sync_file_range(2) — Linux manual page

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, or a directory.

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 5.13 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                          2021-03-22             SYNC_FILE_RANGE(2)

Pages that refer to this page: fsync(2)posix_fadvise(2)syscall(2)syscalls(2)system_data_types(7)