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
       #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 syn-
       chronized, in bytes; if nbytes is zero, then all bytes from offset through to the end of file are synchronized.  Synchro-
       nization  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 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 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 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.

SEE ALSO
       fdatasync(2), fsync(2), msync(2), sync(2), feature_test_macros(7)

COLOPHON
       This page is part of release 3.25 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                                                      2010-01-17                                         SYNC_FILE_RANGE(2)