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PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface
may differ (consult the corresponding Linux manual page for details of Linux behavior), or the interface
may not be implemented on Linux.
NAME
close — close a file descriptor
SYNOPSIS
#include <unistd.h>
int close(int fildes);
DESCRIPTION
The close() function shall deallocate the file descriptor indicated by fildes. To deallocate means to
make the file descriptor available for return by subsequent calls to open() or other functions that
allocate file descriptors. All outstanding record locks owned by the process on the file associated with
the file descriptor shall be removed (that is, unlocked).
If close() is interrupted by a signal that is to be caught, it shall return -1 with errno set to [EINTR]
and the state of fildes is unspecified. If an I/O error occurred while reading from or writing to the
file system during close(), it may return -1 with errno set to [EIO]; if this error is returned, the
state of fildes is unspecified.
When all file descriptors associated with a pipe or FIFO special file are closed, any data remaining in
the pipe or FIFO shall be discarded.
When all file descriptors associated with an open file description have been closed, the open file
description shall be freed.
If the link count of the file is 0, when all file descriptors associated with the file are closed, the
space occupied by the file shall be freed and the file shall no longer be accessible.
If a STREAMS-based fildes is closed and the calling process was previously registered to receive a
SIGPOLL signal for events associated with that STREAM, the calling process shall be unregistered for
events associated with the STREAM. The last close() for a STREAM shall cause the STREAM associated with
fildes to be dismantled. If O_NONBLOCK is not set and there have been no signals posted for the STREAM,
and if there is data on the module's write queue, close() shall wait for an unspecified time (for each
module and driver) for any output to drain before dismantling the STREAM. The time delay can be changed
via an I_SETCLTIME ioctl() request. If the O_NONBLOCK flag is set, or if there are any pending signals,
close() shall not wait for output to drain, and shall dismantle the STREAM immediately.
If the implementation supports STREAMS-based pipes, and fildes is associated with one end of a pipe, the
last close() shall cause a hangup to occur on the other end of the pipe. In addition, if the other end of
the pipe has been named by fattach(), then the last close() shall force the named end to be detached by
fdetach(). If the named end has no open file descriptors associated with it and gets detached, the
STREAM associated with that end shall also be dismantled.
If fildes refers to the master side of a pseudo-terminal, and this is the last close, a SIGHUP signal
shall be sent to the controlling process, if any, for which the slave side of the pseudo-terminal is the
controlling terminal. It is unspecified whether closing the master side of the pseudo-terminal flushes
all queued input and output.
If fildes refers to the slave side of a STREAMS-based pseudo-terminal, a zero-length message may be sent
to the master.
When there is an outstanding cancelable asynchronous I/O operation against fildes when close() is called,
that I/O operation may be canceled. An I/O operation that is not canceled completes as if the close()
operation had not yet occurred. All operations that are not canceled shall complete as if the close()
blocked until the operations completed. The close() operation itself need not block awaiting such I/O
completion. Whether any I/O operation is canceled, and which I/O operation may be canceled upon close(),
is implementation-defined.
If a memory mapped file or a shared memory object remains referenced at the last close (that is, a
process has it mapped), then the entire contents of the memory object shall persist until the memory
object becomes unreferenced. If this is the last close of a memory mapped file or a shared memory object
and the close results in the memory object becoming unreferenced, and the memory object has been
unlinked, then the memory object shall be removed.
If fildes refers to a socket, close() shall cause the socket to be destroyed. If the socket is in
connection-mode, and the SO_LINGER option is set for the socket with non-zero linger time, and the socket
has untransmitted data, then close() shall block for up to the current linger interval until all data is
transmitted.
RETURN VALUE
Upon successful completion, 0 shall be returned; otherwise, -1 shall be returned and errno set to
indicate the error.
ERRORS
The close() function shall fail if:
EBADF The fildes argument is not a open file descriptor.
EINTR The close() function was interrupted by a signal.
The close() function may fail if:
EIO An I/O error occurred while reading from or writing to the file system.
The following sections are informative.
EXAMPLES
Reassigning a File Descriptor
The following example closes the file descriptor associated with standard output for the current process,
re-assigns standard output to a new file descriptor, and closes the original file descriptor to clean up.
This example assumes that the file descriptor 0 (which is the descriptor for standard input) is not
closed.
#include <unistd.h>
...
int pfd;
...
close(1);
dup(pfd);
close(pfd);
...
Incidentally, this is exactly what could be achieved using:
dup2(pfd, 1);
close(pfd);
Closing a File Descriptor
In the following example, close() is used to close a file descriptor after an unsuccessful attempt is
made to associate that file descriptor with a stream.
#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>
#define LOCKFILE "/etc/ptmp"
...
int pfd;
FILE *fpfd;
...
if ((fpfd = fdopen (pfd, "w")) == NULL) {
close(pfd);
unlink(LOCKFILE);
exit(1);
}
...
APPLICATION USAGE
An application that had used the stdio routine fopen() to open a file should use the corresponding
fclose() routine rather than close(). Once a file is closed, the file descriptor no longer exists, since
the integer corresponding to it no longer refers to a file.
Implementations may use file descriptors that must be inherited into child processes for the child
process to remain conforming, such as for message catalog or tracing purposes. Therefore, an application
that calls close() on an arbitrary integer risks non-conforming behavior, and close() can only portably
be used on file descriptor values that the application has obtained through explicit actions, as well as
the three file descriptors corresponding to the standard file streams. In multi-threaded parent
applications, the practice of calling close() in a loop after fork() and before an exec call in order to
avoid a race condition of leaking an unintended file descriptor into a child process, is therefore
unsafe, and the race should instead be combatted by opening all file descriptors with the FD_CLOEXEC bit
set unless the file descriptor is intended to be inherited across exec.
Usage of close() on file descriptors STDIN_FILENO, STDOUT_FILENO, or STDERR_FILENO should immediately be
followed by an operation to reopen these file descriptors. Unexpected behavior will result if any of
these file descriptors is left in a closed state (for example, an [EBADF] error from perror()) or if an
unrelated open() or similar call later in the application accidentally allocates a file to one of these
well-known file descriptors. Furthermore, a close() followed by a reopen operation (e.g., open(), dup(),
etc.) is not atomic; dup2() should be used to change standard file descriptors.
RATIONALE
The use of interruptible device close routines should be discouraged to avoid problems with the implicit
closes of file descriptors by exec and exit(). This volume of POSIX.1‐2017 only intends to permit such
behavior by specifying the [EINTR] error condition.
Note that the requirement for close() on a socket to block for up to the current linger interval is not
conditional on the O_NONBLOCK setting.
The standard developers rejected a proposal to add closefrom() to the standard. Because the standard
permits implementations to use inherited file descriptors as a means of providing a conforming
environment for the child process, it is not possible to standardize an interface that closes arbitrary
file descriptors above a certain value while still guaranteeing a conforming environment.
FUTURE DIRECTIONS
None.
SEE ALSO
Section 2.6, STREAMS, dup(), exec, exit(), fattach(), fclose(), fdetach(), fopen(), fork(), ioctl(),
open(), perror(), unlink()
The Base Definitions volume of POSIX.1‐2017, <unistd.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard
for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics
Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original
IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document.
The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page are most likely to have been introduced
during the conversion of the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 CLOSE(3POSIX)