Free uninitiailized pointer in C

Question

If I dynamically allocated a space for a pointer, list this:

int *a = (int*)malloc(sizeof(int));

should I free a when the code is done? Thanks!

The space for the pointer isn't allocated dynamically, the `int` that it points to is. When the code is done, you should free it with `free(a);`. — Blaze, Mar 07 '19 at 14:56
1. Don't cast `malloc`. 2. It is a good practice to free allocated memory and is always recommended, but it's not necessary if you use the pointer till the absolute end of the program. — Susmit Agrawal, Mar 07 '19 at 14:57
Rule of thumb: anything that has been allocated by `malloc/calloc/realloc/` needs to be freed with `free` at some point. — Jabberwocky, Mar 07 '19 at 14:58
I think I've got your question now. `a` is not uninitialized here. But the `int` _pointed_ by `a` is uninitialized. Yes you need to free `a` a some point. — Jabberwocky, Mar 07 '19 at 15:08
@Jabberwocky: Also `strdup`, `asprintf` and `vasprintf`. Maybe others? — abelenky, Mar 07 '19 at 15:12
Why you shouldn't cast a malloc: https://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc — Jose, Mar 07 '19 at 15:13
@abelenky correct, but `asprintf` is a gcc extension, and `strdup` is posix. — Jabberwocky, Mar 07 '19 at 15:14
Caution: the value passed to `free` must be *exactly the same* as was obtained from `malloc`, not one that has been, for example, incremented. The pointer's **value** is its only identity, as far as `free` is concerned. — Weather Vane, Mar 07 '19 at 15:18

Support Ukraine · Answer 1 · 2019-03-07T15:20:03.830

I think you have a little misunderstanding related to pointer.

Your title says:

Free uninitialized pointer ...

and your code is

int *a = (int*)malloc(sizeof(int));

The problem with this is that there is no uninitialized pointer in the code. The only pointer in the code is the variable a and it is initialized by the value returned by malloc.

Freeing an uninitialized pointer would be bad - example:

int *a;  // a is an uninitialized pointer

free(a);  // Real bad - don't do this

but since you actually initialize the pointer then - Yes, you must call free when your are done using the object/memory pointer a points to. It does not matter whether or not the pointed-to object (aka memory) has been assigned a value.

The general rule: For each call of malloc there must be a call of free

(Exception: If your program terminates, you don't need to call free)

abelenky · Answer 2 · 2019-03-07T15:23:23.443

3

int *a = malloc(sizeof(*a));
if (a) 
{
    /* a is now valid; use it: */
    *a = 1 + 2 + 3;
    printf("The value calculated is %d\n", *a);
}

/* Variable A is done being used; free the memory. */
free(a);  /* If a failed to be allocated, it is NULL, and this call is safe. */

edited Mar 07 '19 at 15:23

answered Mar 07 '19 at 15:14

abelenky

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EvilTeach · Accepted Answer · 2019-03-07T21:30:35.043

0

Yes. If you successfully malloc something is is correct to free it as well.

int *a = (int *) malloc(sizeof int);
if (a != NULL)
{
    /* Do whatever you need to do with a */
    free(a);
}
else
{
    puts("the malloc function failed to allocate an int");
}

edited Mar 07 '19 at 21:30

answered Mar 07 '19 at 15:04

EvilTeach

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It is safe and well-defined to `free(NULL)`. – Weather Vane Mar 07 '19 at 15:04
1

err..... that code looks like rather pointless, and the cast is not needed. – Jabberwocky Mar 07 '19 at 15:04
So for a linked list, as we know the first node of the list is empty, and we should free the first node at the end of the program?? – Liu Mar 07 '19 at 15:05
@weather. That is true in c++. – EvilTeach Mar 07 '19 at 15:05
1

@EvilTeach ... and in C – Jabberwocky Mar 07 '19 at 15:05
1

C17 § 7.22.3.3 *If `ptr` is a null pointer, no action occurs.* – Weather Vane Mar 07 '19 at 15:07
1

I'm considering to DV because the code here is really pointless.... 1. `free(NULL);` is a NOP anyway, 2. the whole code is pointless because the memory is freed right after being allocated, the code is not wrong, it's a bad example. – Jabberwocky Mar 07 '19 at 15:09

score 0 · Answer 4 · answered Mar 07 '19 at 15:25

0

int *a = (int*)malloc(sizeof(int));

should I free a when the code is done?

The question should be

Must I free a when the code is done?

And the answer is YES. A malloc must be accompanied by a a free statement.

free(a);

answered Mar 07 '19 at 15:25

VHS

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Free uninitiailized pointer in C

4 Answers4