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

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

SYNOPSIS
       #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
       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 de‐
       vice (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  cor‐
       rectly  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
       On success, these system calls return zero.  On error, -1 is returned, and errno is set to indicate the error.

ERRORS
       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 re‐
              port 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
       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
       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
       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)

Linux                                                                          2021-03-22                                                                       FSYNC(2)