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Man Pages
CLOSE(2) FreeBSD System Calls Manual CLOSE(2)

close
delete a descriptor

Standard C Library (libc, -lc)

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

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 further 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 processes, the close() system 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 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(2) 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 if the execve 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; the call “fcntl(d, F_SETFD, 0)” restores the default, which is to not close the descriptor.

The close() function returns the value 0 if successful; otherwise the value -1 is returned and the global variable errno is set to indicate the error.

The close() system call will fail if:
[]
The fd argument is not an active descriptor.
[]
An interrupt was received.
[]
The underlying object did not fit, cached data was lost.
[]
The underlying object was a stream socket that was shut down by the peer before all pending data was delivered.
In case of any error except EBADF, the supplied file descriptor is deallocated and therefore is no longer valid.

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

The close() system call is expected to conform to IEEE Std 1003.1-1990 (“POSIX.1”).

The close() function appeared in Version 1 AT&T UNIX.
December 1, 2017 FreeBSD 12.0-RELEASE

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