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

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

       #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

       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.

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

       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.

       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).)

       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.

       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)

       This page is part of release 5.13 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                              2021-03-22                           FSYNC(2)