fsync(2) — Linux manual page


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

NAME         top

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

SYNOPSIS         top

       #include <unistd.h>

       int fsync(int fd);

       int fdatasync(int fd);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

           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
           _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

       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

       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 appropriately.

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.

              fd is bound to a special file (e.g., a pipe, FIFO, or socket)
              which does not support synchronization.

              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

       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 5.08 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

Linux                            2020-06-09                         FSYNC(2)

Pages that refer to this page: sync(1)arm_fadvise(2)arm_fadvise64_64(2)arm_sync_file_range(2)bdflush(2)close(2)creat(2)fadvise64(2)fadvise64_64(2)mount(2)open(2)openat(2)posix_fadvise(2)sync(2)sync_file_range2(2)sync_file_range(2)syncfs(2)syscalls(2)write(2)aio_error(3)aio_fsync(3)aio_return(3)db(3)dbopen(3)fclose(3)fflush(3)cups-files.conf(5)systemd.exec(5)aio(7)signal-safety(7)mount(8)sfdisk(8)xfs_io(8)