How do you implement Coroutines in C++

Question

I doubt it can be done portably, but are there any solutions out there? I think it could be done by creating an alternate stack and reseting SP,BP, and IP on function entry

Answer 1

You should always consider using threads instead; especially in modern hardware. If you have work that can be logically separated in Co-routines, using threads means the work might actually be done concurrently, by separate execution units (processor cores).

But, maybe you do want to use coroutines, perhaps because you have an well tested algorithm that has already been written and tested that way, or because you are porting code written that way.

If you work within Windows, you should take a look at fibers. Fibers will give you a coroutine-like framework with support from the OS.

I am not familiar with other OS's to recommend alternatives there.

Answer 2

Based on macros as well (Duff's device, fully portable, see http://www.chiark.greenend.org.uk/~sgtatham/coroutines.html ) and inspired by the link posted by Mark, the following emulates co-processes collaborating using events as synchronization mechanism (slightly different model than the traditional co-routines/generator style)

// Coprocess.h
#pragma once
#include <vector>

class Coprocess {
  public:
    Coprocess() : line_(0) {}
    void start() { line_ =  0; run(); }
    void end()   { line_ = -1; on_end(); }
    virtual void run() = 0;
    virtual void on_end() {}; 
  protected:
    int line_;
};

class Event {
  public:
    Event() : curr_(0) {}

    void wait(Coprocess* p) { waiters_[curr_].push_back(p); }

    void notify() {
        Waiters& old = waiters_[curr_];
        curr_ = 1 - curr_; // move to next ping/pong set of waiters
        waiters_[curr_].clear();
        for (Waiters::const_iterator I=old.begin(), E=old.end(); I != E; ++I)
            (*I)->run();
    }   
  private:
    typedef std::vector<Coprocess*> Waiters;
    int curr_;
    Waiters waiters_[2];
};

#define corun()   run() { switch(line_) { case 0:
#define cowait(e) line_=__LINE__; e.wait(this); return; case __LINE__:
#define coend     default:; }} void on_end()

An example of use:

// main.cpp
#include "Coprocess.h"
#include <iostream>

Event e;
long sum=0;

struct Fa : public Coprocess {
    int n, i;
    Fa(int x=1) : n(x) {}
    void corun() {
        std::cout << i << " starts\n";
        for (i=0; ; i+=n) {
            cowait(e);
            sum += i;
        }
    } coend {
        std::cout << n << " ended " << i << std::endl;
    }   
};

int main() {
    // create 2 collaborating processes
    Fa f1(5);
    Fa f2(10);

    // start them
    f1.start();
    f2.start();
    for (int k=0; k<=100; k++) { 
        e.notify();
    }   
    // optional (only if need to restart them)
    f1.end();
    f2.end();

    f1.start(); // coprocesses can be restarted
    std::cout << "sum " << sum << "\n";
    return 0;
}

Answer 3

I dont think there are many full-blown, clean implementations in C++. One try that I like is Adam Dunkels' protothread library.

See also Protothreads: simplifying event-driven programming of memory-constrained embedded systems in the ACM Digital Library and discussion in Wikipedia topic Protothread,

Answer 4

It is based on (cringe) macros, but the following site provides an easy-to-use generator implementation: http://www.codeproject.com/KB/cpp/cpp_generators.aspx

Answer 5

I've come up with an implementation without asm code. The idea is to use the system's thread creating function to initialize stack and context, and use setjmp/longjmp to switch context. But It's not portable, see the tricky pthread version if you are interested.

Answer 6

Yes it can be done without a problem. All you need is a little assembly code to move the call stack to a newly allocated stack on the heap.

I would look at the boost::coroutine library.

The one thing that you should watch out for is a stack overflow. On most operating systems overflowing the stack will cause a segfault because virtual memory page is not mapped. However if you allocate the stack on the heap you don't get any guarantee. Just keep that in mind.