NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | VERSIONS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

SYNC_FILE_RANGE(2)        Linux Programmer's Manual       SYNC_FILE_RANGE(2)

NAME         top

       sync_file_range - sync a file segment with disk

SYNOPSIS         top

       #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         top

       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         top

       On success, sync_file_range() returns 0; on failure -1 is returned
       and errno is set to indicate the error.

ERRORS         top

       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         top

       sync_file_range() appeared on Linux in kernel 2.6.17.

CONFORMING TO         top

       This system call is Linux-specific, and should be avoided in portable
       programs.

NOTES         top

   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         top

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

COLOPHON         top

       This page is part of release 3.71 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
       http://www.kernel.org/doc/man-pages/.

Linux                            2014-08-19               SYNC_FILE_RANGE(2)