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 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 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, 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 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().

SEE ALSO

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

COLOPHON

       This  page  is  part  of  release  5.10  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/.