross
..............................

There are actually three different scenarios that could all legitimately
be called a "Cygwin to MinGW" cross compile.  The current (and standard)
definition is when there is a compiler that produces native Windows
libraries and applications, but which itself is a Cygwin application,
just as would be expected in any other cross compile setup.

   However, historically there were two other definitions, which we will
refer to as the _fake_ one, and the _lying_ one.

   In the _fake_ Cygwin to MinGW cross compile case, you actually use a
native MinGW compiler, but you do so from within a Cygwin environment:

     export PATH="/c/MinGW/bin:${PATH}"
     configure --build=i686-pc-cygwin \
     	--host=mingw32 \
     	NM=/c/MinGW/bin/nm.exe

   In this way, the build system "knows" that you are cross compiling,
and the file name conversion logic will be used.  However, because the
tools (‘mingw32-gcc’, ‘nm’, ‘ar’) used are actually native Windows
applications, they will not understand any Cygwin (that is, Unix-like)
absolute file names passed as command line arguments (and, unlike MSYS,
Cygwin does not automatically convert such arguments).  However, so long
as only relative file names are used in the build system, and
non-Windows-supported Unix idioms such as symlinks and mount points are
avoided, this scenario should work.

   If you must use absolute file names, you will have to force Libtool
to convert file names for the toolchain in this case, by doing the
following before you run configure:

     export lt_cv_to_tool_file_cmd=func_convert_file_cygwin_to_w32

   In the _lying_ Cygwin to MinGW cross compile case, you lie to the
build system:

     export PATH="/c/MinGW/bin:${PATH}"
     configure --build=i686-pc-mingw32 \
     	--host=i686-pc-mingw32 \
     	--disable-dependency-tracking

and claim that the build platform is MinGW, even though you are actually
running under _Cygwin_ and not MinGW. In this case, libtool does _not_
know that you are performing a cross compile, and thinks instead that
you are performing a native MinGW build.  However, as described in
(*note Native MinGW File Name Conversion::), that scenario triggers an
"MSYS to Windows" file name conversion.  This, of course, is the wrong
conversion since we are actually running under Cygwin.  Also, the
toolchain is expecting Windows file names (not Cygwin) but unless told
so Libtool will feed Cygwin file names to the toolchain in this case.
To force the correct file name conversions in this situation, you should
do the following _before_ running configure:

     export lt_cv_to_host_file_cmd=func_convert_file_cygwin_to_w32
     export lt_cv_to_tool_file_cmd=func_convert_file_cygwin_to_w32

   Note that this relies on internal implementation details of libtool,
and is subject to change.  Also, ‘--disable-dependency-tracking’ is
required, because otherwise the MinGW GCC will generate dependency files
that contain Windows file names.  This, in turn, will confuse the Cygwin
‘make’ program, which does not accept Windows file names:

     Makefile:1: *** target pattern contains no `%'.  Stop.

   There have also always been a number of other details required for
the _lying_ case to operate correctly, such as the use of so-called
“identity mounts”:

     # CYGWIN-ROOT/etc/fstab
     D:/foo    /foo     some_fs binary 0 0
     D:/bar    /bar     some_fs binary 0 0
     E:/grill  /grill   some_fs binary 0 0

   In this way, top-level directories of each drive are available using
identical names within Cygwin.

   Note that you also need to ensure that the standard Unix directories
(like ‘/bin’, ‘/lib’, ‘/usr’, ‘/etc’) appear in the root of a drive.
This means that you must install Cygwin itself into the ‘C:/’ root
directory (or ‘D:/’, or ‘E:/’, etc)--instead of the recommended
installation into ‘C:/cygwin/’.  In addition, all file names used in the
build system must be relative, symlinks should not be used within the
source or build directory trees, and all ‘-M*’ options to ‘gcc’ except
‘-MMD’ must be avoided.

   This is quite a fragile setup, but it has been in historical use, and
so is documented here.


File: libtool.info,  Node: Windows DLLs,  Prev: File name conversion,  Up: Platform quirks

15.3.7 Windows DLLs
-------------------

This topic describes a couple of ways to portably create Windows Dynamic
Link Libraries (DLLs).  Libtool knows how to create DLLs using GNU tools
and using Microsoft tools.

