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(): _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 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

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

COLOPHON

       This page is part of release 4.04 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/.