FSYNC(2) Linux Programmer's Manual FSYNC(2)
fsync, fdatasync - synchronize a file's in-core state with storage
int fsync(int fd);
int fdatasync(int fd);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
fsync(): _BSD_SOURCE || _XOPEN_SOURCE
|| /* since glibc 2.8: */ _POSIX_C_SOURCE >= 200112L
fdatasync(): _POSIX_C_SOURCE >= 199309L || _XOPEN_SOURCE >= 500
fsync() transfers ("flushes") all modified in-core data of (i.e., modi-
fied buffer cache pages for) the file referred to by the file descrip-
tor fd to the disk device (or other permanent storage device) so that
all changed information can be retrieved even after the system crashed
or was rebooted. This includes writing through or flushing a disk
cache if present. The call blocks until the device reports that the
transfer has completed. It also flushes metadata information associ-
ated with the file (see stat(2)).
Calling fsync() does not necessarily ensure that the entry in the
directory containing the file has also reached disk. For that an
explicit fsync() on a file descriptor for the directory is also needed.
fdatasync() 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 or
st_mtime (respectively, time of last access and time of last modifica-
tion; see stat(2)) do not require flushing because they are not neces-
sary 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 ftruncate(2)),
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 errno is set appropriately.
EBADF fd is not a valid open file descriptor.
EIO An error occurred during synchronization.
fd is bound to a special file which does not support synchro-
POSIX.1-2001, POSIX.1-2008, 4.3BSD.
On POSIX systems on which fdatasync() is available, _POSIX_SYNCHRO-
NIZED_IO is defined in <unistd.h> to a value greater than 0. (See also
On some UNIX systems (but not Linux), fd must be a writable file
In Linux 2.2 and earlier, fdatasync() is equivalent to fsync(), and so
has no performance advantage.
The fsync() implementations in older kernels and lesser used filesys-
tems does not know how to flush disk caches. In these cases disk
caches need to be disabled using hdparm(8) or sdparm(8) to guarantee
bdflush(2), open(2), sync(2), sync_file_range(2), hdparm(8), mount(8),
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Linux 2015-08-08 FSYNC(2)
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