FSYNC(2) Linux Programmer's Manual FSYNC(2)
fsync, fdatasync - synchronize a file's in-core state with storage de-
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
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 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
As well as flushing the file data, fsync() also flushes the metadata
information associated with the file (see inode(7)).
Calling fsync() does not necessarily ensure that the entry in the di-
rectory containing the file has also reached disk. For that an ex-
plicit 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 re-
trieval 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 inode(7)) 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. This error may relate
to data written to some other file descriptor on the same file.
Since Linux 4.13, errors from write-back will be reported to all
file descriptors that might have written the data which trig-
gered the error. Some filesystems (e.g., NFS) keep close track
of which data came through which file descriptor, and give more
precise reporting. Other filesystems (e.g., most local filesys-
tems) will report errors to all file descriptors that were open
on the file when the error was recorded.
ENOSPC Disk space was exhausted while synchronizing.
fd is bound to a special file (e.g., a pipe, FIFO, or socket)
which does not support synchronization.
fd is bound to a file on NFS or another filesystem which does
not allocate space at the time of a write(2) system call, and
some previous write failed due to insufficient storage space.
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 de-
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 do not know how to flush disk caches. In these cases disk caches
need to be disabled using hdparm(8) or sdparm(8) to guarantee safe op-
sync(1), bdflush(2), open(2), posix_fadvise(2), pwritev(2), sync(2),
sync_file_range(2), fflush(3), fileno(3), hdparm(8), mount(8)
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Linux 2020-06-09 FSYNC(2)