Question

I am getting Segmentation Fault for the following code which is weird cuz I don’t see where I am accessing un-initialized memory. I have tried to debug the code and found that this segmentation fault has something to do with *g inside thread procedure. Here is the code:

void *Func(void *arg);

int main()
{
    pthread_t tid;
    void *x;

    pthread_create(&tid,NULL,Func,NULL);
    pthread_join(tid,&x);
    int i=*(int *)x;
    printf("Data returned from the thread %d\n",i);

    return 0;
}

void *Func(void *arg)
{
    int *g;
    int i=2,j=3;
    printf("inside thread\n");      
    *g=i+j;
    printf("%d\n",*g);

    return g;
}

Answer 1

The problem is with the code below

int *g;
....
*g=i+j;

g is an uninitialised pointer. When you dereference it, you are trying to write to an undefined location in memory. The effects of this are undefined but a seg fault is very likely.

There are a number of ways you could address this, including

• allocate memory for g
• point g towards some allocated memory
• declare g on the stack in main and pass a pointer to it into your child thread