NAME
fsync,
fsync_range —
synchronize a file's in-core state with that on disk
LIBRARY
Standard C Library (libc, -lc)
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
fdatasync(2),
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.