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       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
              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 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
       guarantees 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
       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 systems with volatile disk write

   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, or a directory.


       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
       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 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)


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