CLOSE(2)

NAME
close - delete a descriptor

LIBRARY
libc

SYNOPSIS
#include <unistd.h>
int
close(int d);

DESCRIPTION
The close() 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 seek 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 close() 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 fork(). 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

fcntl(d, F_SETFD, 1);

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

fcntl(d, F_SETFD, 0);

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

RETURN VALUES
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In either case, if d was an active descriptor, it is no longer active.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.

ERRORS
close() will fail if:

[EBADF]
    d is not an active descriptor.

[EINTR]
    An interrupt was received.

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

STANDARDS
The close() function conforms to p1003.1-90.

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.

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