| FSYNC(2) | System Calls Manual | FSYNC(2) | 
fsync, fsync_range —
#include <unistd.h>
int
  
  fsync(int
    fd);
int
  
  fsync_range(int
    fd, int how,
    off_t start,
    off_t length);
fsync() causes all modified data and attributes of
  fd to be written to a permanent storage device. This
  normally results in all in-core modified copies of buffers for the associated
  file to be written to a disk.
fsync_range() is similar, but provides
    control over the region of the file to be synchronized, and the method of
    synchronization.
These functions should be used by programs that require a file to be in a known state, for example, in building a simple transaction facility.
Note that writing the data to a permanent storage device does not
    necessarily write the data to permanent storage media within that device;
    for example, after writing data to a disk device, the data might reside in a
    cache within the device, but not yet on more permanent storage within the
    device. Neither fsync() nor the default behavior of
    fsync_range() (without the
    FDISKSYNC flag) will flush disk caches, because they
    assume that storage devices are able to ensure that completed writes are
    transferred to media some time between the write and a power failure or
    system crash.
fsync_range() causes all modified data
    starting at start for length
    length of fd to be written to a
    permanent storage device. If the length parameter is
    zero, fsync_range() will synchronize all of the file
    data.
fsync_range() takes a
    how parameter which contains one or more of the
    following flags:
FDATASYNCFFILESYNCfsync on the specified
      range.FDISKSYNCThe FDATASYNC and
    FFILESYNC flags are mutually exclusive. Either of
    those flags may be combined with the FDISKSYNC
  flag.
Note that fsync_range() requires that the
    file fd must be open for writing, whereas
    fsync() does not.
fsync() or fsync_range() fail
  if:
EBADF]EINVAL]EIO]Additionally, fsync_range() fails if:
fsync_range() call requires
  that the file system containing the file referenced by
  fd support partial synchronization of file data. For
  file systems which do not support partial synchronization, the entire file
  will be synchronized and the call will be the equivalent of calling
  fsync().
fsync() function call appeared in
  4.2BSD.
The fsync_range() function call first
    appeared in NetBSD 2.0 and is modeled after the
    function available in AIX.
| February 17, 2021 | NetBSD 10.0 |