Section: Linux Programmer's Manual (2)
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
is the starting byte of the file range to be synchronized.
specifies the length of the range to be synchronized, in bytes; if
is zero, then all bytes from
through to the end of file are synchronized.
Synchronization is in units of the system page size:
is rounded down to a page boundary;
is 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 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.
Wait upon write-out of all pages in the range
after performing any write.
as 0 is permitted, as a no-op.
This system call is extremely dangerous and should not be used in portable
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.
will detect any
I/O errors or
conditions and will return these to the caller.
Useful combinations of the
- SYNC_FILE_RANGE_WAIT_BEFORE | SYNC_FILE_RANGE_WRITE
Ensures that all pages
in the specified range which were dirty when
was called are placed
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
This is not suitable for data integrity operations.
- SYNC_FILE_RANGE_WAIT_BEFORE (or SYNC_FILE_RANGE_WAIT_AFTER)
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
was called are committed to disk.
returns 0; on failure -1 is returned and
is set to indicate the error.
is not a valid file descriptor.
specifies an invalid bit; or
Out of memory.
Out of disk space.
refers to something other than a regular file, a block device, or
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
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
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
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