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:

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

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

But gcc will give error:

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

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!

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.

Right, that won't work.

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

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();