#include <stdio.h>
#include <stdbool.h>
#include <ucontext.h>

static char stack[2][65536];            // a stack for each coroutine
static ucontext_t coroutine_state[2];   // container to remember context

// switch current coroutine (0 -> 1 -> 0 -> 1 ...)
static inline void
yield_to_next(void) 
{
    static int current = 0;

    int prev = current;
    int next = 1 - current;

    current = next;
    swapcontext(&coroutine_state[prev], &coroutine_state[next]);
}

static void
coroutine(int coroutine_number)
{
    int i;
    for (i = 0; i < 5; i++) {
        printf("Coroutine %d counts i=%d (&i=%p)\n", coroutine_number, i, &i);
        yield_to_next();
    }
}

int
main()
{
    ucontext_t return_to_main;

    // set up
    int i;
    for (i = 0; i < 2; i++) {
        // initialize ucontext_t
        getcontext(&coroutine_state[i]);
        // set up per-context stack
        coroutine_state[i].uc_stack.ss_sp = stack[i];
        coroutine_state[i].uc_stack.ss_size = sizeof(stack[i]);
        // when done, resume 'return_to_main' context
        coroutine_state[i].uc_link = &return_to_main;
        // let context[i] perform a call to coroutine(i) when swapped to
        makecontext(&coroutine_state[i], (void (*)(void))coroutine, 1, i);
    }

    printf("Starting coroutines...\n");
    swapcontext(&return_to_main, coroutine_state);
    printf("Done.\n");
    return 0;
}