Slightly different take on http://dunkels.com/adam/pt/expansion.html (1)

coro.c

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

struct coro {
    unsigned int lc;
    bool done;
};

#define CORO_INIT(co) \
    (co)->lc = 0; \
    (co)->done = false

#define CORO_INITIALIZER \
    (struct coro){ .lc = 0, .done = false }

#define CORO_BEGIN(co) \
    switch ((co)->lc) { case 0:

#define CORO_END(co, res) \
    } (co)->lc = 0; (co)->done = true; return res

#define CORO_YIELD(co, res) \
    (co)->lc = __LINE__; return res; case __LINE__:

static int test(struct coro *co, int n)
{
    static int i;

    CORO_BEGIN(co);

    for (i = 0; i < 5; i++) {
        CORO_YIELD(co, n + i);
    }

    CORO_END(co, n + 5);
}

int main(void)
{
    struct coro co = CORO_INITIALIZER;

    int n = 100;

    while (!co.done) {
        n = test(&co, n);
        printf("Back in main with n = %d\n", n);
    }

    return 0;
}