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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 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 por-
       table programs.  None of these operations writes out the  file's  meta-
       data.   Therefore,  unless the application is strictly performing over-
       writes of already-instantiated disk blocks,  there  are  no  guarantees
       that the data will be available after a crash.  There is no user inter-
       face to know if a write is purely an overwrite.  On  filesystems  using
       copy-on-write  semantics  (e.g.,  btrfs) an overwrite of existing allo-
       cated 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 sys-
       tems 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 call-
       er.
       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, or a directory.
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
   sync_file_range2()
       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  architec-
       ture  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 4.15 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
       https://www.kernel.org/doc/man-pages/.
Linux                             2017-09-15                SYNC_FILE_RANGE(2)