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

static ucontext_t ctx[3];	//0 for main, 1 for f1, 2 for f2.
//-> swap				+-> return to
//main -> f2 -> f1 -> f2 +-> f1 +->main.

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)
{
    char st1[8192];
    char st2[8192];

    getcontext(&ctx[1]);
    ctx[1].uc_stack.ss_sp = st1;
    ctx[1].uc_stack.ss_size = sizeof st1;
    ctx[1].uc_link = &ctx[0];	//f1 returns to main.
    makecontext(&ctx[1], f1, 0);

    getcontext(&ctx[2]);
    ctx[2].uc_stack.ss_sp = st2;
    ctx[2].uc_stack.ss_size = sizeof st2;
    ctx[2].uc_link = &ctx[1];		//f2 returns to f1.
    makecontext(&ctx[2], f2, 0);	// assign ctx[2] to f2.

    swapcontext(&ctx[0], &ctx[2]);	// give CPU to f2.
    return 0;
}
/* OUTPUT
start f2
start f1
finish f2
finish f1
*/