− sync a file segment with disk
_GNU_SOURCE /* See feature_test_macros(7) */
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 with disk.
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
flags bit-mask argument can include any of the
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.
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
SYNC_FILE_RANGE_WAIT_AFTER will detect any I/O errors
or ENOSPC conditions and will return these to the
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
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.
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_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
sync_file_range() returns 0; on failure −1 is
returned and errno is set to indicate the error.
fd is not a valid file
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.
appeared on Linux in kernel 2.6.17.
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:
sync_file_range2(int fd, unsigned int
off64_t offset, off64_t
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
fsync(2), msync(2), sync(2)
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