redhat man page for fsync

FSYNC(2)						     Linux Programmer's Manual							  FSYNC(2)

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

SYNOPSIS
       #include <unistd.h>

       int fsync(int fd);

       int fdatasync(int fd);

DESCRIPTION
       fsync copies all in-core parts of a file to disk, and waits until the device reports that all parts are on stable storage.  It also updates
       metadata stat information. It does not necessarily ensure that the entry in the directory containing the file has also reached  disk.   For
       that an explicit fsync on the file descriptor of the directory is also needed.

       fdatasync does the same as fsync but only flushes user data, not the meta data like the mtime or atime.

RETURN VALUE
       On success, zero is returned.  On error, -1 is returned, and errno is set appropriately.

ERRORS
       EBADF  fd is not a valid file descriptor open for writing.

       EROFS, EINVAL
	      fd is bound to a special file which does not support synchronization.

       EIO    An error occurred during synchronization.

NOTES
       In case the hard disk has write cache enabled, the data may not really be on permanent storage when fsync/fdatasync return.

       When an ext2 file system is mounted with the sync option, directory entries are also implicitly synced by fsync.

       On kernels before 2.4, fsync on big files can be inefficient.  An alternative might be to use the O_SYNC flag to open(2).

CONFORMING TO
       POSIX.1b (formerly POSIX.4)

SEE ALSO
       bdflush(2), open(2), sync(2), mount(8), update(8), sync(8)

Linux 1.3.85							    2001-04-18								  FSYNC(2)