CLOSE(2) System Calls CLOSE(2)


close - close a file descriptor


#include <unistd.h>

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, 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

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

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.


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


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

#include <stdio.h>
#include <unistd.h>
#include <stdlib.h>

#define LOCKFILE "/etc/ptmp"
int pfd;
FILE *fpfd;
if ((fpfd = fdopen (pfd, "w")) == NULL) {


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.


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





creat(2), dup(2), exec(2), fcntl(2), Intro(2), ioctl(2), open(2), pipe(2),
fattach(3C), fclose(3C), fdetach(3C), fopen(3C), signal(3C),
signal.h(3HEAD), streamio(4I), attributes(7), standards(7)

illumos February 5, 2022 illumos