# fsync

## 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 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.
    


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