FUSE's write sequence guarantees

Question

Should write() implementations assume random-access, or can there be some assumptions, like that they'll ever be performed sequentially, and at increasing offsets?

You'll get extra points for a link to the part of a POSIX or SUS specification that describes the VFS interface.

Answer 1

Random, for certain. There's a reason why the read and write interfaces take both size and offset. You'll notice that there isn't a seek field in the fuse_operations struct; when a user program calls seek/lseek on a FUSE file, the offset in the kernel file descriptor is updated, but the FUSE fs isn't notified at all. Later reads and writes just start coming to you with a different offset, and you should be able to handle that. If something about your implementation makes it impossible, you should probably return -EIO on the writes you can't satisfy.