FSYNC(2)

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

LIBRARY
libc

SYNOPSIS
#include <unistd.h>
int
fsync(int fd);
int
fsync_range(int fd, int how, off_t start, off_t length);

DESCRIPTION
fsync() 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.

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

fsync_range() causes all modified data starting at start for length length of fd to be written to permanent storage. Note that fsync_range() requires that the file fd must be open for writing.

fsync_range() may flush the file data in one of two manners:

FDATASYNC
    Synchronize the file data and sufficient meta-data to retrieve the data for the specified range.

FFILESYNC
    Synchronize all modified file data and meta-data for the specified range.

By default, fsync_range() does not flush disk caches, assuming that storage media are able to ensure completed writes are transfered to media. The FDISKSYNC flag may be included in the how parameter to trigger flushing of all disk caches for the file.

If the length parameter is zero, fsync_range() will synchronize all of the file data.

RETURN VALUES
A 0 value is returned on success. A -1 value indicates an error.

ERRORS
fsync() or fsync_range() fail if:

[EBADF]
    fd is not a valid descriptor.

[EINVAL]
    fd refers to a socket, not to a file.

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

Additionally, fsync_range() fails if:

[EBADF]
    fd is not open for writing.

[EINVAL]
    start + length is less than start.

NOTES
For optimal efficiency, the fsync_range() call requires that the file system containing the file referenced by fd support partial synchronization of file data. For file systems which do not support partial synchronization, the entire file will be synchronized and the call will be the equivalent of calling fsync().

SEE ALSO
sync(2), sync(8)

HISTORY
The fsync() function call appeared in 4.2BSD.

The fsync_range() function call first appeared in NetBSD 2.0 and is modeled after the function available in AIX.