   A typical library has a "hidden" implementation with an interface
described in a header file.  On just about every system, the interface
could be something like this:

   Example ‘foo.h’:

     #ifndef FOO_H
     #define FOO_H

     int one (void);
     int two (void);
     extern int three;

     #endif /* FOO_H */

And the implementation could be something like this:

   Example ‘foo.c’:

     #include "foo.h"

     int one (void)
     {
       return 1;
     }

     int two (void)
     {
       return three - one ();
     }

     int three = 3;

   When using contemporary GNU tools to create the Windows DLL, the
above code will work there too, thanks to its auto-import/auto-export
features.  But that is not the case when using older GNU tools or
perhaps more interestingly when using proprietary tools.  In those cases
the code will need additional decorations on the interface symbols with
‘__declspec(dllimport)’ and ‘__declspec(dllexport)’ depending on whether
the library is built or it's consumed and how it's built and consumed.
However, it should be noted that it would have worked also with
Microsoft tools, if only the variable ‘three’ hadn't been there, due to
the fact the Microsoft tools will automatically import functions (but
sadly not variables) and Libtool will automatically export non-static
symbols as described next.

   With Microsoft tools, Libtool digs through the object files that make
up the library, looking for non-static symbols to automatically export.
I.e., Libtool with Microsoft tools tries to mimic the auto-export
feature of contemporary GNU tools.  It should be noted that the GNU
auto-export feature is turned off when an explicit
‘__declspec(dllexport)’ is seen.  The GNU tools do this to not make more
symbols visible for projects that have already taken the trouble to
decorate symbols.  There is no similar way to limit what symbols are
visible in the code when Libtool is using Microsoft tools.  In order to
limit symbol visibility in that case you need to use one of the options
‘-export-symbols’ or ‘-export-symbols-regex’.

   No matching help with auto-import is provided by Libtool, which is
why variables must be decorated to import them from a DLL for everything
but contemporary GNU tools.  As stated above, functions are
automatically imported by both contemporary GNU tools and Microsoft
tools, but for other proprietary tools the auto-import status of
functions is unknown.

   When the objects that form the library are built, there are generally
two copies built for each object.  One copy is used when linking the DLL
and one copy is used for the static library.  On Windows systems, a pair
of defines are commonly used to discriminate how the interface symbols
should be decorated.  The first define is ‘-DDLL_EXPORT’, which is
automatically provided by Libtool when ‘libtool’ builds the copy of the
object that is destined for the DLL. The second define is
‘-DLIBFOO_BUILD’ (or similar), which is often added by the package
providing the library and is used when building the library, but not
when consuming the library.

   However, the matching double compile is not performed when consuming
libraries.  It is therefore not possible to reliably distinguish if the
consumer is importing from a DLL or if it is going to use a static
library.

   With contemporary GNU tools, auto-import often saves the day, but see
the GNU ld documentation and its ‘--enable-auto-import’ option for some
corner cases when it does not (*note ‘--enable-auto-import’:
(ld)Options.).

   With Microsoft tools you typically get away with always compiling the
code such that variables are expected to be imported from a DLL and
functions are expected to be found in a static library.  The tools will
then automatically import the function from a DLL if that is where they
are found.  If the variables are not imported from a DLL as expected,
but are found in a static library that is otherwise pulled in by some
function, the linker will issue a warning (LNK4217) that a locally
defined symbol is imported, but it still works.  In other words, this
scheme will not work to only consume variables from a library.  There is
also a price connected to this liberal use of imports in that an extra
indirection is introduced when you are consuming the static version of
the library.  That extra indirection is unavoidable when the DLL is
consumed, but it is not needed when consuming the static library.

   For older GNU tools and other proprietary tools there is no generic
way to make it possible to consume either of the DLL or the static
library without user intervention, the tools need to be told what is
intended.  One common assumption is that if a DLL is being built
(‘DLL_EXPORT’ is defined) then that DLL is going to consume any
dependent libraries as DLLs.  If that assumption is made everywhere, it
is possible to select how an end-user application is consuming libraries
by adding a single flag ‘-DDLL_EXPORT’ when a DLL build is required.
This is of course an all or nothing deal, either everything as DLLs or
everything as static libraries.

