FSYNC(3)                   POSIX Programmer's Manual                  FSYNC(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
       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
       transfer 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()
       function 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
              operation 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
       modifications to a file to be completed before continuing; for example,
       a program which contains a simple transaction facility might use it to
       ensure that all modifications to a file or files caused by a
       transaction 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
       permitted. 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
       actually cause data to be written where it is safe from a power
       failure. The conformance document should identify at least that one
       configuration 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 configured to allow the data to be written to non-volatile
       storage.

       It is reasonable to assert that the key aspects of fsync() are
       unreasonable 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
       automated 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.1-2017, <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                             FSYNC(3)
