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)