C++ thread conversion error [duplicate] - c++

Question

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pthread function from a class
(9 answers)
Closed 9 years ago.
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);