   To sum up the above, the header file of the foo library needs to be
changed into something like this:

   Modified ‘foo.h’:

     #ifndef FOO_H
     #define FOO_H

     #if defined _WIN32 && !defined __GNUC__
     # ifdef LIBFOO_BUILD
     #  ifdef DLL_EXPORT
     #   define LIBFOO_SCOPE            __declspec (dllexport)
     #   define LIBFOO_SCOPE_VAR extern __declspec (dllexport)
     #  endif
     # elif defined _MSC_VER
     #  define LIBFOO_SCOPE
     #  define LIBFOO_SCOPE_VAR  extern __declspec (dllimport)
     # elif defined DLL_EXPORT
     #  define LIBFOO_SCOPE             __declspec (dllimport)
     #  define LIBFOO_SCOPE_VAR  extern __declspec (dllimport)
     # endif
     #endif
     #ifndef LIBFOO_SCOPE
     # define LIBFOO_SCOPE
     # define LIBFOO_SCOPE_VAR extern
     #endif

     LIBFOO_SCOPE     int one (void);
     LIBFOO_SCOPE     int two (void);
     LIBFOO_SCOPE_VAR int three;

     #endif /* FOO_H */

   When the targets are limited to contemporary GNU tools and Microsoft
tools, the above can be simplified to the following:

   Simplified ‘foo.h’:

     #ifndef FOO_H
     #define FOO_H

     #if defined _WIN32 && !defined __GNUC__ && !defined LIBFOO_BUILD
     # define LIBFOO_SCOPE_VAR extern __declspec (dllimport)
     #else
     # define LIBFOO_SCOPE_VAR extern
     #endif

     int one (void);
     int two (void);
     LIBFOO_SCOPE_VAR int three;

     #endif /* FOO_H */

   This last simplified version can of course only work when Libtool is
used to build the DLL, as no symbols would be exported otherwise (i.e.,
when using Microsoft tools).

   It should be noted that there are various projects that attempt to
relax these requirements by various low level tricks, but they are not
discussed here.  Examples are FlexDLL (https://github.com/ocaml/flexdll)
and edll (https://edll.sourceforge.net/).


File: libtool.info,  Node: libtool script contents,  Next: Cheap tricks,  Prev: Platform quirks,  Up: Maintaining

15.4 ‘libtool’ script contents
==============================

Since version 1.4, the ‘libtool’ script is generated by ‘configure’
(*note Configuring::).  In earlier versions, ‘configure’ achieved this
by calling a helper script called ‘ltconfig’.  From libtool version 0.7
to 1.0, this script simply set shell variables, then sourced the libtool
backend, ‘ltmain.sh’.  ‘ltconfig’ from libtool version 1.1 through 1.3
inlined the contents of ‘ltmain.sh’ into the generated ‘libtool’, which
improved performance on many systems.  The tests that ‘ltconfig’ used to
perform are now kept in ‘libtool.m4’ where they can be written using
Autoconf.  This has the runtime performance benefits of inlined
‘ltmain.sh’, _and_ improves the build time a little while considerably
easing the amount of raw shell code that used to need maintaining.

   The convention used for naming variables that hold shell commands for
delayed evaluation, is to use the suffix ‘_cmd’ where a single line of
valid shell script is needed, and the suffix ‘_cmds’ where multiple
lines of shell script *may* be delayed for later evaluation.  By
convention, ‘_cmds’ variables delimit the evaluation units with the ‘~’
character where necessary.

   Here is a listing of each of the configuration variables, and how
they are used within ‘ltmain.sh’ (*note Configuring::):

 -- Variable: AR
     The name of the system library archiver.

 -- Variable: CC
     The name of the compiler used to configure libtool.  This will
     always contain the compiler for the current language (*note
     Tags::).

 -- Variable: ECHO
     An ‘echo’ program that does not interpret backslashes as an escape
     character.  It may be given only one argument, so due quoting is
     necessary.

 -- Variable: LD
     The name of the linker that libtool should use internally for
     reloadable linking and possibly shared libraries.

 -- Variable: LTCC
 -- Variable: LTCFLAGS
     The name of the C compiler and C compiler flags used to configure
     libtool.

