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FSYNC(2)                    BSD System Calls Manual                   FSYNC(2)

NAME
     fsync -- synchronize a file's in-core state with that on disk

SYNOPSIS
     #include <unistd.h>

     int
     fsync(int fildes);

DESCRIPTION
     Fsync() causes all modified data and attributes of fildes to be moved to
     a permanent storage device.  This normally results in all in-core modi-fied modified
     fied copies of buffers for the associated file to be written to a disk.

     Note that while fsync() will flush all data from the host to the drive
     (i.e. the "permanent storage device"), the drive itself may not physi-cally physically
     cally write the data to the platters for quite some time and it may be
     written in an out-of-order sequence.

     Specifically, if the drive loses power or the OS crashes, the application
     may find that only some or none of their data was written.  The disk
     drive may also re-order the data so that later writes may be present,
     while earlier writes are not.

     This is not a theoretical edge case.  This scenario is easily reproduced
     with real world workloads and drive power failures.

     For applications that require tighter guarantees about the integrity of
     their data, Mac OS X provides the F_FULLFSYNC fcntl.  The F_FULLFSYNC
     fcntl asks the drive to flush all buffered data to permanent storage.
     Applications, such as databases, that require a strict ordering of writes
     should use F_FULLFSYNC to ensure that their data is written in the order
     they expect.  Please see fcntl(2) for more detail.

RETURN VALUES
     The fsync() function returns the value 0 if successful; otherwise the
     value -1 is returned and the global variable errno is set to indicate the
     error.

ERRORS
     The fsync() system call will fail if:

     [EBADF]            fildes is not a valid descriptor.

     [EINTR]            Its execution is interrupted by a signal.

     [EINVAL]           fildes refers to a file type (e.g., a socket) that
                        does not support this operation.

     [EIO]              An I/O error occurred while reading from or writing to
                        the file system.

     If a queued I/O operation fails, fsync() may fail with any of the errors
     defined for read(2) or write(2).

SEE ALSO
     fcntl(2), read(2), sync(2), write(2), sync(8), update(8)

HISTORY
     The fsync() function call appeared in 4.2BSD.

4.2 Berkeley Distribution        June 4, 1993        4.2 Berkeley Distribution
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