close(2) - OpenBSD manual pages

SYNOPSIS

#include <unistd.h>

int
close(int d);

The close() call deletes a descriptor d 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; and on the last close of a file holding an advisory lock, the lock is released (see flock(2)). However, the semantics of System V and IEEE Std 1003.1-1988 (“POSIX.1”) dictate that all fcntl(2) advisory record locks associated with a file for a given process are removed when any file descriptor for that file is closed by that process.

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

When a process forks (see 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 image is to then 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(2) is attempted, but since some of these descriptors may still be needed should the execve(2) fail, it is necessary to arrange for them to be closed when the execve(2) succeeds. For this reason, the call fcntl(d, F_SETFD, FD_CLOEXEC) is provided, which arranges that a descriptor will be closed after a successful execve(2); the call fcntl(d, F_SETFD, 0) restores the default, which is to not close the descriptor.