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 st_atime or st_mtime (respectively, time of last access and time of last modification; see stat(2)).

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.

EIO An error occurred during synchronization.

EROFS, EINVAL
    fd is bound to a special file which does not support 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), sync(8), update(8)