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CLOSE(P) CLOSE(P)
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 regis-
tered to receive a SIGPOLL signal for events associated with that STREAM, the call-
ing 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 disman-
tled. 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 dis-
mantling 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 immedi-
ately.
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 unspeci-
fied 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 shared memory object or a memory mapped file 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 shared memory object or a memory mapped file 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 valid 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.
RATIONALE
The use of interruptible device close routines should be discouraged to avoid prob-
lems with the implicit closes of file descriptors by exec and exit(). This volume
of IEEE Std 1003.1-2001 only intends to permit such behavior by specifying the
[EINTR] error condition.
FUTURE DIRECTIONS
None.
SEE ALSO
STREAMS , fattach() , fclose() , fdetach() , fopen() , ioctl() , open() , the Base
Definitions volume of IEEE Std 1003.1-2001, <unistd.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1, 2003 Edition, Standard for Information Technology -- Portable Operating
System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C)
2001-2003 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 .
POSIX 2003 CLOSE(P)
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