CLOSE(3)                   POSIX Programmer's Manual                  CLOSE(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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 memory mapped file or a shared memory object 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 memory mapped
       file or a shared memory object 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 open 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 aa 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 aa 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.

       Implementations may use file descriptors that must be inherited into
       child processes for the child process to remain conforming, such as for
       message catalog or tracing purposes. Therefore, an application that
       calls close() on an arbitrary integer risks non-conforming behavior,
       and close() can only portably be used on file descriptor values that
       the application has obtained through explicit actions, as well as the
       three file descriptors corresponding to the standard file streams. In
       multi-threaded parent applications, the practice of calling close() in
       a loop after fork() and before an exec call in order to avoid a race
       condition of leaking an unintended file descriptor into a child
       process, is therefore unsafe, and the race should instead be combatted
       by opening all file descriptors with the FD_CLOEXEC bit set unless the
       file descriptor is intended to be inherited across exec.

       Usage of close() on file descriptors STDIN_FILENO, STDOUT_FILENO, or
       STDERR_FILENO should immediately be followed by an operation to reopen
       these file descriptors. Unexpected behavior will result if any of these
       file descriptors is left in a closed state (for example, an [EBADF]
       error from perror()) or if an unrelated open() or similar call later in
       the application accidentally allocates a file to one of these well-
       known file descriptors. Furthermore, a close() followed by a reopen
       operation (e.g., open(), dup(), etc.) is not atomic; dup2() should be
       used to change standard file descriptors.

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 POSIX.1-2017 only intends to permit such
       behavior by specifying the [EINTR] error condition.

       Note that the requirement for close() on a socket to block for up to
       the current linger interval is not conditional on the O_NONBLOCK
       setting.

       The standard developers rejected a proposal to add closefrom() to the
       standard. Because the standard permits implementations to use inherited
       file descriptors as a means of providing a conforming environment for
       the child process, it is not possible to standardize an interface that
       closes arbitrary file descriptors above a certain value while still
       guaranteeing a conforming environment.

FUTURE DIRECTIONS
       None.

SEE ALSO
       Section 2.6, STREAMS, dup(), exec, exit(), fattach(), fclose(),
       fdetach(), fopen(), fork(), ioctl(), open(), perror(), unlink()

       The Base Definitions volume of POSIX.1-2017, <unistd.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                             CLOSE(3)
