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RedHat 9 (Linux i386) - man page for fdatasync (redhat section 2)

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

fdatasync - synchronize a file's in-core data with that on disk
#include <unistd.h> #ifdef _POSIX_SYNCHRONIZED_IO int fdatasync(int fd); #endif
fdatasync flushes all data buffers of a file to disk (before the system call returns). It resembles fsync but is not required to update the metadata such as access time. Applications that access databases or log files often write a tiny data fragment (e.g., one line in a log file) and then call fsync immedi- ately in order to ensure that the written data is physically stored on the harddisk. Unfortunately, fsync will always initiate two write operations: one for the newly written data and another one in order to update the modification time stored in the inode. If the modifica- tion time is not a part of the transaction concept fdatasync can be used to avoid unnecessary inode disk write operations.
On success, zero is returned. On error, -1 is returned, and errno is set appropriately.
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.
Currently (Linux 2.2) fdatasync is equivalent to fsync.
POSIX1b (formerly POSIX.4)
fsync(2), B.O. Gallmeister, POSIX.4, O'Reilly, pp. 220-223 and 343. Linux 1.3.86 1996-04-13 FDATASYNC(2)