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Linux 2.6 - man page for fsync (linux section 3posix)

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

       fsync - synchronize changes to a file

       #include <unistd.h>

       int fsync(int fildes);

       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  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.

       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.

       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.


       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.

       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 configu-
       ration 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 config-
       ured 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) dur-
       ing  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.


       sync() , the Base Definitions volume of IEEE Std 1003.1-2001, <unistd.h>

       Portions of this text are reprinted and	reproduced  in	electronic  form  from	IEEE  Std
       1003.1,	2003  Edition,	Standard  for Information Technology -- Portable Operating System
       Interface (POSIX), The Open Group Base Specifications 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
       Standard, the original IEEE and The Open Group Standard is the referee document. The orig-
       inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .

IEEE/The Open Group			       2003					 FSYNC(P)

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