 -- Variable: NM
     The name of a BSD- or MS-compatible program that produces listings
     of global symbols.  For BSD ‘nm’, the symbols should be in one the
     following formats:

          ADDRESS C GLOBAL-VARIABLE-NAME
          ADDRESS D GLOBAL-VARIABLE-NAME
          ADDRESS T GLOBAL-FUNCTION-NAME

     For MS ‘dumpbin’, the symbols should be in one of the following
     formats:

          COUNTER SIZE    UNDEF    notype       External     | GLOBAL-VAR
          COUNTER ADDRESS SECTION  notype       External     | GLOBAL-VAR
          COUNTER ADDRESS SECTION  notype ()    External     | GLOBAL-FUNC

     The SIZE of the global variables are not zero and the SECTION of
     the global functions are not "UNDEF". Symbols in "pick any"
     sections ("pick any" appears in the section header) are not global
     either.

 -- Variable: RANLIB
     Set to the name of the ‘ranlib’ program, if any.

 -- Variable: allow_undefined_flag
     The flag that is used by ‘archive_cmds’ to declare that there will
     be unresolved symbols in the resulting shared library.  Empty, if
     no such flag is required.  Set to ‘unsupported’ if there is no way
     to generate a shared library with references to symbols that aren't
     defined in that library.

 -- Variable: always_export_symbols
     Whether libtool should automatically generate a list of exported
     symbols using ‘export_symbols_cmds’ before linking an archive.  Set
     to ‘yes’ or ‘no’.  Default is ‘no’.

 -- Variable: archive_cmds
 -- Variable: archive_expsym_cmds
 -- Variable: old_archive_cmds
     Commands used to create shared libraries, shared libraries with
     ‘-export-symbols’ and static libraries, respectively.

 -- Variable: archiver_list_spec
     Specify filename containing input files for ‘AR’.

 -- Variable: old_archive_from_new_cmds
     If the shared library depends on a static library,
     ‘old_archive_from_new_cmds’ contains the commands used to create
     that static library.  If this variable is not empty,
     ‘old_archive_cmds’ is not used.

 -- Variable: old_archive_from_expsyms_cmds
     If a static library must be created from the export symbol list to
     correctly link with a shared library,
     ‘old_archive_from_expsyms_cmds’ contains the commands needed to
     create that static library.  When these commands are executed, the
     variable ‘soname’ contains the name of the shared library in
     question, and the ‘$objdir/$newlib’ contains the path of the static
     library these commands should build.  After executing these
     commands, libtool will proceed to link against ‘$objdir/$newlib’
     instead of ‘soname’.

 -- Variable: lock_old_archive_extraction
     Set to ‘yes’ if the extraction of a static library requires locking
     the library file.  This is required on Darwin.

 -- Variable: build
 -- Variable: build_alias
 -- Variable: build_os
     Set to the specified and canonical names of the system that libtool
     was built on.

 -- Variable: build_libtool_libs
     Whether libtool should build shared libraries on this system.  Set
     to ‘yes’ or ‘no’.

 -- Variable: build_old_libs
     Whether libtool should build static libraries on this system.  Set
     to ‘yes’ or ‘no’.

 -- Variable: compiler_c_o
     Whether the compiler supports the ‘-c’ and ‘-o’ options
     simultaneously.  Set to ‘yes’ or ‘no’.

 -- Variable: compiler_needs_object
     Whether the compiler has to see an object listed on the command
     line in order to successfully invoke the linker.  If ‘no’, then a
     set of convenience archives or a set of object file names can be
     passed via linker-specific options or linker scripts.

 -- Variable: dlopen_support
     Whether ‘dlopen’ is supported on the platform.  Set to ‘yes’ or
     ‘no’.

 -- Variable: dlopen_self
     Whether it is possible to ‘dlopen’ the executable itself.  Set to
     ‘yes’ or ‘no’.

 -- Variable: dlopen_self_static
     Whether it is possible to ‘dlopen’ the executable itself, when it
     is linked statically (‘-all-static’).  Set to ‘yes’ or ‘no’.

 -- Variable: exclude_expsyms
     List of symbols that should not be listed in the preloaded symbols.

 -- Variable: export_dynamic_flag_spec
     Compiler link flag that allows a dlopened shared library to
     reference symbols that are defined in the program.

 -- Variable: export_symbols_cmds
     Commands to extract exported symbols from ‘libobjs’ to the file
     ‘export_symbols’.

 -- Variable: extract_expsyms_cmds
     Commands to extract the exported symbols list from a shared
     library.  These commands are executed if there is no file
     ‘$objdir/$soname-def’, and should write the names of the exported
     symbols to that file, for the use of
     ‘old_archive_from_expsyms_cmds’.

