You don't need to start spawning new threads to use io_uring as a backend for synchronous IO APIs. You just need to set up the rings once, then when the program does an fwrite or whatever, that gets implemented as sending a submission queue entry followed by a single io_uring_enter syscall that informs the kernel there's something in the submission queue, and using the arguments indicating that the calling process wants to block until there's something in the completion queue.