FSYNC(2)

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

SYNOPSIS
#include <unistd.h>
int
fsync(int fd);
int
fdatasync(int fd);

DESCRIPTION
The fsync() function causes all modified data and attributes of fd 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.

The fdatasync() function is similar to fsync() except that it only guarantees modified data (and metadata necessary to read that data) is committed to storage. Other file modifications may be left unsynchronized.

fsync() and fdatasync() should be used by programs that require a file to be in a known state, for example, in building a simple transaction facility.

If fsync() or fdatasync() fail with EIO, the state of the on-disk data may have been only partially written. To guard against potential inconsistency, future calls will continue failing until all references to the file are closed.

RETURN VALUES

ERRORS
The fsync() and fdatasync() functions fail if:

[EBADF]
    fd is not a valid descriptor.

[EINVAL]
    fd does not refer to a file which can be synchronized.

[EIO]
    An I/O error occurred while reading from or writing to the file system.

SEE ALSO
sync(2), sync(8)

STANDARDS
The fsync() and fdatasync() functions conform to p1003.1-2008.

HISTORY
The fsync() system call first appeared in 4.1cBSD, and the fdatasync() function has been available since OpenBSD 5.4.

BUGS
The fdatasync() function is currently a wrapper around fsync(), so it synchronizes more state than necessary.