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
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:
rounded down to a page boundary;
(offset+nbytes-1) is rounded
up to a page boundary.
flags bit-mask argument can include any of the following
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 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.
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.
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
nbytes is invalid.
Out of memory.
Out of disk space.
fd refers to something other than a regular file, a block
device, or a directory.
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 sync_file_range() shown in the SYNOPSIS
would force a register to be wasted as padding between the
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().
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