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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 dif-
       fer (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 trans-
       ferred to the storage device associated with the file described by fildes in an implementation-defined  manner.
       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 crit-
       ical  data.  It  is not intended that an exhaustive list is required, but rather sufficient information is pro-
       vided 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 IEEE Std 1003.1-2001, <unistd.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Stan-
       dard  for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifica-
       tions Issue 6, Copyright (C) 2001-2003 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 Stan-
       dard, 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 .



IEEE/The Open Group                  2003                            FSYNC(3P)