close(3p) — Linux manual page


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

PROLOG         top

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

NAME         top

       close — close a file descriptor

SYNOPSIS         top

       #include <unistd.h>

       int close(int fildes);

DESCRIPTION         top

       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

       When all file descriptors associated with an open file
       description have been closed, the open file description shall be

       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

       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

RETURN VALUE         top

       Upon successful completion, 0 shall be returned; otherwise, -1
       shall be returned and errno set to indicate the error.

ERRORS         top

       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         top

   Reassigning 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

           #include <unistd.h>
           int pfd;

       Incidentally, this is exactly what could be achieved using:

           dup2(pfd, 1);

   Closing 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 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

       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         top

       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

       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



SEE ALSO         top

       Section 2.6, STREAMS, dup(3p), exec(1p), exit(3p), fattach(3p),
       fclose(3p), fdetach(3p), fopen(3p), fork(3p), ioctl(3p),
       open(3p), perror(3p), unlink(3p)

       The Base Definitions volume of POSIX.1‐2017, unistd.h(0p)

COPYRIGHT         top

       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 .

       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 .

IEEE/The Open Group               2017                         CLOSE(3P)

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