Free memory from a pointer to pointer - C destructor

Question

If I have the next function

Stack *construct_stack(void) {
  Stack *instance = (Stack *) malloc(sizeof(Stack));

  // Stuff...

  return instance;
}

It's sort of "constructor". But I'v been having problems with the "destructor".

If I do this I get an error.

void destruct_stack(Stack **st) {
  // Stuff...

  free(st);
}

Some idea?

Why you pass pointer to pointer ? just pass a pointer and free it . — ameyCU, Sep 05 '15 at 06:14
'Cause my code looks like this: `Stack *s = construct_stack(); // Stuff... destruct_stack(&s);` — mishelashala, Sep 05 '15 at 06:21

score 3 · Answer 1 · edited May 23 '17 at 12:22

3

void destruct_stack(Stack **st) {
  // Stuff...

  free(*st);//free the pointer which st points to
  *st = NULL;//set it to null
}

And You don't need to cast the result of malloc: Do I cast the result of malloc?

edited May 23 '17 at 12:22

Community

1
1

answered Sep 05 '15 at 06:25

Viktor Simkó

2,607
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ameyCU · Accepted Answer · 2015-09-05T06:22:23.867

void destruct_stack(Stack **st)

Here st is pointer to pointer(i.e instance) to which you allocate memory (st holds address of pointer instance).

 free(st); // you didn't allocate memory to st therefore gives error

So you should free *st like this

   free(*st);
   *st=NULL;  // just a good practice

Or another way instead of address of the pointer(i.e &instance) you can pass the pointer instance itself and then free it but you will need to change the type of parameter which is passed to function.

Free memory from a pointer to pointer - C destructor

2 Answers2