FSYNC(2) | System Calls Manual | FSYNC(2) |
fsync
, fsync_range
—
#include <unistd.h>
int
fsync
(int
fd);
int
fsync_range
(int
fd, int how,
off_t start,
off_t length);
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
FFILESYNC
FDISKSYNC
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.
fsync
() or fsync_range
() fail
if:
EBADF
]EINVAL
]EIO
]Additionally, fsync_range
() fails if:
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
().
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.
September 22, 2013 | NetBSD 9.0 |