pointer of a pointer in linked list append

Question

I normally program in python. To increase performance of my simulations, I am learning C. I have a problem to understand the use of a pointer of a pointer when implementing

Answer 1

You don't need the if/else; in both cases you link the new node to a pointer that was NULL before the operation. This could be the root node, or the ->next node of the last node in the chain. Both are pointers to struct node, and you need a pointer to these pointers in order to assign to them.

void append( struct node **q, int num){

    while (*q){ q = &(*q)->link; }

    *q = malloc(sizeof **q);
    (*q)->data = num;
    (*q)->link = NULL;

}

Why would anyone do this? Basically because it is shorter, it uses only one loop and no additional conditions, uses no additional variables, and it can be proven to be correct. There should of course a test be added for the result of malloc, which would need an addtional condition.