fsync(2) - Linux manual page (original) (raw)

fsync(2) System Calls Manual fsync(2)

NAME top

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

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#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 top

   **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)](../man7/inode.7.html)).

   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 _statime_ or _stmtime_ (respectively, time of last access
   and time of last modification; see [inode(7)](../man7/inode.7.html)) 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 (_stsize_, as made by say [ftruncate(2)](../man2/ftruncate.2.html)), 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 top

   On success, these system calls return zero.  On error, -1 is
   returned, and _[errno](../man3/errno.3.html)_ is set to indicate the error.

ERRORS top

   **EBADF** _fd_ is not a valid open file descriptor.

   **EINTR** The function was interrupted by a signal; see [signal(7)](../man7/signal.7.html).

   **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)](../man2/write.2.html) system
          call, and some previous write failed due to insufficient
          storage space.

VERSIONS top

   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)](../man3/sysconf.3.html).)

STANDARDS top

   POSIX.1-2008.

HISTORY top

   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)](../man8/hdparm.8.html) 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 top

   [sync(1)](../man1/sync.1.html), [bdflush(2)](../man2/bdflush.2.html), [open(2)](../man2/open.2.html), [posix_fadvise(2)](../man2/posix%5Ffadvise.2.html), [pwritev(2)](../man2/pwritev.2.html),
   [sync(2)](../man2/sync.2.html), [sync_file_range(2)](../man2/sync%5Ffile%5Frange.2.html), [fflush(3)](../man3/fflush.3.html), [fileno(3)](../man3/fileno.3.html), [hdparm(8)](../man8/hdparm.8.html),
   [mount(8)](../man8/mount.8.html)

COLOPHON top

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   man-pages@man7.org

Linux man-pages 6.10 2024-07-23 fsync(2)

Pages that refer to this page:pv(1), sync(1), bdflush(2), close(2), io_uring_enter2(2), io_uring_enter(2), mount(2), open(2), posix_fadvise(2), statx(2), sync(2), sync_file_range(2), syscalls(2), write(2), aio_error(3), aio_fsync(3), aio_return(3), dbopen(3), fclose(3), fflush(3), io_uring_prep_fsync(3), cups-files.conf(5), systemd.exec(5), aio(7), signal-safety(7), mount(8), sfdisk(8), xfs_io(8)