C++ thread conversion error

Question

I'm from Colombia so the error is in Spanish. This seems the fastest place to get an answer...

I've been trying to make a simple program that creates threads and saves the information in a vector but when I build the code the next error appears:

...Lanzador.cpp|19|error: no se puede convertir ‘void* (Hilo::*)(void*)’ a ‘void* (*)(void*)’ para el argumento ‘3’ para ‘int pthread_create(pthread_t*, const pthread_attr_t*, void* (*)(void*), void*)’|

I have three files: the header for Hilos that says:

    #ifndef HILO_H
    #define HILO_H
    using namespace std;

    class Hilo
    {
        public:
            Hilo();
            virtual ~Hilo();
            void addHilo(int);
            void* ImprimirHilo(void*);

        protected:
        private:
        };

    #endif // HILO_H

Hilo.cpp that says:

    #include <iostream>
    #include <vector>
    #include <cstdlib>
    #include <pthread.h>
    #include <unistd.h>
    #include "Hilo.h"
    using namespace std;
    vector<int> info (1);
    Hilo::Hilo()
    {
    //ctor
    }

    Hilo::~Hilo()
    {
     //dtor
    }

    void Hilo::addHilo(int tiempo){

         info.push_back(tiempo);

    }


    void* Hilo::ImprimirHilo(void *threadid)
    {
         long tid;
         tid = (long) threadid;
        int n =info.at(tid);
        for (int i=n; i>0; i-- ){
        info.at(tid)=i;
        cout << "El hilo numero: " << tid << " tiene " << i <<" segundos"<< endl;
        sleep(1);
        }
        pthread_exit(NULL);
      }

And the class that has the main Lanzador.cpp

    #include <iostream>
    #include <cstdlib>
    #include <pthread.h>
    #include "Hilo.h"
    using namespace std;
    int main (){
        Hilo h;
        pthread_t threads;
        int tiempo=0;
        int rc;
        int contador=0;
        cout << "Para salir oprima 0 \n"<<endl;
        cout << "Escriba el tiempo del hilo" << endl;
        while (true){
            cin >> tiempo;
            if (tiempo>0){
                contador++;
                h.addHilo(tiempo);
                rc = pthread_create(&threads, NULL,&h.ImprimirHilo, (void*)contador);
                if (rc){
                     cout << "Error:unable to create thread," << rc << endl;
                     exit(-1);
                }
           }
      }


      pthread_exit(NULL);

   }

I hope that you people can help me. Sorry for the bad English and thanks for the help

Answer 1

Based on the signatures mentioned in the error message you are trying to use a member function where a void() function or pointer thereof is needed: using a member function directly won't work! You need to realize that a member function apparently taking no argument actually does have a hidden argument: the this pointer! Thus, an object needs to be provided. Since you use pthread_create() there isn't any way around that: you need to come up with a normal function taking no arguments. Since the function probably needs to be declared extern "C" you can't use a static member function either.

Something doesn't seem right, however, as pthread_create() actually takes a void*(*)(void*) as argument, not a void*(*)(void) as is claimed in the error message: the void* can be used to pass information to the thread entry function, e.g., a pointer to an object which could be restored to the correct type before calling a suitable member function.

Answer 2

You cannot use non static methods in threading. You will have to use the following pattern to get an instance aware method multi-threaded.

baseclass.h

class baseclass
{
    int a;
    static void __stdcall threaded(void* params);
}

baseclass.cpp

void __stdcall 
threaded(void* params)
{
    // this is your class instance
    pThis = static_cast<baseclass*>(params);
    printf("%d", pThis->a);
}

main.cpp

// class instance is entered as the parameter
rc = pthread_create(&threads, NULL, Hilo::ImprimirHilo, (void*)&h);