NAME
fsync, fsync_range
— synchronize a file's in-core
state with that on disk
LIBRARY
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
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.
fsync_range()
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
fsync() 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.
fsync_range()
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.
fsync_range()
takes a how parameter which contains one or more of
the following flags:
FDATASYNC- 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.
FFILESYNC- Synchronize all modified file data and meta-data for the specified range. This is equivalent to fsync(2) on the specified range.
FDISKSYNC- 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
fsync_range()
requires that the file fd must be open for writing,
whereas fsync() does not.
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
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.