Show HN: Async tasks in 350 lines of C

[rkaehn]

Tasks are a primitive for asynchronous programming and an alternative to working with threads directly. Instead of creating a thread, performing work, then waiting on it using synchronization primitives, you only define a unit of work and let a scheduler decide when and where to execute it. The threads that the scheduler uses are created once and reused for many tasks throughout the program's lifetime. In most cases, creating and executing a task requires not even a single syscall and is a significantly faster operation than creating a thread, resulting in lower latency. Furthermore, tasks from different subsystems can be automatically interleaved by the scheduler, which is difficult to achieve manually with threads without communication between the subsystems.

The library is written in standard C11 and only depends on the C POSIX library. It is designed to be easy to integrate into a project by just copying the header and simple enough to be understood in an afternoon.

[j1elo]

Oh boy, this reminds me of the good (not-so-)old times! Some years ago I was writing embedded C for Atmel AVR32 chips, and needed them to do several things "at the same time".

Started from learning about Protothreads (cooperative concurrency) as described by Adam Dunkels [1], and ended up devising a Task manager/runner library with a main loop, so multiple protothreads could be scheduled easily. At any given point in time, any of the scheduled tasks could be running or yielding on some continuation point.

There's some beauty in grasping these concepts from the ground up. Some time later I had to learn JavaScript, and the concept of async/await clicked so fast and nicely in my mind. Saving distances, the core idea is fundamentally the same, and that was enlightening (and fun!)

[1]: https://dunkels.com/adam/pt/

[swingingFlyFish]

God I love C. Thanks for reminding me.

[nineteen999]

Right with you there. We are going to cause the universe to implode tomorrow afternoon if we don't rewrite everything in Rust though.

[JonChesterfield]

Looks exactly like one would expect it to. Clear, boring C. Good stuff.

C11 has a <threads> header that should be usable in place of the pthreads api.

[LoganDark]

FWIW, I opened a PR that adds Windows support: https://github.com/rkaehn/cr_task.h/pull/2

I may play with the C11-standard `threads.h`, but note that it was not implemented by MSVC at all until quite recently: https://devblogs.microsoft.com/cppblog/c11-threads-in-visual...

Edit: Made a C11 threads.h implementation as well. https://github.com/rkaehn/cr_task.h/pull/3

[LoganDark]

I expected it to include yielding, but I'm actually glad it doesn't. This looks like a good base layer to build higher-level abstractions on. Useful stuff.

[Cieric]

I've actually been looking for something like this to translate over to zig. Thanks for sharing! The only thing it seems to be missing for my purposes is a way of returning a value.

[Cloudef]

For zig take a look at https://github.com/mitchellh/libxev and https://github.com/rsepassi/zigcoro

[rad_gruchalski]

How would an asynchronous system return a value? This is a job for something akin to a JS promise or golang channel.

[o11c]

Presumably you're supposed to include space for that as part of the argument when you construct the task in the first place, then just run until the task is complete. Given the awkwardness of returning bigger-than-pointer types anyway, this is actually easier than the alternative.

That said, the API here is undocumented and the chosen names are very confusing (implementations do not do what I would expect functions of those names to do).

[kccqzy]

How is this different from Apple's Grand Central Dispatch aka libdispatch? Based on my superficial understanding they appear to be quite similar.

[krackers]

Yeah to me they seem to fill a similar/same niche, both are implementations of dispatch queues. cr_task_run_sync mapping to dispatch_sync and cr_task_wait_request_signal + cr_task_run mapping to dispatch_async. This library seems to be slightly more low level in that by default you have to explicitly "run" the task whereas with GCD blocks are scheduled and run as soon as you enqueue them.

GCD is a lot more heavyweight though (but it brings a lot of niceties), whereas this is going to be much more portable.

[crest]

GCD does a lot more.

[samatman]

This is meant to be a friendly comment, because I think what you're doing here is cool and useful.

When I see "impressive thing in only a few lines of C", I think someone has invented a new way to crash my computer if I don't color inside invisible, underspecified lines.

What I'd want to see, if I were looking for a library like this, is tests. I see that you do have a test suite: that's good. I also see that it covers the basic functionality: that's a good start. There are a number of excellent tools for testing for the various woes which C code is prone to, consider integrating them.

