sync_file_range - sync a file segment with disk
#define _GNU_SOURCE /* See feature_test_macros(7) */
int sync_file_range(int fd, off64_t offset, off64_t nbytes,
unsigned int flags);
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:
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.
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
Wait upon write-out of all pages in the range after performing
Specifying flags as 0 is permitted, as a no-op.
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.
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
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.
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
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 |
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.
On success, sync_file_range() returns 0; on failure -1 is returned and
errno is set to indicate the error.
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.
sync_file_range() appeared on Linux in kernel 2.6.17.
This system call is Linux-specific, and should be avoided in portable
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
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
fdatasync(2), fsync(2), msync(2), sync(2)
This page is part of release 3.65 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.