Section: Linux Programmer's Manual (2)
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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
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 descriptor
to the disk device (or other permanent storage device) so that all
changed information can be retrieved even if the system crashes or
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,
also flushes the metadata information associated with the file (see
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
but does not flush modified metadata unless that metadata
is needed in order to allow a subsequent data retrieval to be
For example, changes to
(respectively, time of last access and
time of last modification; see
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
as made by say
would require a metadata flush.
The aim of
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.
is not a valid open file descriptor.
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 triggered
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 filesystems) will report errors to
all file descriptors that where open on the file when the error was recorded.
Disk space was exhausted while synchronizing.
- EROFS, EINVAL
is bound to a special file (e.g., a pipe, FIFO, or socket)
which does not support synchronization.
- ENOSPC, EDQUOT
is bound to a file on NFS or another filesystem which does not allocate
space at the time of a
system call, and some previous write failed due to insufficient
POSIX.1-2001, POSIX.1-2008, 4.3BSD.
On POSIX systems on which
is defined in
to a value greater than 0.
On some UNIX systems (but not Linux),
must be a
In Linux 2.2 and earlier,
is equivalent to
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
to guarantee safe operation.
This page is part of release 4.16 of the Linux
A description of the project,
information about reporting bugs,
and the latest version of this page,
can be found at