changing pointers in functions, c

Question

I'll say in advance that I'm asking about pointers to pointers, so my words may be a bit vague, but try to stay with me :)

I am trying to understand how changing a pointer passed as an argument takes effect outside the scope of the function. So far this is what I understand: if a pointer is passed as an argument to a function, I can only change what the pointer points to, and not the pointer itself (I'm talking about a change that will take effect outside the scope of the function, as I said). And if I want to change what the pointer is pointing to, I have to pass a pointer to a pointer.

Am I right so far? Also, I've noticed that when I have a struct that holds some pointers, if I want to initialize those pointers I have to pass the struct to the initialization function as a pointer to pointer. Is this for the same reason?

Answer 1

You are right in the first bit but if you've allocated the struct then you can pass a pointer to it. However, if the function allocated the struct, then either you use the function return to collect the value of the newly allocated struct or you pass in a pointer to a pointer in the parameter list.

(I've not got a c compiler to hand but I've tried to write some examples).

• You've allocated the pointer

    int main() {
       struct x *px = malloc(...);
       initx(px); 
    }

    void intix(struct x* px){
        px-> ....
    }
 

• The function allocated the pointer

     int main() {
       struct x *px = initx(); 
     }

     struct x* intix(){
        struct x *px = malloc(...);
        px-> ....
        return px;
     }
 

• The function allocated the pointer

     int main() {
       struct x *px;
       initx(&px); 
     }

     void intix(struct x** ppx){
        struct x *px = malloc(...);
        px-> ....
        *ppx = px;
     }