CLOSE(2) System Calls Manual CLOSE(2)

close — delete a descriptor

Standard C Library (libc, -lc)

#include <unistd.h>

int
close(int d);

The () system call deletes a descriptor from the per-process object reference table. If this is the last reference to the underlying object, the object will be deactivated. For example, on the last close of a file the current pointer associated with the file is lost; on the last close of a socket(2) associated naming information and queued data are discarded; on the last close of a file holding an advisory lock the lock is released (see flock(2)).

When a process exits, all associated descriptors are freed, but since there is a limit on active descriptors per processes, the () system call is useful when a large quantity of file descriptors are being handled.

When a process calls fork(2), all descriptors for the new child process reference the same objects as they did in the parent before the (). If a new process is then to be run using execve(2), the process would normally inherit these descriptors. Most of the descriptors can be rearranged with dup2(2) or deleted with close() before the execve() is attempted, but if some of these descriptors will still be needed if the execve() fails, it is necessary to arrange for them to be closed only if the execve() succeeds. For this reason, the system call

is provided, which arranges that a descriptor “d” will be closed after a successful (); the system call

restores the default, which is to not close descriptor “d”.

Upon successful completion, a value of 0 is returned. Otherwise, a value of -1 is returned and errno is set to indicate the error.

In either case, if d was an active descriptor, it is no longer active. That is, close() always closes the file descriptor, and, if it was the last reference to the underlying object, frees the associated resources — even if some underlying I/O fails or it is interrupted by a signal.

Callers must not retry failed close(), even on EINTR. Retrying may inadvertently close a descriptor that was created by another thread concurrently after the first call to close() failed.

close() will fail if:

[]
d is not an active descriptor.
[]
An interrupt was received.

accept(2), execve(2), fcntl(2), flock(2), open(2), pipe(2), socket(2), socketpair(2)

The close() function conforms to IEEE Std 1003.1-1990 (“POSIX.1”).

The finality of close(), even on error, is not specified by POSIX, but most operating systems, including FreeBSD, OpenBSD, Linux, and Solaris, implement the same semantics.

The close() function appeared in Version 1 AT&T UNIX.

September 1, 2019 NetBSD 11.0