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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 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 physically 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_FULLF-SYNC F_FULLFSYNC
     SYNC 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