Warning¶

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.

Some details¶

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:

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 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.

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 sync_file_range() was called are committed to disk.

sync_file_range2()¶

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().