fsync(2)
NAME
fsync, fdatasync - synchronize a file's in-core state with storage device
SYNOPSIS
#include <unistd.h>
int fsync(int fd);
int fdatasync(int fd);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
fsync():
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
fdatasync():
_POSIX_C_SOURCE >= 199309L || _XOPEN_SOURCE >= 500
DESCRIPTION
fsync() transfers ("flushes") all modified in-core data of (i.e.,
modified buffer cache pages for) the file referred to by the file
descriptor 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
associated 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
modification; see stat(2)) 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
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.
RETURN VALUE
On success, these system calls return zero. On error, -1 is returned, and errno is set appropriately.
ERRORS
EBADF fd is not a valid open file descriptor.
EIO 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.
CONFORMING TO
POSIX.1-2001, POSIX.1-2008, 4.3BSD.
AVAILABILITY
On POSIX systems on which fdatasync() is available, _POSIX_SYNCHRONIZED_IO is defined in <unistd.h> to a value greater than 0. (See also sysconf(3).)
NOTES
On some UNIX systems (but not Linux), fd must be a writable file descriptor. 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 filesystems 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 safe operation.
SEE ALSO
sync(1), bdflush(2), open(2), pwritev(2), sync(2), sync_file_range(2), fflush(3), fileno(3), hdparm(8), mount(8)
COLOPHON
This page is part of release 4.09 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.
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