pointer to pointers in C

Question

I need help with pointers in C. I've two structures a struct made like this:

    typedef struct s{
        s2** d;
        struct s* next;
    }s;


 typedef struct s2{
        c* fi;
        struct s2* next;
    }s2;

And I have a function like this one:

void modify(c* a, s2* b){   //c* is a pointer to a struct
    s* rd = malloc(sizeof(s);
    rd->next = NULL;
    //need also to initialize the field "d" of the "s" struct
}

This is generating error. I need the structure rd to point to b as in the example. I need to store a double pointer because s2 are linked in list-like fashion, so I need the ** pointer to have the possibility to remove the first element of the list. I was doing the assignment in the comment like rd->d = &b but when I try to deference the field "d" in a function I have an invalid memory read and I can't understand why.

Could you explain why you need a double pointer on the 'd' type? It isn't obvious at all from your description. — jv42
– jv42, Commented Mar 19, 2011 at 14:23
what is d in struct s? Why are you trying to assign a s2* to a d**? — Ptival
– Ptival, Commented Mar 19, 2011 at 14:25
@Ptival: Sorry, It was s2**, I made a mistake trying to semplify things to let you understand the question. @jv42: because the s2 is stored inside another struct (like s2*) and I need to be able to set this pointer to NULL. I know this is quite complicated.. — Raffo
– Raffo, Commented Mar 19, 2011 at 14:32

ig2r · Accepted Answer · 2011-03-19 14:47:44Z

I suppose the problem is this:

You pass in s2* b as an argument to modify, thus this pointer resides on the stack. Now when you assign rd->d = &b, you take that location on the stack, which will only be valid until execution leaves the scope of modify. Thus, when you dereference rd->d later on, you access that (now invalid) location on the stack, which yields garbage or a crash. (However, in this scenario, you should be able to dereference rd->d correctly while still in modify.)

Probably you'd want to alter how b is passed into modify, most likely into something like s2** b, so that you can correctly pass in a pointer to a pointer to s2 in another structure, instead of making it a pointer to s2 sitting on the stack for modify.

Basically like this:

void modify(c* a, s2** b) {
    s* rd = malloc(sizeof(s));
    rd->next = NULL;
    rd->d = b;
}

and call it like

s2* myS2 = ...;
modify(<whatever>, &myS2->next);

This should allow you to pass the location of a pointer to a s2 instance which you can store away and dereference even after modify finishes (untested, though).

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