longjmp usage.

jmp.c

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
#include <string.h>
#include <setjmp.h>
#include <assert.h>
#include <unistd.h>
#include <time.h>
#include "list.h"

struct task {
	jmp_buf env;
	struct list_head list;
};

static LIST_HEAD(tasklist);
static void (*tasks[2])(void *);
static int ntasks;
static jmp_buf sched;

static void task_add(struct list_head *tasklist, jmp_buf env)
{
	struct task *t = malloc(sizeof(*t));
	memcpy(t->env, env, sizeof(jmp_buf));
	INIT_LIST_HEAD(&t->list);
	list_add_tail(&t->list, tasklist);
}

static void task_switch(struct list_head *tasklist)
{
	jmp_buf env;

	if (!list_empty(tasklist)) {
		struct task *t = list_first_entry(tasklist, struct task, list);
		list_del(&t->list);
		memcpy(env, t->env, sizeof(jmp_buf));
		free(t);
		longjmp(env, 1);
	}
}

static void task_join(struct list_head *tasklist)
{
	jmp_buf env;

	while (!list_empty(tasklist)) {
		struct task *t = list_first_entry(tasklist, struct task, list);
		list_del(&t->list);
		memcpy(env, t->env, sizeof(jmp_buf));
		free(t);
		longjmp(env, 1);
	}
}

void schedule(void)
{
	static int i;

	srand(time(NULL));

	setjmp(sched);

	while (ntasks-- > 0) {
		int n = rand() % 5;
		tasks[i++](&n);
		printf("Never reached\n"); 
	}

	task_join(&tasklist);
}

// A task yields control n times

void task0(void *arg)
{
	jmp_buf env;
	static int n;
	static int i;
	n = *(int *)arg;

	printf("Task 0: n = %d\n", n);

	if (setjmp(env) == 0) {
		task_add(&tasklist, env);
		// Jump back to scheduler
		longjmp(sched, 1);
	}

	for (i = 0; i < n; i++) {
		if (setjmp(env) == 0) {
			task_add(&tasklist, env);
			task_switch(&tasklist);
		}
		printf("Task 0: resume\n");
	}

	printf("Task 0: complete\n");
	longjmp(sched, 1);
}

void task1(void *arg)
{
	jmp_buf env;
	static int n;
	static int i;
	n = *(int *)arg;

	printf("Task 1: n = %d\n", n);

	if (setjmp(env) == 0) {
		task_add(&tasklist, env);
		// Jump back to scheduler
		longjmp(sched, 1);
	}

	for (i = 0; i < n; i++) {
		if (setjmp(env) == 0) {
			task_add(&tasklist, env);
			task_switch(&tasklist);
		}
		printf("Task 1: resume\n");
	}

	printf("Task 1: complete\n");
	longjmp(sched, 1);
}

int main(void)
{
	tasks[0] = task0;
	tasks[1] = task1;
	ntasks = 2;
	schedule();

	return 0;
}