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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 .