context.c

/*

  $ cc context.c && ./a.out

	foo() - 1
	bar() - 1
	foo() - 2
	bar() - 2
	foo() - 3
	bar() - 3
	bar() - 4
	foo() - 4

*/

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

void
foo(ucontext_t* thisctx, ucontext_t* nextctx) {
	printf("foo() - 1\n");
	swapcontext(thisctx, nextctx);
	printf("foo() - 2\n");
	swapcontext(thisctx, nextctx);
	printf("foo() - 3\n");
	swapcontext(thisctx, nextctx);
	printf("foo() - 4\n");
	swapcontext(thisctx, nextctx);
}

void
bar(ucontext_t* thisctx, ucontext_t* nextctx) {
	printf("bar() - 1\n");
	swapcontext(thisctx, nextctx);
	printf("bar() - 2\n");
	swapcontext(thisctx, nextctx);
	printf("bar() - 3\n");
	swapcontext(thisctx, nextctx);
	printf("bar() - 4\n");
	swapcontext(thisctx, nextctx);
}

int
main() {
	const STACK_SIZE = 512;
	ucontext_t foo_ctx, bar_ctx, blank_ctx;
	uint8_t foo_stack[STACK_SIZE];
	uint8_t bar_stack[STACK_SIZE];

	getcontext(&foo_ctx);
	foo_ctx.uc_stack.ss_sp = foo_stack;
	foo_ctx.uc_stack.ss_size = STACK_SIZE;

	getcontext(&bar_ctx);
	bar_ctx.uc_stack.ss_sp = bar_stack;
	bar_ctx.uc_stack.ss_size = STACK_SIZE;

	makecontext(&foo_ctx, foo, 2, &foo_ctx, &blank_ctx);
	makecontext(&bar_ctx, bar, 2, &bar_ctx, &blank_ctx);
	swapcontext(&blank_ctx, &foo_ctx);
	swapcontext(&blank_ctx, &bar_ctx);
	swapcontext(&blank_ctx, &foo_ctx);
	swapcontext(&blank_ctx, &bar_ctx);
	swapcontext(&blank_ctx, &foo_ctx);
	swapcontext(&blank_ctx, &bar_ctx);
	swapcontext(&blank_ctx, &bar_ctx);
	swapcontext(&blank_ctx, &foo_ctx);
}