Async.h - asynchronous, stackless subroutines

Taking inspiration from protothreads and async/await as found in C#, Rust and JS, this is a header-only async/await implementation for C based on Duff's device.

Features

1. It's 100% pure, 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.

API

Function	Description
async_begin(state)	Mark the beginning of an async subroutine
async_end	Mark the end of an async subroutine
async_yield	Yield execution until it's invoked again
await(cond)	Block progress until cond is true
await_while(cond)	Block progress while cond is true
async_exit	Terminate the current async subroutine
async_call(func, state)	Asynchronously call func(state) and return true if done executing (optional). You can also simply call func(state)directly which returns true/false.
async_init(state)	Initialize async subroutine state
async_done(state)	Returns true if async subroutine has completed execution, otherwise false

Examples

I ported the examples found in the protothreads distribution to async.h. Here is the async.h equivalent of the protothreads sample on the home page:

#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;
}

Most of the code looks very similar, with the main exceptions being the more concise names, and the fact that the async.h calls mostly don't need to accept the async structure/local continuation as an argument.

Here is the same example as above, but where the timer is lifted to a local parameter:

#include "async.h"

typedef struct { 
    async_state;    // declare the asynchronous state
    timer timer;    // declare local state
} example_state;
example_state pt;

async example(example_state *pt) {
    async_begin(pt);
    
    while(1) {
        if(initiate_io()) {
            timer_start(&pt->timer);
            await(io_completed() || timer_expired(&pt->timer));
            read_data();
        }
    }
    async_end;
}

So local parameters simply need to be lifted to members of the async state structure.

Nested Async Calls

You can also execute nested async subroutines in a manner reminiscent of fork-join 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));
    // OR call directly:
    //await(nested(&pt->nested1) & 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));
    // OR call the subroutines directly:
    //await(nested(&pt->nested1) | nested(&pt->nested2));

    async_end;
}

Caveats

1. Due to compile-time bug, MSVC requires changing: Project Properties > Configuration Properties > C/C++ > General > Debug Information Format From "Program Database for Edit And Continue" to "Program Database".
2. As with protothreads, you have to be careful with switch statements within an async subroutine. Stick to this simple rule and you'll never have trouble: place every switch in its own function. This is generally a good practice anyway.
3. As with protothreads, you can't make blocking system calls and preserve the async semantics. These must be changed into non-blocking calls that test a condition.