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 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 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 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.

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 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.

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  5.05  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/.