#define _GNU_SOURCE         /* See feature_test_macros(7) */
       #include <fcntl.h>

       int sync_file_range(int fd, off64_t offset, off64_t nbytes,
                           unsigned int flags);

       sync_file_range() permits fine control when synchronizing the open file
       referred to by the file descriptor fd with disk.

       offset is the starting byte of  the  file  range  to  be  synchronized.
       nbytes  specifies the length of the range to be synchronized, in bytes;
       if nbytes is zero, then all bytes from offset through  to  the  end  of
       file  are synchronized.  Synchronization is in units of the system page
       size: offset is rounded down to a page boundary;  (offset+nbytes-1)  is
       rounded up to a page boundary.

       The flags bit-mask argument can include any of the following values:

              Wait  upon  write-out  of  all pages in the specified range that
              have already been submitted to the device driver  for  write-out
              before performing any write.

              Initiate  write-out  of  all  dirty pages in the specified range
              which are not presently submitted  write-out.   Note  that  even
              this  may  block if you attempt to write more than request queue

              Wait upon write-out of all pages in the range  after  performing
              any write.

       Specifying flags as 0 is permitted, as a no-op.

       This  system call is extremely dangerous and should not be used in por-
       table programs.  None of these operations writes out the  file's  meta-
       data.   Therefore,  unless the application is strictly performing over-
       writes of already-instantiated disk blocks,  there  are  no  guarantees
       that the data will be available after a crash.  There is no user inter-
       face to know if a write is purely an overwrite.  On file systems  using
       copy-on-write  semantics  (e.g.,  btrfs) an overwrite of existing allo-
       cated blocks is impossible.  When writing into preallocated space, many
       file  systems  also  require calls into the block allocator, which this
       system call does not sync out to disk.  This system call does not flush
       disk  write caches and thus does not provide any data integrity on sys-
       tems with volatile disk write caches.

   Some details
              are  not  presently  under  write-out.   This is an asynchronous
              flush-to-disk  operation.   This  is  not  suitable   for   data
              integrity operations.

              Wait  for  completion of write-out of all pages in the specified
              range.     This    can    be    used    after     an     earlier
              wait for completion of that operation, and obtain its result.

              This  is  a  write-for-data-integrity operation that will ensure
              that all pages in the specified  range  which  were  dirty  when
              sync_file_range() was called are committed to disk.

       On  success, sync_file_range() returns 0; on failure -1 is returned and
       errno is set to indicate the error.

       EBADF  fd is not a valid file descriptor.

       EINVAL flags specifies an invalid bit; or offset or nbytes is invalid.

       EIO    I/O error.

       ENOMEM Out of memory.

       ENOSPC Out of disk space.

       ESPIPE fd refers to something  other  than  a  regular  file,  a  block
              device, a directory, or a symbolic link.

       sync_file_range() appeared on Linux in kernel 2.6.17.

       This  system  call is Linux-specific, and should be avoided in portable

       Some architectures (e.g., PowerPC, ARM) need  64-bit  arguments  to  be
       aligned  in  a  suitable pair of registers.  On such architectures, the
       call signature of sync_file_range() is flawed, since it forces a regis-
       ter  to  be  wasted  as  padding  between  the fd and offset arguments.
       Therefore, these architectures define  a  different  system  call  that
       orders the arguments suitably:

           int sync_file_range2(int fd, unsigned int flags,
                                off64_t offset, off64_t nbytes);

       The  behavior  of  this  system  call  is otherwise exactly the same as
       This  page  is  part of release 3.35 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

Linux                             2010-10-09                SYNC_FILE_RANGE(2)
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