 -- Variable: fast_install
     Determines whether libtool will privilege the installer or the
     developer.  The assumption is that installers will seldom run
     programs in the build tree, and the developer will seldom install.
     This is only meaningful on platforms where
     ‘shlibpath_overrides_runpath’ is not ‘yes’, so ‘fast_install’ will
     be set to ‘needless’ in this case.  If ‘fast_install’ set to ‘yes’,
     libtool will create programs that search for installed libraries,
     and, if a program is run in the build tree, a new copy will be
     linked on-demand to use the yet-to-be-installed libraries.  If set
     to ‘no’, libtool will create programs that use the
     yet-to-be-installed libraries, and will link a new copy of the
     program at install time.  The default value is ‘yes’ or ‘needless’,
     depending on platform and configuration flags, and it can be turned
     from ‘yes’ to ‘no’ with the configure flag
     ‘--disable-fast-install’.

     On some systems, the linker always hardcodes paths to dependent
     libraries into the output.  In this case, ‘fast_install’ is never
     set to ‘yes’, and relinking at install time is triggered.  This
     also means that ‘DESTDIR’ installation does not work as expected.

 -- Variable: file_magic_glob
     How to find potential files when ‘deplibs_check_method’ is
     ‘file_magic’.  ‘file_magic_glob’ is a ‘sed’ expression, and the
     ‘sed’ instance is fed potential file names that are transformed by
     the ‘file_magic_glob’ expression.  Useful when the shell does not
     support the shell option ‘nocaseglob’, making ‘want_nocaseglob’
     inappropriate.  Normally disabled (i.e.  ‘file_magic_glob’ is
     empty).

 -- Variable: finish_cmds
     Commands to tell the dynamic linker how to find shared libraries in
     a specific directory.  These commands can be disabled during
     testing local changes to shared libraries with ‘--no-finish’.

 -- Variable: finish_eval
     Same as ‘finish_cmds’, except the commands are not displayed.

 -- Variable: global_symbol_pipe
     A pipeline that takes the output of ‘NM’, and produces a listing of
     raw symbols followed by their C names.  For example:

          $ eval "$NM progname | $global_symbol_pipe"
          D SYMBOL1 C-SYMBOL1
          T SYMBOL2 C-SYMBOL2
          C SYMBOL3 C-SYMBOL3
          ...
          $

     The first column contains the symbol type (used to tell data from
     code) but its meaning is system dependent.

 -- Variable: global_symbol_to_cdecl
     A pipeline that translates the output of ‘global_symbol_pipe’ into
     proper C declarations.  Since some platforms, such as HP/UX, have
     linkers that differentiate code from data, data symbols are
     declared as data, and code symbols are declared as functions.

 -- Variable: hardcode_action
     Either ‘immediate’ or ‘relink’, depending on whether shared library
     paths can be hardcoded into executables before they are installed,
     or if they need to be relinked.

 -- Variable: hardcode_direct
     Set to ‘yes’ or ‘no’, depending on whether the linker hardcodes
     directories if a library is directly specified on the command line
     (such as ‘DIR/libNAME.a’) when ‘hardcode_libdir_flag_spec’ is
     specified.

 -- Variable: hardcode_direct_absolute
     Some architectures hardcode "absolute" library directories that
     cannot be overridden by ‘shlibpath_var’ when ‘hardcode_direct’ is
     ‘yes’.  In that case set ‘hardcode_direct_absolute’ to ‘yes’, or
     otherwise ‘no’.

 -- Variable: hardcode_into_libs
     Whether the platform supports hardcoding of run-paths into
     libraries.  If enabled, linking of programs will be much simpler
     but libraries will need to be relinked during installation.  Set to
     ‘yes’ or ‘no’.

 -- Variable: hardcode_libdir_flag_spec
     Flag to hardcode a ‘libdir’ variable into a binary, so that the
     dynamic linker searches ‘libdir’ for shared libraries at runtime.
     If it is empty, libtool will try to use some other hardcoding
     mechanism.

 -- Variable: hardcode_libdir_separator
     If the compiler only accepts a single ‘hardcode_libdir_flag’, then
     this variable contains the string that should separate multiple
     arguments to that flag.

 -- Variable: hardcode_minus_L
     Set to ‘yes’ or ‘no’, depending on whether the linker hardcodes
     directories specified by ‘-L’ flags into the resulting executable
     when ‘hardcode_libdir_flag_spec’ is specified.

