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Linux 2.6 - man page for sync_file_range (linux section 2)

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 sub-
	      mitted 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 portable programs.  None
       of these operations writes out the file's metadata.  Therefore, unless the application  is
       strictly  performing  overwrites of already-instantiated disk blocks, there are no guaran-
       tees that the data will be available after a crash.  There is no user interface to know if
       a  write  is  purely  an  overwrite.   On filesystems using copy-on-write semantics (e.g.,
       btrfs) an overwrite of existing allocated blocks is impossible.	When writing into  preal-
       located	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 systems 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 caller.

       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 differ-
       ent 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 architecture 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 3.55 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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