Tribblix: manual page: close.2

CLOSE(2) System Calls CLOSE(2)

NAME

close - close a file descriptor

SYNOPSIS

#include <unistd.h>

int
close(int fildes);

DESCRIPTION

The close() function deallocates the file descriptor indicated by
fildes. To deallocate means to make the file descriptor available for
return by subsequent calls to open(2) or other functions that allocate
file descriptors. All outstanding record locks owned by the process on
the file associated with the file descriptor will be removed (that is,
unlocked).

If close() is interrupted by a signal that is to be caught, it will
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 returns -1, sets errno to EIO, and
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 will be discarded.

When all file descriptors associated with an open file description have
been closed the open file description will 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
will be freed and the file will no longer be accessible.

If a streams-based (see Intro(2)) fildes is closed and the calling
process was previously registered to receive a SIGPOLL signal (see
signal(3C)) for events associated with that stream (see I_SETSIG in
streamio(4I)), the calling process will be unregistered for events
associated with the stream. The last close() for a stream causes the
stream associated with fildes to be dismantled. If O_NONBLOCK and
O_NDELAY are not set and there have been no signals posted for the
stream, and if there is data on the module's write queue, close() waits
up to 15 seconds (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(2) request (see streamio(4I)). If the O_NONBLOCK or
O_NDELAY flag is set, or if there are any pending signals, close() does
not wait for output to drain, and dismantles the stream immediately.

If fildes is associated with one end of a pipe, the last close() causes
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(3C), then the last
close() forces the named end to be detached by fdetach(3C). If the
named end has no open file descriptors associated with it and gets
detached, the stream associated with that end is also dismantled.

If fildes refers to the manager side of a pseudo-terminal, a SIGHUP
signal is sent to the session leader, if any, for which the subsidiary
side of the pseudo-terminal is the controlling terminal. It is
unspecified whether closing the manager side of the pseudo-terminal
flushes all queued input and output.

If fildes refers to the subsidiary side of a streams-based pseudo-
terminal, a zero-length message may be sent to the manager.

When there is an outstanding cancelable asynchronous I/O operation
against fildes when close() is called, that I/O operation is 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
will complete as if the close() blocked until the operations completed.

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 will 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 will be removed.

If fildes refers to a socket, close() causes the socket to be
destroyed. If the socket is 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() will block for up to the current
linger interval until all data is transmitted.

RETURN VALUES

The close() function returns the value 0 if successful; otherwise the
value -1 is returned and the global variable errno is set to indicate
the error.

EXAMPLES

Example 1 Reassign 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);

Example 2 Close 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);
}
...

ERRORS

The close() function will fail if:

EBADF The fildes argument is not a valid file descriptor.

EINTR The close() function was interrupted by a signal.

ENOLINK The fildes argument is on a remote machine and the
link to that machine is no longer active.

ENOSPC There was no free space remaining on the device
containing the file.

The close() function may fail if:

EIO An I/O error occurred while reading from or writing
to the file system.

USAGE

An application that used the stdio(3C) function fopen(3C) to open a
file should use the corresponding fclose(3C) function rather than
close().

INTERFACE STABILITY

Committed

MT-LEVEL
Async-Signal-Safe