async.h - asynchronous, stackless subroutines in C

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:

1. It's 100% portable C.
2. It requires very little state (2 bytes).
3. It's not dependent on an OS.
4. 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":

#include "async.h"

typedef struct { 
    async_state;
    struct async nested1;
    struct async nested2;
} example_state;
example_state pt;

async nested(struct async *pt){
    async_begin(pt);
    ...
    async_end;
}

async example(example_state *pt) {
    async_begin(pt);

    // fork two nested async subroutines and wait until both complete
    async_init(&pt->nested1);
    async_init(&pt->nested2);
    await(async_call(nested, &pt->nested1) & async_call(nested, &pt->nested2));
    
    // fork two nested async subroutines and wait until at least one completes
    async_init(&pt->nested1);
    async_init(&pt->nested2);
    await(async_call(nested, &pt->nested1) | async_call(nested, &pt->nested2));

    async_end;
}