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FSYNC(2)                   Linux Programmer's Manual                  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(): _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) where that file resides.  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 con-
       taining 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 file descriptor open for writing.

       EIO    An error occurred during synchronization.

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

CONFORMING TO
       4.3BSD, POSIX.1-2001.

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
       Applications that access databases or log files often write a  tiny  data  fragment
       (e.g., one line in a log file) and then call fsync() immediately in order to ensure
       that the written data is physically stored on the harddisk.  Unfortunately, fsync()
       will  always  initiate  two  write  operations:  one for the newly written data and
       another one in order to update the modification time stored in the inode.   If  the
       modification  time is not a part of the transaction concept fdatasync() can be used
       to avoid unnecessary inode disk write operations.

       If the underlying hard disk has write caching enabled, then the data may not really
       be on permanent storage when fsync() / fdatasync() return.

       When  an  ext2  file  system is mounted with the sync option, directory entries are
       also implicitly synced by fsync().

       On kernels before 2.4, fsync() on big files can  be  inefficient.   An  alternative
       might be to use the O_SYNC flag to open(2).

       In  Linux 2.2 and earlier, fdatasync() is equivalent to fsync(), and so has no per-
       formance advantage.

SEE ALSO
       bdflush(2), open(2), sync(2),  sync_file_range(2),  hdparm(8),  mount(8),  sync(8),
       update(8)

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of
       the project, and information about reporting bugs, can be found at  http://www.ker-
       nel.org/doc/man-pages/.



Linux                             2008-11-07                          FSYNC(2)

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