sync_file_range — sync a file segment with disk
#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); |
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_BEFOREWait 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_WRITEInitiate 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_AFTERWait upon write-out of all pages in the range after performing any write.
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 caller.
Useful combinations of the flags bits are:
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_WRITEStart 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.
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.
fd is not a
valid file descriptor.
flags
specifies an invalid bit; or offset or nbytes is invalid.
I/O error.
Out of memory.
Out of disk space.
fd refers to
something other than a regular file, a block device, a
directory, or a symbolic link.
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().