int
     fsync(int fd);

DESCRIPTION

     Fsync() causes all modified data and attributes of fd to be moved to a
     permanent storage device.  This normally results in all in-core modified
     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 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 guarantess about the integrity of
     their data, MacOS 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 their data is written in the order they
     expect.  Please see fcntl(2) for more detail.

RETURN VALUES

     A 0 value is returned on success.  A -1 value indicates an error.

ERRORS

     The fsync() fails if:

     [EBADF]            fd is not a valid descriptor.

     [EINVAL]           fd refers to a socket, not to a file.

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

SEE ALSO

     sync(2), sync(8), update(8), fcntl(2)

HISTORY

     The fsync() function call appeared in 4.2BSD.

4.2 Berkeley Distribution        June 4, 1993        4.2 Berkeley Distribution