C code doesn't have to be full of goblins which will venture forth at midnight and eat the candy out of your nose. But it should be suspected of such goblins until demonstrated otherwise. Few lines is actually good! And the test suite suggests you've implemented a good surface area here.

And 350 lines means you probably only need a couple thousand more to really, very thoroughly test the code. It will take some time but it's well worth it.

[Arelius]

Awesome, always great to see more task/job libraries...

I have used this sort of thing a lot though, and something I often find both essential, and overlooked is you really need something like `cr_task_sync_and_do_work` that isn't just entirely blocking on the current thread. Since as you build systems on this sort of API, you very quickly get bound by threads just stuck waiting on their children jobs.

Having said that, I realize that complicates the implementation quite a bit. The most elegant approach I have seen is one where you put tasks on fibers, and when you sync, instead of entering a sync loop, or waiting on a mutex, you instead suspend the fiber in the awaiting job, and put it back on the task queue. This was detailed by Naughty Dog in their Job System talk at GDC: https://www.gdcvault.com/play/1022186/Parallelizing-the-Naug...

[Cloudef]

I wrote implementation of "javascript promises" for c++ that is still used by the company. It needs event loop implementation for platform to work (few lines of code usually), but other than that it works pretty well. However, nowadays I probably would instead use coroutines, promises while cool the callbacky catch / then API isn't very nice. One difference to JS promises is that I implemented cancel-ability.

[gonzus]

How do you release all the resources created? I could not find any instances of `free()` in the code (and there are two calls to `malloc()`). Are you supposed to call `free()` yourself directly on the executor / task pointers? If so, I would suggest exposing a documented API to dispose of resources.

[rkaehn]

The tasks themselves come from a pool and are reused automatically, so no call to `free` is needed there. As for the executor, destroying it is a bit involved, because you need to wait for the currently executed task to finish before joining the threads and releasing the resources. It is a feature I do want to work on in the future though.

[marmaduke]

Since performance was a consideration for you, how does it compare to TBB or even say OpenMP? I've used those as well as the task system that comes with ISPC for HPC stuff, but I like the lightweight C11 approach here.

[sakras]

I'm almost certain this will be slower than OpenMP because it uses a centralized task queue that gets locked. OpenMP uses a decentralized work-stealing task queue called the Chase-Lev Deque. There's a C implementation in this paper:

https://fzn.fr/readings/ppopp13.pdf

[cryptonector]

This is a very well designed API. Well done u/rkaehn!

[up2isomorphism]

Posts like this realistically remind people that C won’t go away any time soon.

[mm007emko]

If no government bans it and mandates Linux kernel to be rewritten in Rust (and all higher language FFIs to be compatible with Rust rather than C), it's here to stay.

[eqvinox]

Purely out of curiosity — the indentation style is very unusual. Where did you pick it up?

[thenewwazoo]

It looks like bog-standard K&R style[0] to me. Exceedingly common, in my experience. What about it strikes you as odd?

[0] https://en.wikipedia.org/wiki/Indentation_style#K&R_style

[antirez]

That's effectively the most obvious indentation style used for C.

[colejohnson66]

It looks like nothing more than 1TBS[0] plus no line breaks inside functions.

[0]: https://softwareengineering.stackexchange.com/a/99546/54480

[dc-programmer]

What do lines 7-21 do?

[rkaehn]

The library is an "stb-style" header-only library, so the header has two modes: when you define CR_TASK_IMPL, it includes the implementation (starting from line 20), otherwise it just includes declarations (the first 19 lines). The implementation should only be included in one translation unit, but the declarations can be included in multiple units.

[salamander014]

Historically, C compilers attempted to build the list of all symbols in one pass of the files.

Sometimes, functions may call other functions in the same code file.

This required that functions be declared before they are referenced so C knew it existed.

You can also see this on lines 84-92.

[hinkley]

My first job our software ran on Windows, Linux and Mac. This was well before OS X, and I was horrified to learn that all the 'multitasking' in the Mac version was a message pump and longjmp calls all-the-fuck-over. Decode a few blocks of gzip compressed data, then check for network traffic. Then decode part of a JPEG. I don't know how they did it.

The were, however, the only people in the world with multiple monitors (outside of SGI).

[lexicality]

Huh, I didn't think the Obfuscated C Contest was open for submissions yet

[adamrezich]

What's obfuscated about it? Seems pretty straightforward to me at first glance.