 -- Variable: hardcode_shlibpath_var
     Set to ‘yes’ or ‘no’, depending on whether the linker hardcodes
     directories by writing the contents of ‘$shlibpath_var’ into the
     resulting executable when ‘hardcode_libdir_flag_spec’ is specified.
     Set to ‘unsupported’ if directories specified by ‘$shlibpath_var’
     are searched at run time, but not at link time.

 -- Variable: host
 -- Variable: host_alias
 -- Variable: host_os
     Set to the specified and canonical names of the system that libtool
     was configured for.

 -- Variable: include_expsyms
     List of symbols that must always be exported when using
     ‘export_symbols’.

 -- Variable: inherit_rpath
     Whether the linker adds runtime paths of dependency libraries to
     the runtime path list, requiring libtool to relink the output when
     installing.  Set to ‘yes’ or ‘no’.  Default is ‘no’.

 -- Variable: install_override_mode
     Permission mode override for installation of shared libraries.  If
     the runtime linker fails to load libraries with wrong permissions,
     then it may fail to execute programs that are needed during
     installation, because these need the library that has just been
     installed.  In this case, it is necessary to pass the mode to
     ‘install’ with ‘-m INSTALL_OVERRIDE_MODE’.

 -- Variable: libext
     The standard old archive suffix (normally ‘a’).

 -- Variable: libname_spec
     The format of a library name prefix.  On all Unix systems, static
     libraries are called ‘libNAME.a’, but on some systems (such as OS/2
     or MS-DOS), the library is just called ‘NAME.a’.

 -- Variable: library_names_spec
     A list of shared library names.  The first is the name of the file,
     the rest are symbolic links to the file.  The name in the list is
     the file name that the linker finds when given ‘-lNAME’.

 -- Variable: link_all_deplibs
     Whether libtool must link a program against all its dependency
     libraries.  Set to ‘yes’ or ‘no’.  Default is ‘unknown’, which is a
     synonym for ‘yes’.

 -- Variable: link_static_flag
     Linker flag (passed through the C compiler) used to prevent dynamic
     linking.

 -- Variable: macro_version
 -- Variable: macro_revision
     The release and revision from which the libtool.m4 macros were
     taken.  This is used to ensure that macros and ‘ltmain.sh’
     correspond to the same Libtool version.

 -- Variable: max_cmd_len
     The approximate longest command line that can be passed to ‘$SHELL’
     without being truncated, as computed by ‘LT_CMD_MAX_LEN’.

 -- Variable: need_lib_prefix
     Whether we can ‘dlopen’ modules without a ‘lib’ prefix.  Set to
     ‘yes’ or ‘no’.  By default, it is ‘unknown’, which means the same
     as ‘yes’, but documents that we are not really sure about it.  ‘no’
     means that it is possible to ‘dlopen’ a module without the ‘lib’
     prefix.

 -- Variable: need_version
     Whether versioning is required for libraries, i.e. whether the
     dynamic linker requires a version suffix for all libraries.  Set to
     ‘yes’ or ‘no’.  By default, it is ‘unknown’, which means the same
     as ‘yes’, but documents that we are not really sure about it.

 -- Variable: need_locks
     Whether files must be locked to prevent conflicts when compiling
     simultaneously.  Set to ‘yes’ or ‘no’.

 -- Variable: nm_file_list_spec
     Specify filename containing input files for ‘NM’.

 -- Variable: no_builtin_flag
     Compiler flag to disable builtin functions that conflict with
     declaring external global symbols as ‘char’.

 -- Variable: no_undefined_flag
     The flag that is used by ‘archive_cmds’ to declare that there will
     be no unresolved symbols in the resulting shared library.  Empty,
     if no such flag is required.

 -- Variable: objdir
     The name of the directory that contains temporary libtool files.

 -- Variable: objext
     The standard object file suffix (normally ‘o’).

 -- Variable: pic_flag
     Any additional compiler flags for building library object files.

 -- Variable: postinstall_cmds
 -- Variable: old_postinstall_cmds
     Commands run after installing a shared or static library,
     respectively.

 -- Variable: postuninstall_cmds
 -- Variable: old_postuninstall_cmds
     Commands run after uninstalling a shared or static library,
     respectively.

