SYNC_FILE_RANGE(2)        (2017-09-15)         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 synchro-
          nized.  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.  Syn-
          chronization 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

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          if a write is purely an overwrite.  On filesystems using
          copy-on-write semantics (e.g., btrfs) an overwrite of exist-
          ing allocated blocks is impossible.  When writing into pre-
          allocated 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 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_AFTER
          SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE |
               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

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               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 argu-
          ments 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 archi-
          tectures where glibc support is provided, glibc transpar-
          ently 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

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          reporting bugs, and the latest version of this page, can be
          found at https://www.kernel.org/doc/man-pages/.

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