fsync(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

FSYNC(2)                Linux Programmer's Manual               FSYNC(2)

NAME         top

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

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)).

       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         top

       On success, these system calls return zero.  On error, -1 is
       returned, and errno is set to indicate the error.

ERRORS         top

       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         top

       POSIX.1-2001, POSIX.1-2008, 4.3BSD.

       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         top

       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         top

       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         top

       This page is part of release 5.12 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                          2021-03-22                       FSYNC(2)

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