NAME

       fsync, fdatasync - synchronize a file's in-core state with storage device

LIBRARY

       Standard C library (libc, -lc)

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  if the system crashes or is rebooted.  This includes
       writing through or flushing a disk cache if present.  The call blocks until the device reports  that  the
       transfer has completed.

       As well as flushing the file data, fsync() also flushes the metadata information associated with the file
       (see inode(7)).

       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 inode(7)) 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 to  indicate  the
       error.

ERRORS

       EBADF  fd is not a valid open file descriptor.

       EINTR  The function was interrupted by a signal; see signal(7).

       EIO    An  error  occurred  during  synchronization.  This error may relate to data written to some other
              file descriptor on the same file.  Since Linux 4.13, errors from write-back will  be  reported  to
              all file descriptors that might have written the data which triggered the error.  Some filesystems
              (e.g., NFS) keep close track of which data came through  which  file  descriptor,  and  give  more
              precise  reporting.   Other  filesystems  (e.g., most local filesystems) will report errors to all
              file descriptors that were open on the file when the error was recorded.

       ENOSPC Disk space was exhausted while synchronizing.

       EROFS
       EINVAL fd is bound  to  a  special  file  (e.g.,  a  pipe,  FIFO,  or  socket)  which  does  not  support
              synchronization.

       ENOSPC
       EDQUOT fd  is bound to a file on NFS or another filesystem which does not allocate space at the time of a
              write(2) system call, and some previous write failed due to insufficient storage space.

VERSIONS

       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).)

STANDARDS

       POSIX.1-2008.

HISTORY

       POSIX.1-2001, 4.2BSD.

       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 do 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.

       Under  AT&T  UNIX  System  V Release 4 fd needs to be opened for writing.  This is by itself incompatible
       with the original BSD interface and forbidden by POSIX, but nevertheless survives in HP-UX and AIX.

SEE ALSO

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