The fdatasync() 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.
The functionality shall be equivalent to fsync() with the symbol
_POSIX_SYNCHRONIZED_IO defined, with the exception that all I/O operations
shall be completed as defined for synchronized I/O data integrity completion.
If successful, the fdatasync() function shall return the value 0;
otherwise, the function shall return the value -1 and set errno to
indicate the error. If the fdatasync() function fails, outstanding I/O
operations are not guaranteed to have been completed.
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 original
Standard can be obtained online at http://www.opengroup.org/unix/online.html