fsync(2)                                                System Calls Manual                                                fsync(2)

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

       Standard C library (libc, -lc)

       #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  re‐
       trieved  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.

       EINTR  The function was interrupted by a signal; see signal(7).

       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 er‐
              rors 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 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.

       sync(1),  bdflush(2),  open(2),  posix_fadvise(2), pwritev(2), sync(2), sync_file_range(2), fflush(3), fileno(3), hdparm(8),

Linux man-pages 6.03                                         2023-02-05                                                    fsync(2)