FSYNC(2) System Calls Manual FSYNC(2)

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

Standard C Library (libc, -lc)

#include <unistd.h>

int
fsync(int fd);

int
fsync_range(int fd, int how, off_t start, off_t length);

() causes all modified data and attributes of fd to be written 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.

() is similar, but provides control over the region of the file to be synchronized, and the method of synchronization.

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

Note that writing the data to a permanent storage device does not necessarily write the data to permanent storage media within that device; for example, after writing data to a disk device, the data might reside in a cache within the device, but not yet on more permanent storage within the device. Neither () nor the default behavior of fsync_range() (without the FDISKSYNC flag) will flush disk caches, because they assume that storage devices are able to ensure that completed writes are transferred to media some time between the write and a power failure or system crash.

() causes all modified data starting at start for length length of fd to be written to a permanent storage device. If the length parameter is zero, fsync_range() will synchronize all of the file data.

() takes a how parameter which contains one or more of the following flags:

Synchronize the file data and sufficient meta-data to retrieve the data for the specified range. This is equivalent to fdatasync(2) on the specified range.
Synchronize all modified file data and meta-data for the specified range. This is equivalent to fsync on the specified range.
Request the destination device to ensure that the relevant data and meta-data is flushed from any cache to permanent storage media. In the present implementation, the entire cache on the affected device will be flushed, and this may have a significant impact on performance.

The FDATASYNC and FFILESYNC flags are mutually exclusive. Either of those flags may be combined with the FDISKSYNC flag.

Note that () requires that the file fd must be open for writing, whereas fsync() does not.

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

fsync() or fsync_range() fail if:

[]
fd is not a valid descriptor.
[]
fd refers to a socket, not to a file.
[]
An I/O error occurred while reading from or writing to the file system.

Additionally, fsync_range() fails if:

[]
fd is not open for writing.
[]
start is less than zero, or start + length is less than start or triggers an integer overflow; or how contains an invalid value.

For optimal efficiency, the () 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().

fdatasync(2), sync(2), sync(8)

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.

February 17, 2021 NetBSD 11.0