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PROLOG

       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

       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 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);

   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) {
               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 .