sync_file_range

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
       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 5.08 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)