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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 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
       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  filesystem
       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 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 3.24 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/.