#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

       fsync() transfers ("flushes") all modified in-core data of (i.e., modi-
       fied  buffer cache pages for) the file referred to by the file descrip-
       tor 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  associ-
       ated 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  modifica-
       tion;  see stat(2)) do not require flushing because they are not neces-
       sary 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 file descriptor open for writing.

       EIO    An error occurred during synchronization.

              fd is bound to a special file which does  not  support  synchro-

       4.3BSD, POSIX.1-2001.
       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()

       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 performance advantage.

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

       This page is part of release 3.35 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://man7.org/linux/man-pages/.

Linux                             2008-11-07                          FSYNC(2)
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