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

NAME
       sync_file_range - sync a file segment with disk

SYNOPSIS
       #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);

DESCRIPTION
       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:

       SYNC_FILE_RANGE_WAIT_BEFORE
	      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.

       SYNC_FILE_RANGE_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
	      size.

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

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

   Warning
       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  filesystems	 using
       copy-on-write  semantics	 (e.g.,	 btrfs) an overwrite of existing allo‐
       cated blocks is impossible.  When writing into preallocated space, many
       filesystems  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
       SYNC_FILE_RANGE_WAIT_BEFORE and SYNC_FILE_RANGE_WAIT_AFTER will	detect
       any  I/O errors or ENOSPC conditions and will return these to the call‐
       er.

       Useful combinations of the flags bits are:

       SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE
	      Ensures that all pages in the specified range which  were	 dirty
	      when  sync_file_range()  was  called are placed under write-out.
	      This is a start-write-for-data-integrity operation.

       SYNC_FILE_RANGE_WRITE
	      Start write-out of all dirty pages in the specified range	 which
	      are  not	presently  under  write-out.   This is an asynchronous
	      flush-to-disk  operation.	  This	is  not	 suitable   for	  data
	      integrity operations.

       SYNC_FILE_RANGE_WAIT_BEFORE (or SYNC_FILE_RANGE_WAIT_AFTER)
	      Wait  for	 completion of write-out of all pages in the specified
	      range.	 This	 can	be    used    after	an     earlier
	      SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE operation to
	      wait for completion of that operation, and obtain its result.

       SYNC_FILE_RANGE_WAIT_BEFORE	 |	 SYNC_FILE_RANGE_WRITE	     |
       SYNC_FILE_RANGE_WAIT_AFTER
	      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.

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

ERRORS
       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.

VERSIONS
       sync_file_range() appeared on Linux in kernel 2.6.17.

CONFORMING TO
       This  system  call is Linux-specific, and should be avoided in portable
       programs.

NOTES
       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() shown in the SYNOPSIS would force a
       register	 to  be wasted as padding between the fd and offset arguments.
       (See syscall(2) for details.)  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
       sync_file_range().

       A system call with this signature first appeared on the	ARM  architec‐
       ture  in	 Linux	2.6.20,	 with  the name arm_sync_file_range().	It was
       renamed in Linux 2.6.22, when the analogous system call was  added  for
       PowerPC.	  On  architectures  where  glibc  support  is provided, glibc
       transparently	wraps	 sync_file_range2()	under	  the	  name
       sync_file_range().

SEE ALSO
       fdatasync(2), fsync(2), msync(2), sync(2)

COLOPHON
       This  page  is  part of release 3.54 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2013-04-01		    SYNC_FILE_RANGE(2)
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