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., 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 associ-
       ated 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  meta-
       data 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
       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 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  descrip-
              tors 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 sup-
              port 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 insuffi-
              cient storage space.

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), 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)