PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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

       fsync — synchronize changes to a file

SYNOPSIS         top

       #include <unistd.h>

       int fsync(int fildes);

DESCRIPTION         top

       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         top

       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         top

       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         top

       None.

APPLICATION USAGE         top

       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         top

       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         top

       None.

SEE ALSO         top

       sync(3p)

       The Base Definitions volume of POSIX.1‐2008, unistd.h(0p)

COPYRIGHT         top

       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 Electrical 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.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                 2013                           FSYNC(3P)