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

       sync_file_range - sync a file segment with disk

       #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 sub‐
              mitted 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 size.

              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 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:

              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.

              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.

              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.

              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

       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 programs.

       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().

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

       This  page  is  part of release 3.74 of the Linux man-pages project.  A description of the
       project, information about reporting bugs, and the latest version of  this  page,  can  be
       found at http://www.kernel.org/doc/man-pages/.

Linux                                       2014-08-19                         SYNC_FILE_RANGE(2)

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