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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 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 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 performance advantage.

SEE ALSO

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

COLOPHON

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       description  of  the project, and information about reporting bugs, can
       be found at http://man7.org/linux/man-pages/.