CLOSE(2) System Calls CLOSE(2)

close — close a file descriptor

#include <unistd.h>

int
close(int fildes);

The () 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 () 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 () 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 () 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 () 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, () 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 error.

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);
}
...

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 file.

The close() function may fail if:

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 ().

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)

February 5, 2022 OmniOS