Why were the original read(2) and write(2) system calls written to offer synchronous I/O only? It's relatively easy to create synchronous I/O functions given asynchronous I/O primitives but it's impossible to do the opposite. Multics (at least) supported asynchronous I/O so the concept wasn't novel. And any multi-tasking kernel has to support asynchronous I/O internally so suitable code exists in the kernel. -- Peter Jeremy