Passing class member function to pthread_create

Hi,
I 'd like to pass a class member function to pthread_create which is called by another class member function of the same class.

Code:

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class A{
public:
void * func1(void *parm);
void  func2();
};

void A::func2(){
...
pthread_create(threads, NULL, & func1, (void *)(parm));
...
}

---

But gcc will give error:

Quote:

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error: ISO C++ forbids taking the address of an unqualified or parenthesized non-static member function to form a pointer to member function.

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How to fix this error? Thanks!

You will not be able to do that directly, since pthread_create() only allows you to specify one argument, but you need two -- the "this" pointer and the arg itself. The general pattern is:

Code:

---

void *LaunchMemberFunction(void *obj)
{
    MyClass *myObj = reinterpret_cast<MyClass *>(obj);
    return myObj->function();
}

---

But since you need to pass a parameter, you'll need a second level of indirection:

Code:

---

class MyLauncher
{
public:
    MyLauncher(MyClass *myObj, void *arg) : mObj(myObj), mArg(arg) {}

    void *launch() { return mObj->function(mArg); }

    MyClass *mObj;
    void *mArg;
};

void *LaunchMemberFunction(void *obj)
{
    MyLauncher *myLauncher = reinterpret_cast<MyLauncher *>(obj);
    return myLauncher->launch();
}

MyClass myObj;
void *myArg;
MyLauncher launcher(&myObj, myArg);
pthread_create(&thr, NULL, LaunchMemberFunction, &launcher);

---

It kinda makes you want to rethink what you're doing, doesn't it? ;)

Thanks brewbuck!
Since the way you gave is indirect, are there other solutions?

1. I've tried to make func1 as static member function, but since it uses non-static data member, so error follows.

2. is there some wrapper on pthread library or its equivalence for C++? Is pthread library the best choice for multithreading in C++?

Thanks!

Quote:

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Originally Posted by lehe

1. I've tried to make func1 as static member function, but since it uses non-static data member, so error follows.

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Right, that won't work.

Quote:

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2. is there some wrapper on pthread library or its equivalence for C++? Is pthread library the best choice for multithreading in C++?

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I'd say pthreads is still the right choice. The method I gave above could be made general by some careful templatizing.

You can of course also provide the LaunchMemberFunction() as a static function - but you still need to translate the static call and void * into object and member function call.

--
Mats

IMO the best choice is Boost.Thread, which supports member functions through Boost.Bind without any problems.

I usually create an abstract class of sorts, and each thread that I create inherits from that class. Since a thread is implemented via a subclass, having to pass parameters directly to the thread function isn't necessary. Example:

Code:

---


// Obviously not complete, but this should demonstrate the basic idea....
class thread
{
      public:
            thread() : code_(0) { }
            void start() { pthread_create(&pth_, NULL, thread::thread_router, reinterpret_cast<void*>(this)); }
            void join() { pthread_join(&pth_, NULL); }
           
            virtual int exec() = 0;
            int exit_code() { return code_; }

      private:
            static void* thread_router(void* arg);
            void exec_thread() { code_ = exec(); }
            pthread_t pth_;
            int code_;
};

void*
thread::thread_router(void* arg)
{
    reinterpret_cast<thread*>(arg)->exec_thread();
    return NULL;
}


class my_thread : public thread
{
    public:
        my_thread(int arg1, int arg2) : arg1_(arg1), arg2_(arg2) { }
        virtual int exec();
   
    private:
        int arg1_, arg2_;
};

int
my_thread::exec()
{
    // use arg1_, arg2_....
    return 0;
}


my_thread thr(1, 2);
thr.start();

// Wait for it to complete...
thr.join();

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