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);
() permits fine control when synchronizing the open file
referred to by the file descriptor fd
is the starting byte of the file range to be synchronized.
specifies the length of the range to be synchronized, in bytes;
is zero, then all bytes from offset
through to the end
of file are synchronized. Synchronization is in units of the system page size:
is rounded down to a page boundary; (offset+nbytes-1)
rounded up to a page boundary.
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
- 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.
- Wait upon write-out of all pages in the range after performing any
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.,
) 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.
detect any I/O errors or ENOSPC
conditions and will return these to the
Useful combinations of the flags
- 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-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
- 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
is set to indicate the error.
- fd is not a valid file descriptor.
- flags specifies an invalid bit; or offset or nbytes
- I/O error.
- Out of memory.
- Out of disk space.
- fd refers to something other than a regular file, a block device,
or a directory.
() appeared on Linux in kernel 2.6.17.
This system call is Linux-specific, and should be avoided in portable programs.
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
() shown in the SYNOPSIS would force a register to be
wasted as padding between the fd
(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
() under the name sync_file_range