# parse-options API

The parse-options API is used to parse and massage options in Git and to provide a usage help with consistent look.

## Basics

The argument vector `argv[]` may usually contain mandatory or optional non-option arguments, e.g. a filename or a branch, and options. Options are optional arguments that start with a dash and that allow to change the behavior of a command.

- There are basically three types of options: boolean options, options with (mandatory) arguments and options with optional arguments (i.e. a boolean option that can be adjusted).

- There are basically two forms of options: Short options consist of one dash (`-`) and one alphanumeric character. Long options begin with two dashes (`--`) and some alphanumeric characters.

- Options are case-sensitive. Please define lower-case long options only.

The parse-options API allows:

- sticked and separate form of options with arguments. `-oArg` is sticked, `-o Arg` is separate form. `--option=Arg` is sticked, `--option Arg` is separate form.

- Long options may be abbreviated, as long as the abbreviation is unambiguous.

- Short options may be bundled, e.g. `-a -b` can be specified as `-ab`.

- Boolean long options can be negated (or unset) by prepending `no-`, e.g. `--no-abbrev` instead of `--abbrev`. Conversely, options that begin with `no-` can be negated by removing it.

- Options and non-option arguments can clearly be separated using the `--` option, e.g. `-a -b --option -- --this-is-a-file` indicates that `--this-is-a-file` must not be processed as an option.

## Steps to parse options

1. `#include "parse-options.h"`

2. define a NULL-terminated `static const char * const builtin_foo_usage[]` array containing alternative usage strings

3. define `builtin_foo_options` array as described below in section Data Structure.

4. in `cmd_foo(int argc, const char **argv, const char *prefix)` call

       argc = parse_options(argc, argv, prefix, builtin_foo_options, builtin_foo_usage, flags);

   `parse_options()` will filter out the processed options of `argv[]` and leave the non-option arguments in `argv[]`. `argc` is updated appropriately because of the assignment.

   You can also pass NULL instead of a usage array as the fifth parameter of parse_options(), to avoid displaying a help screen with usage info and option list. This should only be done if necessary, e.g. to implement a limited parser for only a subset of the options that needs to be run before the full parser, which in turn shows the full help message.

   Flags are the bitwise-or of:

   `PARSE_OPT_KEEP_DASHDASH`

     Keep the `--` that usually separates options from non-option arguments.

   `PARSE_OPT_STOP_AT_NON_OPTION`

     Usually the whole argument vector is massaged and reordered. Using this flag, processing is stopped at the first non-option argument.

   `PARSE_OPT_KEEP_ARGV0`

     Keep the first argument, which contains the program name. It’s removed from argv[] by default.

   `PARSE_OPT_KEEP_UNKNOWN`

     Keep unknown arguments instead of erroring out. This doesn’t work for all combinations of arguments as users might expect it to do. E.g. if the first argument in `--unknown --known` takes a value (which we can’t know), the second one is mistakenly interpreted as a known option. Similarly, if `PARSE_OPT_STOP_AT_NON_OPTION` is set, the second argument in `--unknown value` will be mistakenly interpreted as a non-option, not as a value belonging to the unknown option, the parser early. That’s why parse_options() errors out if both options are set.

   `PARSE_OPT_NO_INTERNAL_HELP`

     By default, parse_options() handles `-h`, `--help` and `--help-all` internally, by showing a help screen. This option turns it off and allows one to add custom handlers for these options, or to just leave them unknown.

## Data Structure

The main data structure is an array of the `option` struct, say `static struct option builtin_add_options[]`. There are some macros to easily define options:

`OPT__ABBREV(&int_var)`

  Add `--abbrev[=<n>]`.

`OPT__COLOR(&int_var, description)`

  Add `--color[=<when>]` and `--no-color`.

`OPT__DRY_RUN(&int_var, description)`

  Add `-n, --dry-run`.

`OPT__FORCE(&int_var, description)`

  Add `-f, --force`.

`OPT__QUIET(&int_var, description)`

  Add `-q, --quiet`.

`OPT__VERBOSE(&int_var, description)`

  Add `-v, --verbose`.

`OPT_GROUP(description)`

  Start an option group. `description` is a short string that describes the group or an empty string. Start the description with an upper-case letter.

`OPT_BOOL(short, long, &int_var, description)`

  Introduce a boolean option. `int_var` is set to one with `--option` and set to zero with `--no-option`.

