fibers.c

#define _XOPEN_SOURCE
#include <ucontext.h>
#include <stdio.h>
#include <stdlib.h>
#include <signal.h>

// If I make this 16KiB the program crashes. why?
#define STACK_SIZE (32 * 1024)

void task(int);
typedef void (task_entry_f)(int);

struct task;
struct task {
	int 			id;
	ucontext_t		ctx;
	struct task 	*prev;
	struct task 	*next;
};

int next_task_id = 1;
struct task *active = 0;

void spawn(task_entry_f fn, int arg) {
	struct task *t = malloc(sizeof(struct task));

	t->id = next_task_id++;

	if (getcontext(&t->ctx) != 0) {
		printf("getcontext failed\n");
		exit(1);
	}

	t->ctx.uc_link = 0;
	t->ctx.uc_stack.ss_sp = malloc(STACK_SIZE);
	t->ctx.uc_stack.ss_size = STACK_SIZE;
	t->ctx.uc_stack.ss_flags = 0;
	if (t->ctx.uc_stack.ss_sp == 0) {
		printf("no stack\n");
		exit(1);
	}

	makecontext(&t->ctx, fn, 1, arg);

	if (active == 0) {
		t->next = t;
		t->prev = t;
		active = t;
	} else {
		t->next = active;
		t->prev = active->prev;
		active->prev->next = t;
		active->prev = t;
	}

	printf("spawned task %d\n", t->id);
}

void yield() {
	struct task *yielder = active;
	active = yielder->next;
	if (yielder == active) {
		return;
	}
	swapcontext(&yielder->ctx, &active->ctx);
}

void task(int n) {
	printf("initialising task %d\n", n);
	for (;;) {
		printf("hello task %d\n", n);
		yield();
	}
}

void start() {
	printf("starting...\n");
	setcontext(&active->ctx);
	printf("you will never see this.\n");
}

int main(int argc, char *argv[]) {
	spawn(&task, 1);
	spawn(&task, 2);
	spawn(&task, 3);
	spawn(&task, 4);
	start();
	printf("you will never see me either!\n");
	return 0;
}