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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!


  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) {
    while(1) {
        if(initiate_io()) {
            await(io_completed() || timer_expired(&timer));

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

async nested(struct async *pt){

async example(example_state *pt) {

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



przemub said…
'Tis more than cool! Yet another reminder that preprocessor enables some real magic.

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