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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 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 problems
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 .