sync_file_range — sync a file segment with disk
#define _GNU_SOURCE #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 synchronising the open file referred to by
the file descriptor fd
with disk.
offset
is the
starting byte of the file range to be synchronised.
nbytes
specifies the
length of the range to be synchronised, in bytes; if
nbytes
is zero, then
all bytes from offset
through to the end of file are synchronised. Synchronisation
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.
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.
None of these operations write 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.
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_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.
This is a traditional fdatasync(2)
operation. It 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.
I/O error.
flags
specifies an invalid bit; or offset
or nbytes
is invalid.
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.
fdatasync(2), fsync(2), msync(2), sync(2), feature_test_macros(7)
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