使用 ucontext 模拟协程切换

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ucontext 定义如下

The ucontext_t type is a structure type suitable for holding the context for a user thread of execution. A thread's context includes its stack, saved registers, and list of blocked signals.

可以用来保存当前线程的堆栈和寄存器，保存执行现场，这就为在一个线程内进行任务切换提供了可能，这种多任务之间的切换就是协程。

示例

#include <stdio.h>
#include <ucontext.h>
#include <unistd.h>
#include <iostream>
using namespace std;

void coroutine_1(void* arg) 
{
	puts("This is from coroutine 1");
}

void coroutine_2(void* arg) 
{
	puts("This is from coroutine 2");
}

定义两个任务 coroutine_1 和 coroutine_2

void context_test()
{
	char stack_1[1024*128];
	ucontext_t co_1, co_save_1;

	getcontext(&co_1);
	co_1.uc_stack.ss_sp = stack_1;
	co_1.uc_stack.ss_size = sizeof(stack_1);
	co_1.uc_stack.ss_flags = 0;
	co_1.uc_link = &co_save_1;

	makecontext(&co_1, (void(*)(void))coroutine_1, 0); // To coroutine_1
	swapcontext(&co_save_1, &co_1); 

	puts("here 33 line");

	char stack_2[1024*128];
	ucontext_t co_2, co_save_2;

	getcontext(&co_2);
	co_2.uc_stack.ss_sp = stack_2;
	co_2.uc_stack.ss_size = sizeof(stack_2);
	co_2.uc_stack.ss_flags = 0;
	co_2.uc_link = &co_save_2;

	makecontext(&co_2, (void(*)(void))coroutine_2, 0); // To coroutine_2
	swapcontext(&co_save_2, &co_2); 

	puts("here 45 line");
}

int main() {
	context_test();
        puts("here 50 line");
	return 0;
}

用 co_1 表示一个新的 context，它指向下一个执行的 context 是 co_save_1。

makecontext 函数修改目前的上下文，去执行 coroutine_1 函数。swapcontext 触发任务切换，同时保存当前上下文到 co_save_1 中，一旦 coroutine_1 执行完会回到当前位置。

同理，用 co_2 表示另一个 context，它指向下一个执行的 context 是 co_save_2。

这样就可以看到如下执行顺序

 context_test——>  coroutine_1
coroutine_1 ——>   context_test
  context_test——>  coroutine_2
coroutine_2 ——>   context_test
 context_test  ——>  main 结束

运行结果

This is from coroutine 1
here 33 line
This is from coroutine 2
here 45 line
here 50 line

转载于:https://my.oschina.net/lvyi/blog/795658