fsync, fdatasync - synchronize a file's in-core state with storage device
int fsync(int fd);
int fdatasync(int fd);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)
Glibc 2.16 and later:
No feature test macros need be defined
Glibc up to and including 2.15:
_BSD_SOURCE || _XOPEN_SOURCE
|| /* since glibc 2.8: */ _POSIX_C_SOURCE >= 200112L
_POSIX_C_SOURCE >= 199309L ||
_XOPEN_SOURCE >= 500
() transfers ("flushes") all modified in-core data of
(i.e., modified buffer cache pages for) the file referred to by the file
to the disk device (or other permanent storage device) so
that all changed information can be retrieved even if the system crashes or is
rebooted. This includes writing through or flushing a disk cache if present.
The call blocks until the device reports that the transfer has completed.
As well as flushing the file data, fsync
() also flushes the metadata
information associated with the file (see inode(7)
() does not necessarily ensure that the entry in the
directory containing the file has also reached disk. For that an explicit
() on a file descriptor for the directory is also needed.
() is similar to fsync
(), but does not flush modified
metadata unless that metadata is needed in order to allow a subsequent data
retrieval to be correctly handled. For example, changes to st_atime
(respectively, time of last access and time of last
modification; see inode(7)
) do not require flushing because they are
not necessary for a subsequent data read to be handled correctly. On the other
hand, a change to the file size (st_size
, as made by say
), would require a metadata flush.
The aim of fdatasync
() is to reduce disk activity for applications that
do not require all metadata to be synchronized with the disk.
On success, these system calls return zero. On error, -1 is returned, and
is set appropriately.
- fd is not a valid open file descriptor.
- An error occurred during synchronization.
- EROFS, EINVAL
- fd is bound to a special file (e.g., a pipe, FIFO,
or socket) which does not support synchronization.
POSIX.1-2001, POSIX.1-2008, 4.3BSD.
On POSIX systems on which fdatasync
() is available,
is defined in <unistd.h>
to a value
greater than 0. (See also sysconf(3)
On some UNIX systems (but not Linux), fd
must be a writable
In Linux 2.2 and earlier, fdatasync
() is equivalent to fsync
and so has no performance advantage.
() implementations in older kernels and lesser used filesystems
does not know how to flush disk caches. In these cases disk caches need to be
disabled using hdparm(8)
to guarantee safe
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