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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():
           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., 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)  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  modifica-
       tion; see inode(7)) 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.
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.  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 trig-
              gered 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 filesys-
              tems) will report errors to all file descriptors that where 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.
CONFORMING TO
       POSIX.1-2001, POSIX.1-2008, 4.3BSD.
AVAILABILITY
       On  POSIX  systems  on  which fdatasync() is available, _POSIX_SYNCHRO-
       NIZED_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 filesys-
       tems 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),  posix_fadvise(2),  pwritev(2),  sync(2),
       sync_file_range(2), fflush(3), fileno(3), hdparm(8), mount(8)
COLOPHON
       This  page  is  part of release 4.15 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/.
Linux                             2017-09-15                          FSYNC(2)