llco

A low-level coroutine library for C.

The main purpose of this project is to power sco and neco, which are more general purpose coroutine libraries.

Features

• Stackful coroutines.
• No allocations (bring your own stack)
• Cross-platform. Linux, Mac, Webassembly, iOS, Android, FreeBSD, Windows, RaspPi, RISC-V
• Fast context switching. Uses assembly in most cases
• No built-in scheduler. You are in charge of the coroutine priority
• Single file amalgamation. No dependencies.

Example

void cleanup(void *stack, size_t stack_size, void *udata) {
    // Free the coroutine stack
    free(stack);
}

void entry(void *udata) {
    printf("Coroutine started\n");
    // Switch back to the main thread and cleanup this coroutine
    llco_switch(0, true);
}

int main(void) {
    // Start a coroutine from the main function using an newly allocated stack.
    struct llco_desc desc = {
        .stack = malloc(LLCO_MINSTACKSIZE),
        .stack_size = LLCO_MINSTACKSIZE,
        .entry = entry,
        .cleanup = cleanup,
    };
    llco_start(&desc, false);
    printf("Back to main\n");
}

API

// Switch to another coroutine. Set the `co` param to NULL to switch to the 
// main function. Use the final param to tell the program that you are done
// with the current coroutine, at which point it's respective `cleanup` 
// callback will be called.
void llco_switch(struct llco *co, bool final);

// Start a new coroutine. This can be called from the main function or a 
// nested coroutine.
void llco_start(struct llco_desc *desc, bool final);

// Return the current coroutine or NULL if not currently running in a
// coroutine.
struct llco *llco_current(void);

// Returns a string that indicates which coroutine method is being used by
// the program. Such as "asm" or "ucontext", etc.
const char *llco_method(void *caps);

Caveats

• Windows: Only x86_64 is supported at this time. The Windows Fibers API is not currently suitable as a fallback due to the CreateFiber call needing to allocate memory dynamically.
• Webassembly: Must be compiled with Emscripten using the -sASYNCIFY flag.
• All other platforms may fallback to using ucontext when the assembly method is not available. The uco_method(0) function can be used to see if assembly or ucontext is being used.
• The ucontext fallback method only uses the ucontext API when starting a coroutine. Once the coroutine has been started, setjmp and longjmp take over the switching duties.

Compiler Options

• -DLLCO_NOASM: Disable assembly. Use ucontext fallback instead.
• -DLLCO_STACKJMP: Use setjmp and longjmp for jumping between stacks, even with the assembly method.
• -DLLCO_VALGRIND: Enable support for Valgrind
• -DLLCO_NOUNWIND: Disable support for stack unwinding

Running test

# Basic test
cc llco.c test.c && ./a.out

# Using Valgrind
cc -DLLCO_VALGRIND llco.c test.c && valgrind ./a.out

# Using Emscripten (Web Assembly)
emcc -sASYNCIFY llco.c test.c && node ./a.out.js

Acknowledgements

Much of the assembly code was adapted from the minicoro project by Eduardo Bart, which was originally adapted from the Lua Coco project by Mike Pall.

License

Public Domain or MIT No Attribution, your choice.