#define _XOPEN_SOURCE 600
#include <stdio.h>
#include <stdlib.h>
#include <ucontext.h>
#define MEM 64000
#include <stdio.h>
#include <ucontext.h>

static ucontext_t ctx[3];

static void f1 (void) {
	puts("start f1");
	swapcontext(&ctx[1], &ctx[2]);
	puts("finish f1");
}

static void f2 (void) {
	puts("start f2");
	swapcontext(&ctx[2], &ctx[1]);
	puts("finish f2");
}

int main (void) {
	puts("start main");
	char st1[8192];
	char st2[8192];

	/* Prepares ctx[1]. */
	getcontext(&ctx[1]);
	// Setup stack
	ctx[1].uc_stack.ss_sp = st1;
	ctx[1].uc_stack.ss_size = sizeof st1;
	// When ctx[1] ends swap to ctx[0]
	ctx[1].uc_link = &ctx[0];
	makecontext(&ctx[1], f1, 0);

	/* Prepares ctx[2]. When ctx[2] ends swap to ctx[0]. */
	getcontext(&ctx[2]);
	ctx[2].uc_stack.ss_sp = st2;
	ctx[2].uc_stack.ss_size = sizeof st2;
	// When ctx[2] ends swap to ctx[1]
	ctx[2].uc_link = &ctx[1];
	// ctx[2].uc_link = &ctx[0]; If we do this, we never print "finish f1".
	makecontext(&ctx[2], f2, 0);

	/* Finally run ctx[2] */
	swapcontext(&ctx[0], &ctx[2]);
	puts("finish main");
	return 0;
}