NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | AVAILABILITY | NOTES | SEE ALSO | COLOPHON

FSYNC(2)                  Linux Programmer's Manual                 FSYNC(2)

NAME         top

       fsync,  fdatasync  -  synchronize a file's in-core state with storage
       device

SYNOPSIS         top

       #include <unistd.h>

       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

DESCRIPTION         top

       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         top

       On success, these system calls return zero.  On error, -1 is
       returned, and errno is set appropriately.

ERRORS         top

       EBADF  fd is not a valid open file descriptor.

       EIO    An error occurred during synchronization.

       EROFS, EINVAL
              fd is bound to a special file which does not support
              synchronization.

CONFORMING TO         top

       4.3BSD, POSIX.1-2001.

AVAILABILITY         top

       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         top

       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         top

       bdflush(2), open(2), sync(2), sync_file_range(2), hdparm(8),
       mount(8), sync(1)

COLOPHON         top

       This page is part of release 3.71 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
       http://www.kernel.org/doc/man-pages/.

Linux                            2014-08-19                         FSYNC(2)