core dump on pthread_mutex_lock

Question

I'm trying to simulate a multi-threading environment, with each thread calling the Speak function of the Kevin class. However, there's a segmentation fault (after/during) the pthread_mutex_lock function and I couldn't find out why. The line after pthread_mutex_lock will never execute. Some says it could be a declaration problem of pthread_mutex_t, but I had it globally declared just after my include statements. EDIT: it's running on redhat linux 64bit using gcc version 4.1.2

#include <iostream>
#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
using namespace std;


pthread_mutex_t mutexsum;

class Kevin
{
public:
    Kevin();
    static void* Speak(void* value);
};

Kevin::Kevin()
{
    cout << "new instance of Kevin is created\n";
}

void* Kevin::Speak(void* value)
{
        cout<<"before lock"<<endl;
        pthread_mutex_lock (&mutexsum);
    cout << "Name: Kevin" << *((int*)value) << "\n" << "Seconds since epoch:" << "\nThread id:" << pthread_self() << endl;
        pthread_mutex_unlock (&mutexsum);
}

int main (int argc, char *argv[])
{
cout << "HI1" << endl;
int threadsNumb = atoi(argv[1]);
pthread_t callThd[threadsNumb];
long i;
void *status;
pthread_attr_t attr;

/* Assign storage and initialize values */


pthread_mutex_init(&mutexsum, NULL);

cout << "HI2" << endl;
/* Create threads to perform the dotproduct  */
pthread_attr_init(&attr);
pthread_attr_setdetachstate(&attr, PTHREAD_CREATE_JOINABLE);

cout << "HI3" << endl;
for(i=0;i<threadsNumb;i++)
  {
cout << "HI4" << endl;
   pthread_create(&callThd[i], &attr, Kevin::Speak, (void *)i);
   }

   cout <<"HI6"<<endl;
pthread_attr_destroy(&attr);
/* Wait on the other threads */

   cout <<"HI7"<<endl;
for(i=0;i<threadsNumb;i++) {

    cout <<"HI8"<<endl;
    pthread_join(callThd[i], NULL);

    cout <<"HI9"<<endl;
  }

pthread_mutex_destroy(&mutexsum);
pthread_exit(NULL);
}

The sample outputs with the 1st argument(thread number) 8 :

$ ./a.out 8
HI1
HI2
HI3
HI4
HI4
HI4
HI4
HI4
HI4
HI4
HI4
before lock
before lock
before lockbefore lock
before lock
before lock
Segmentation fault (core dumped)
$ ./a.out 8
HI1
HI2
HI3
HI4
HI4
HI4
HI4
HI4
HI4
HI4
HI4
HI6
HI7
HI8
before lock
Segmentation fault (core dumped)

Answer 1

It is not the mutex call, it is the cout after that. You reference the value as if it is a pointer while it is actually a long. Try the below changes:

void* Kevin::Speak(void* value)
{
    long i = (long) value; // cast from pointer to long
    cout<<"before lock"<<endl;
    pthread_mutex_lock (&mutexsum);
    cout << "Name: Kevin" << i << "\n" << "Seconds since epoch:" << "\nThread id:" << pthread_self() << endl;
    pthread_mutex_unlock (&mutexsum);
}

Answer 2

pthread_create(&callThd[i], &attr, Kevin::Speak, (void *)&i) // should be variable pointer
cout << "Name: Kevin" << *((long*)value) ... // int => long

Unlucky, even modified like this, This program is still wrong. Because:

The variable i is shared by all speak threads and the main thread, but the main thread uses i with no mutex lock.