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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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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.
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.
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.
None.
Section 2.6, STREAMS, dup(3p), exec(1p), exit(3p), fattach(3p),
fclose(3p), fdetach(3p), fopen(3p), fork(3p), ioctl(3p), open(3p),
perror(3p), unlink(3p)
The Base Definitions volume of POSIX.1‐2017, unistd.h(0p)
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(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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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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