C/Pointer - Declaring a local variable pointer without initializing it

Question

So, I tried doing something like this :

void place(struct node * list, int elem){                                                                                                                                                         
    struct node *tmp = list;                                                                                                                                                                      
    struct node *prev ;                                                                                                                                                                                                                                                                                                                                             
    while(tmp && tmp->info <= elem){                                                                                                                                                              
        prev = tmp;                                                                                                                                                                               
        tmp = tmp->next;                                                                                                                                                                          
    }                                                                                                                                                                                             
    struct node *new = (struct node *)malloc(sizeof(struct node));                                                                                                                                
    new->info = elem;                                                                                                                                                                             
    new->next = prev->next;                                                                                                                                                                       
    prev->next = new;                                                                                                                                                                             
}

And it gave me a segmentation fault. gdb didn't help - showed a backtrace full of 000000 and ??.

But when I tried this :

void place(struct node * list, int elem){                                                                                                                                                         
    struct node *tmp = list;                                                                                                                                                                      
    struct node *prev = tmp;                                                                                                                                                                                                                                                                                                                                             
    while(tmp && tmp->info <= elem){                                                                                                                                                              
        prev = tmp;                                                                                                                                                                               
        tmp = tmp->next;                                                                                                                                                                          
    }                                                                                                                                                                                             
    struct node *new = (struct node *)malloc(sizeof(struct node));                                                                                                                                
    new->info = elem;                                                                                                                                                                             
    new->next = prev->next;                                                                                                                                                                       
    prev->next = new;                                                                                                                                                                             
}

It worked fine ! The only difference between the two is that I'm initializing the local variable pointer prev in the second case, while I'm not doing so in the first case. But I can't see why the first case should be a segmentation fault ?

Can someone please explain this ?

Thanks!

Answer 1

Consider what happens if the while() condition is false when it's first encountered. prev will never be assigned anything sensible.