`OPT_COUNTUP(short, long, &int_var, description)`

  Introduce a count-up option. `int_var` is incremented on each use of `--option`, and reset to zero with `--no-option`.

`OPT_BIT(short, long, &int_var, description, mask)`

  Introduce a boolean option. If used, `int_var` is bitwise-ored with `mask`.

`OPT_NEGBIT(short, long, &int_var, description, mask)`

  Introduce a boolean option. If used, `int_var` is bitwise-anded with the inverted `mask`.

`OPT_SET_INT(short, long, &int_var, description, integer)`

  Introduce an integer option. `int_var` is set to `integer` with `--option`, and reset to zero with `--no-option`.

`OPT_SET_PTR(short, long, &ptr_var, description, ptr)`

  Introduce a boolean option. If used, set `ptr_var` to `ptr`.

`OPT_STRING(short, long, &str_var, arg_str, description)`

  Introduce an option with string argument. The string argument is put into `str_var`.

`OPT_INTEGER(short, long, &int_var, description)`

  Introduce an option with integer argument. The integer is put into `int_var`.

`OPT_DATE(short, long, &int_var, description)`

  Introduce an option with date argument, see `approxidate()`. The timestamp is put into `int_var`.

`OPT_CALLBACK(short, long, &var, arg_str, description, func_ptr)`

  Introduce an option with argument. The argument will be fed into the function given by `func_ptr` and the result will be put into `var`. See Option Callbacks below for a more elaborate description.

`OPT_FILENAME(short, long, &var, description)`

  Introduce an option with a filename argument. The filename will be prefixed by passing the filename along with the prefix argument of `parse_options()` to `prefix_filename()`.

`OPT_ARGUMENT(long, description)`

  Introduce a long-option argument that will be kept in `argv[]`.

`OPT_NUMBER_CALLBACK(&var, description, func_ptr)`

  Recognize numerical options like -123 and feed the integer as if it was an argument to the function given by `func_ptr`. The result will be put into `var`. There can be only one such option definition. It cannot be negated and it takes no arguments. Short options that happen to be digits take precedence over it.

`OPT_COLOR_FLAG(short, long, &int_var, description)`

  Introduce an option that takes an optional argument that can have one of three values: "always", "never", or "auto". If the argument is not given, it defaults to "always". The `--no-` form works like `--long=never`; it cannot take an argument. If "always", set `int_var` to 1; if "never", set `int_var` to 0; if "auto", set `int_var` to 1 if stdout is a tty or a pager, 0 otherwise.

`OPT_NOOP_NOARG(short, long)`

  Introduce an option that has no effect and takes no arguments. Use it to hide deprecated options that are still to be recognized and ignored silently.

The last element of the array must be `OPT_END()`.

If not stated otherwise, interpret the arguments as follows:

- `short` is a character for the short option (e.g. `'e'` for `-e`, use `0` to omit),

- `long` is a string for the long option (e.g. `"example"` for `--example`, use `NULL` to omit),

- `int_var` is an integer variable,

- `str_var` is a string variable (`char *`),

- `arg_str` is the string that is shown as argument (e.g. `"branch"` will result in `<branch>`). If set to `NULL`, three dots (`...`) will be displayed.

- `description` is a short string to describe the effect of the option. It shall begin with a lower-case letter and a full stop (`.`) shall be omitted at the end.

## Option Callbacks

The function must be defined in this form:

    int func(const struct option *opt, const char *arg, int unset)

The callback mechanism is as follows:

- Inside `func`, the only interesting member of the structure given by `opt` is the void pointer `opt->value`. `*opt->value` will be the value that is saved into `var`, if you use `OPT_CALLBACK()`. For example, do `*(unsigned long *)opt->value = 42;` to get 42 into an `unsigned long` variable.

- Return value `0` indicates success and non-zero return value will invoke `usage_with_options()` and, thus, die.

- If the user negates the option, `arg` is `NULL` and `unset` is 1.

## Sophisticated option parsing

If you need, for example, option callbacks with optional arguments or without arguments at all, or if you need other special cases, that are not handled by the macros above, you need to specify the members of the `option` structure manually.

This is not covered in this document, but well documented in `parse-options.h` itself.

## Examples

See `test-parse-options.c` and `builtin-add.c`, `builtin-clone.c`, `builtin-commit.c`, `builtin-fetch.c`, `builtin-fsck.c`, `builtin-rm.c` for real-world examples.