 -- Variable: postlink_cmds
     Commands necessary for finishing linking programs.  ‘postlink_cmds’
     are executed immediately after the program is linked.  Any
     occurrence of the string ‘@OUTPUT@’ in ‘postlink_cmds’ is replaced
     by the name of the created executable (i.e. not the wrapper, if a
     wrapper is generated) prior to execution.  Similarly,
     ‘@TOOL_OUTPUT@’ is replaced by the toolchain format of ‘@OUTPUT@’.
     Normally disabled (i.e. ‘postlink_cmds’ empty).

 -- Variable: reload_cmds
 -- Variable: reload_flag
     Commands to create a reloadable object.  Set ‘reload_cmds’ to
     ‘false’ on systems that cannot create reloadable objects.

 -- Variable: runpath_var
     The environment variable that tells the linker what directories to
     hardcode in the resulting executable.

 -- Variable: shlibpath_overrides_runpath
     Indicates whether it is possible to override the hard-coded library
     search path of a program with an environment variable.  If this is
     set to no, libtool may have to create two copies of a program in
     the build tree, one to be installed and one to be run in the build
     tree only.  When each of these copies is created depends on the
     value of ‘fast_install’.  The default value is ‘unknown’, which is
     equivalent to ‘no’.

 -- Variable: shlibpath_var
     The environment variable that tells the dynamic linker where to
     find shared libraries.

 -- Variable: soname_spec
     The name coded into shared libraries, if different from the real
     name of the file.

 -- Variable: striplib
 -- Variable: old_striplib
     Command to strip a shared (‘striplib’) or static (‘old_striplib’)
     library, respectively.  If these variables are empty, the strip
     flag in the install mode will be ignored for libraries (*note
     Install mode::).

 -- Variable: sys_lib_dlsearch_path_spec
     Expression to get the run-time system library search path.
     Directories that appear in this list are never hard-coded into
     executables.

 -- Variable: sys_lib_search_path_spec
     Expression to get the compile-time system library search path.
     This variable is used by libtool when it has to test whether a
     certain library is shared or static.  The directories listed in
     ‘shlibpath_var’ are automatically appended to this list, every time
     libtool runs (i.e., not at configuration time), because some
     linkers use this variable to extend the library search path.
     Linker switches such as ‘-L’ also augment the search path.

 -- Variable: thread_safe_flag_spec
     Linker flag (passed through the C compiler) used to generate
     thread-safe libraries.

 -- Variable: to_host_file_cmd
     If the toolchain is not native to the build platform (e.g. if you
     are using MSYS to drive the scripting, but are using the MinGW
     native Windows compiler) this variable describes how to convert
     file names from the format used by the build platform to the format
     used by host platform.  Normally set to ‘func_convert_file_noop’,
     libtool will autodetect most cases where other values should be
     used.  On rare occasions, it may be necessary to override the
     autodetected value (*note Cygwin to MinGW Cross::).

 -- Variable: to_tool_file_cmd
     If the toolchain is not native to the build platform (e.g. if you
     are using some Unix to drive the scripting together with a Windows
     toolchain running in Wine) this variable describes how to convert
     file names from the format used by the build platform to the format
     used by the toolchain.  Normally set to ‘func_convert_file_noop’.

 -- Variable: version_type
     The library version numbering type.  One of ‘libtool’,
     ‘freebsd-aout’, ‘freebsd-elf’, ‘irix’, ‘linux’, ‘osf’, ‘sunos’,
     ‘windows’, or ‘none’.

 -- Variable: want_nocaseglob
     Find potential files using the shell option ‘nocaseglob’, when
     ‘deplibs_check_method’ is ‘file_magic’.  Normally set to ‘no’.  Set
     to ‘yes’ to enable the ‘nocaseglob’ shell option when looking for
     potential file names in a case-insensitive manner.

 -- Variable: whole_archive_flag_spec
     Compiler flag to generate shared objects from convenience archives.

 -- Variable: wl
     The C compiler flag that allows libtool to pass a flag directly to
     the linker.  Used as: ‘${wl}SOME-FLAG’.

   Variables ending in ‘_cmds’ or ‘_eval’ contain a ‘~’-separated list
of commands that are ‘eval’ed one after another.  If any of the commands
return a nonzero exit status, libtool generally exits with an error
message.

   Variables ending in ‘_spec’ are ‘eval’ed before being used by
libtool.

