When and why to use multiple indirection ?

Hi all,

I'm new to C programming and I'm reading a book about C data structures to get more expertise with the language. I'm a Java Programmer and I really can't figure out why multiple indirection is used :(

Such the linked list example below.:

Code:

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int list_rem_next(List *list, ListElmt *element, void **data);

---

Why "data" is a pointer to pointer ? Wouldn't be a normal pointer *data enough ? Since a pointer is a reference to an address ...

Thank you very much
Marcos

Why do pointers exist? So that you can change the value of a variable from another scope, for one thing. What if the variable you wanted to change was a pointer itself?

Code:

---

char name[] = "Anonymous";
set_name(&name);

void set_name(char **name) {
    strcpy(*name, "Samuel");
}

---

That's the main reason for pointers-to-pointers. If pointers to variables are a good thing, then sooner or later you're going to need a pointer to another pointer.

void pointers are a special case. Because a void pointer can hold the value of any pointer, of any indirection level, this code is perfectly valid:

Code:

---

int x;
void *a = &x, *b = &a, *c = &c;
*(int *)a = 0;
**(int **)b = 0;
***(int ***)c = 0;

---

You just have to remember what you stored in the void pointer when you decide to go and retrieve it.

There are issues with pointers-to-pointers, of course. const doesn't play well with them, for reasons I don't feel like explaining. Many people would rather return a modified pointer rather than take a pointer-to-pointer as a parameter to a function:

Code:

---

char name[] = "Anonymous";
strcpy(name, set_name(name));

char *set_name(char *name) {
    return "Samuel";
}

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But pointers-to-pointers definitely have their place. If you do a board search you might turn up some more information.