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CLOSE(3P) POSIX Programmer's Manual CLOSE(3P)
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.
close — close a file descriptor
#include <unistd.h>
int close(int fildes);
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.
Upon successful completion, 0 shall be returned; otherwise, −1 shall
be returned and errno set to indicate the error.
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.
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);
}
...
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.
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.1‐2008 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.
None.
Section 2.6, STREAMS, exec(1p), fattach(3p), fclose(3p), fdetach(3p),
fopen(3p), ioctl(3p), open(3p), unlink(3p)
The Base Definitions volume of POSIX.1‐2008, unistd.h(0p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/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 2013 CLOSE(3P)
Pages that refer to this page: stropts.h(0p), unistd.h(0p), aio_error(3p), aio_read(3p), aio_return(3p), aio_write(3p), connect(3p), dup(3p), exec(3p), fclose(3p), fcntl(3p), getsockopt(3p), ioctl(3p), lio_listio(3p), lockf(3p), open(3p), posix_spawn(3p), posix_spawn_file_actions_addclose(3p), posix_typed_mem_open(3p), shm_open(3p), shm_unlink(3p), unlink(3p)
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