The async/await idiom is becoming increasingly popular. The first widely used language to include it was C#, and it has now spread into JavaScript and Rust. Now C/C++ programmers don't have to feel left out, because async.h is a header-only library that brings async/await to C!

Features:

It's 100% portable C. It requires very little state (2 bytes). It's not dependent on an OS. It's a bit simpler to understand than protothreads because the async state is caller-saved rather than callee-saved.

#include "async.h" struct async pt; struct timer timer; async example(struct async *pt) { async_begin(pt); while(1) { if(initiate_io()) { timer_start(&timer); await(io_completed() || timer_expired(&timer)); read_data(); } } async_end; }

This library is basically a modified version of the idioms found in the Protothreads library by Adam Dunkels, so it's not truly ground breaking. I've made a few tweaks that make it more understandable and generate more compact code, and I also think it more cleanly maps to the async/await semantics than it does to true threading.

Protothreads and async.h are both based around local continuations, but where protothreads are callee-saved, async.h is caller-saved. This eliminates the need to pass in the local continuation to any async operations except async_begin . This simplifies the macros that implement the async/await idiom, and even simplifies code that uses async.h.

Here's a simple example of fork-join style "parallelism":