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():
       Glibc 2.16 and later:
           No feature test macros need be defined
       Glibc up to and including 2.15:
           _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 (e.g., a pipe,  FIFO,  or  socket)
          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

   sync(1), bdflush(2), open(2), pwritev(2), sync(2),  sync_file_range(2),
   fflush(3), fileno(3), hdparm(8), mount(8)

COLOPHON

   This  page  is  part of release 4.09 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
   https://www.kernel.org/doc/man-pages/.

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