a simple coroutine implementation

coroutine.c

#include <setjmp.h>
#include <stdio.h>
#include <stdlib.h>

#define COROUTINE_START int r = setjmp(current->context);\
switch (r) {\
    case 0:

#define COROUTINE_PREEMPT if (current->iter % 500 == 0) {\
    longjmp(m, 1);\
}\
case 1:;

#define COROUTINE_END }\
longjmp(m, 1);

struct Coroutine {
	jmp_buf context;
	void (*func)();
	int started;
	int iter;
	int id;
};

struct Coroutine* current;
jmp_buf m;

void func()
{
    COROUTINE_START;
    while (1) {
        printf ("function: %s, id: %d, iter: %d \n", __func__, current->id, ++current->iter); 
        COROUTINE_PREEMPT;
    }
    COROUTINE_END;
}

#define CO_NUMS 5

int main()
{
    struct Coroutine *coroutines = malloc(sizeof(struct Coroutine) * CO_NUMS);

    for (int i = 0; i < CO_NUMS; i++) {
        coroutines[i].id = i;
        coroutines[i].started = 0;
        coroutines[i].iter = 0;
        coroutines[i].func = func;
    }

    switch (setjmp(m)) {
        case 0:
            while (1) {
                for (int i = 0; i < CO_NUMS; i++) {
                    current = &coroutines[i];

                    if (!current->started) {
                        current->started = 1;
                        current->func();
                    } else {
                        longjmp(current->context, 1);
                    }

                    case 1:;
                }
            }
    }
}