CLOSE(2) System Calls CLOSE(2)


close - close a file descriptor


#include <unistd.h>

int close(int fildes);


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,

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(7I)), 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(7I)). 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 master side of a pseudo-terminal, a SIGHUP signal
is sent to the session leader, 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 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.


Upon successful completion, 0 is returned. Otherwise, -1 is returned and
errno is set to indicate the error.


The close() function will fail if:

The fildes argument is not a valid file descriptor.

The close() function was interrupted by a signal.

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that machine is no longer active.

There was no free space remaining on the device containing the

The close() function may fail if:

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


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;

Incidentally, this is exactly what could be achieved using:

dup2(pfd, 1);

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) {


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


See attributes(5) for descriptions of the following attributes:

|Interface Stability | Standard |
|MT-Level | Async-Signal-Safe |


Intro(2), creat(2), dup(2), exec(2), fcntl(2), ioctl(2), open(2) pipe(2),
fattach(3C), fclose(3C), fdetach(3C), fopen(3C), signal(3C),
signal.h(3HEAD), attributes(5), standards(5), streamio(7I)

October 18, 2005 CLOSE(2)