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FSYNC(3P)                  POSIX Programmer's Manual                 FSYNC(3P)
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
       fsync -- synchronize changes to a file
SYNOPSIS
       #include <unistd.h>
       int fsync(int fildes);
DESCRIPTION
       The fsync() function shall request that all  data  for  the  open  file
       descriptor  named  by fildes is to be transferred to the storage device
       associated with the file described by fildes.  The nature of the trans-
       fer  is  implementation-defined.  The fsync() function shall not return
       until the system has  completed  that  action  or  until  an  error  is
       detected.
       If  _POSIX_SYNCHRONIZED_IO is defined, the fsync() function shall force
       all currently queued I/O operations associated with the file  indicated
       by file descriptor fildes to the synchronized I/O completion state. All
       I/O operations shall be completed as defined for synchronized I/O  file
       integrity completion.
RETURN VALUE
       Upon successful completion, fsync() shall return 0. Otherwise, -1 shall
       be returned and errno set to indicate the error. If the  fsync()  func-
       tion  fails, outstanding I/O operations are not guaranteed to have been
       completed.
ERRORS
       The fsync() function shall fail if:
       EBADF  The fildes argument is not a valid descriptor.
       EINTR  The fsync() function was interrupted by a signal.
       EINVAL The fildes argument does not refer to a file on which this oper-
              ation is possible.
       EIO    An  I/O error occurred while reading from or writing to the file
              system.
       In the event that any of the queued I/O operations fail, fsync()  shall
       return the error conditions defined for read() and write().
       The following sections are informative.
EXAMPLES
       None.
APPLICATION USAGE
       The fsync() function should be used by programs which require modifica-
       tions to a file to be completed before continuing; for example, a  pro-
       gram  which  contains  a  simple  transaction  facility might use it to
       ensure that all modifications to a file or files caused by  a  transac-
       tion are recorded.
RATIONALE
       The fsync() function is intended to force a physical write of data from
       the buffer cache, and to assure that after  a  system  crash  or  other
       failure that all data up to the time of the fsync() call is recorded on
       the disk. Since the concepts of  ``buffer  cache'',  ``system  crash'',
       ``physical  write'', and ``non-volatile storage'' are not defined here,
       the wording has to be more abstract.
       If _POSIX_SYNCHRONIZED_IO is not defined, the wording relies heavily on
       the conformance document to tell the user what can be expected from the
       system. It is explicitly intended that a null implementation is permit-
       ted.  This  could  be  valid in the case where the system cannot assure
       non-volatile storage under any circumstances  or  when  the  system  is
       highly  fault-tolerant  and  the  functionality is not required. In the
       middle ground between these extremes, fsync() might or might not  actu-
       ally  cause  data  to be written where it is safe from a power failure.
       The conformance document should identify at least that  one  configura-
       tion  exists  (and  how to obtain that configuration) where this can be
       assured for at least some files that the user can  select  to  use  for
       critical  data. It is not intended that an exhaustive list is required,
       but rather sufficient information is provided so that if critical  data
       needs  to be saved, the user can determine how the system is to be con-
       figured to allow the data to be written to non-volatile storage.
       It is reasonable to assert that the key aspects of fsync()  are  unrea-
       sonable  to  test  in a test suite. That does not make the function any
       less valuable, just more difficult to test. A formal  conformance  test
       should  probably  force a system crash (power shutdown) during the test
       for this condition, but it needs to be done in such a  way  that  auto-
       mated  testing  does  not  require this to be done except when a formal
       record of the results is being made. It would also not be  unreasonable
       to omit testing for fsync(), allowing it to be treated as a quality-of-
       implementation issue.
FUTURE DIRECTIONS
       None.
SEE ALSO
       sync()
       The Base Definitions volume of POSIX.1-2008, <unistd.h>
COPYRIGHT
       Portions of this text are reprinted and reproduced in  electronic  form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri-
       cal and Electronics Engineers,  Inc  and  The  Open  Group.   (This  is
       POSIX.1-2008  with  the  2013  Technical Corrigendum 1 applied.) 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.unix.org/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.ker-
       nel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group                  2013                            FSYNC(3P)