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NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | AVAILABILITY | NOTES | SEE ALSO | COLOPHON |
FSYNC(2) Linux Programmer's Manual FSYNC(2)
fsync, fdatasync - synchronize a file's in-core state with storage
device
#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
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 inode(7)).
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 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 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.
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,
_POSIX_SYNCHRONIZED_IO 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 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.
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)
This page is part of release 4.12 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/.
Linux 2017-05-03 FSYNC(2)
Pages that refer to this page: bdflush(2), close(2), open(2), posix_fadvise(2), sync(2), sync_file_range(2), syscalls(2), write(2), aio_error(3), aio_fsync(3), aio_return(3), dbopen(3), fclose(3), fflush(3), cups-files.conf(5), aio(7), signal-safety(7), mount(8), xfs_io(8)