实现一个简单的C++协程库

之前看协程相关的东西时,曾一念而过想着怎么自己来实现一个给 C++ 用,但在保存现场恢复现场之类的细节上被自己的想法吓住,也没有深入去研究,后面一丢开就忘了。近来微博上看人在讨论怎么实现一个 user space 上的线程库,有人提到了 setcontext,swapcontext 之类的函数,说可以用来保存和切换上下文,我忽然觉得这应该也能用来实现协程,回头搜,果然已经有人曾用这些函数做过相关的事情,略略看了几个,觉得到底不大好用,还不如自己搞一个简单点的。

说到 c++ 上的协程,boost 里其实已经有相关的实现了,不过接口上看用起来有些麻烦,单纯从语法上来说,我觉得 Lua 的协程最简洁易用了,概念上也比较直接,为什么不做一个类似的呢?所以我就打算照着 Lua 来山寨一个,只需要支持四个接口就够了:

1)create coroutine。

2)run/resume coroutine。

3)Yield running corouinte。

4)IsCoroutineAlive。 

保存与恢复上下文

实现协程/线程,最麻烦莫过于保存和切换上下文了,好在 makecontext,swapcontext 这几个函数相当好用,已经完全帮忙解决了这个难题:makecontext 可以帮我们建立起协程的上下文,swapcontext 则可以切换不同的上下文,从而实现那种把当前函数暂时停住,切换出去执行别的函数然后再切换回来继续执行的效果:

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

static char g_stack[2048];
static ucontext_t ctx,ctx_main;

void func()
{
    // do something.
    cout << "enter func" << endl;

    swapcontext(&ctx, &ctx_main);

    cout << "func1 resume from yield" << endl;
    // continue to do something.
}

int main()
{
   getcontext(&ctx);
   ctx.uc_stack.ss_sp = g_stack;
   ctx.uc_stack.ss_size = sizeof g_stack;
   ctx.uc_link = &ctx_main;
    
   makecontext(&ctx, func, 0);

   cout << "in main, before coroutine starts" << endl;

   swapcontext(&ctx_main, &ctx);

   cout << "back to main" << endl;

   swapcontext(&ctx_main, &ctx);
   
   cout << "back to main again" << endl;
   return 0;
}

如上代码所示,显然我们只要简单包装一下 swapcontext,很容易就可以实现 Yield 和 Resume,有了它们的帮助协程做起来就容易多了。

使用与实现

在使用 makecontext,swapcontext 的基础上,我花了一个多小时简单实现了一个协程库,参看这里,代码写下来总共才200多行,出乎意料的简单,用起来也很方便了:

#include "coroutine.h"

#include <iostream>

using namespace std;

CoroutineScheduler* sched = NULL;

void func1(void* arg)
{
    uintptr_t ret;
    cout << "function1 a now!,arg:" << arg << ", start to yield." << endl;
    ret = sched->Yield((uintptr_t)"func1 yield 1");
    cout << "1.fun1 return from yield:" << (const char*)ret << endl;
    ret = sched->Yield((uintptr_t)"func1 yield 2");
    cout << "2.fun1 return from yield:" << (const char*)ret << ", going to stop" << endl;

}

void func2(void* s)
{
    cout << "function2 a now!, arg:" << s << ", start to yield." << endl;
    const char* y = (const char*)sched->Yield((uintptr_t)"func2 yield 1");
    cout << "fun2 return from yield:" << y <<", going to stop" << endl;
}

int main()
{
    sched = new CoroutineScheduler();

    bool stop = false;
    int f1 = sched->CreateCoroutine(func1, (void*)111);
    int f2 = sched->CreateCoroutine(func2, (void*)222);

    while (!stop)
    {
        stop = true;
        if (sched->IsCoroutineAlive(f1))
        {
            stop = false;
            const char* y1 = (const char*)sched->ResumeCoroutine(f1, (uintptr_t)"resume func1");
            cout << "func1 yield:" << y1 << endl;
        }

        if (sched->IsCoroutineAlive(f2))
        {
            stop = false;
            const char* y2 = (const char*)sched->ResumeCoroutine(f2, (uintptr_t)"resume func2");
            cout << "func2 yield:" << y2 << endl;
        }
    }

    delete sched;
    return 0;
}

如上所示,Yield 里传的参数会在调用 Resume 时被返回,同理 Resume 里的第二个参数,会在 Yield 里被返回,这种机制也是模仿 Lua 来的,有些时候可以用来在协程间传递一些参数,很方便,看起来也挺酷的,但在实现上却相当地简洁,核心代码如下:

// static function
void CoroutineScheduler::SchedulerImpl::Schedule(void* arg)
{
    assert(arg);
    SchedulerImpl* sched = (SchedulerImpl*) arg;

    int running = sched->running_;

    coroutine* cor = sched->id2routine_[running];
    assert(cor);

    cor->func(cor->arg);

    sched->running_ = -1;
    cor->status = CO_FINISHED;
}

// resume coroutine.
uintptr_t CoroutineScheduler::SchedulerImpl::ResumeCoroutine(int id, uintptr_t y)
{
    coroutine* cor = id2routine_[id];
    if (cor == NULL || cor->status == CO_RUNNING) return 0;

    cor->yield = y;
    switch (cor->status)
    {
        case CO_READY:
            {
                getcontext(&cor->cxt);

                cor->status = CO_RUNNING;
                cor->cxt.uc_stack.ss_sp = cor->stack;
                cor->cxt.uc_stack.ss_size = stacksize_;
                // sucessor context.
                cor->cxt.uc_link = &mainContext_;

                running_ = id;
                makecontext(&cor->cxt, (void (*)())Schedule, 1, this);
                swapcontext(&mainContext_, &cor->cxt);
            }
            break;
        case CO_SUSPENDED:
            {
                running_ = id;
                cor->status = CO_RUNNING;
                swapcontext(&mainContext_, &cor->cxt);
            }
            break;
        default:
            assert(0);
    }

    uintptr_t ret = cor->yield;

    if (running_ == -1 && cor->status == CO_FINISHED) DestroyCoroutine(id);

    return ret;
}

uintptr_t CoroutineScheduler::SchedulerImpl::Yield(uintptr_t y)
{
    if (running_ < 0) return 0;

    int cur = running_;
    running_ = -1;

    coroutine* cor = id2routine_[cur];

    cor->yield = y;
    cor->status = CO_SUSPENDED;

    swapcontext(&cor->cxt, &mainContext_);
    return cor->yield;
}

单就代码量和程序结构而言,以上的实现很简洁,但细节上看,每个协程都要分配一个一定大小的栈空间,空间效率上可能不大好,不够轻量;运行效率上来说,swapcontext 的执行效率如何,现在也未知,只是出于学习的目的,就先这样吧,可以再了解了解别人是怎么做的。 

原文地址:https://www.cnblogs.com/catch/p/3617962.html