Assigning a pointer to a pointer in C results in a null pointer

Question

In C, I am creating a datastructure that includes an array of pointers to another structure. The array is defined like this (the structure is called Bucket).

Bucket *b_array[];

This array is part of another structure, called Object for the sake of this question. The entire typedef then looks like this:

typedef struct Object {
    Bucket *b_array[];
} Object;

Now, a bit further in the program, I am using this array and I am trying to create a variable that holds a pointer to one of the buckets in the array. The function gets a pointer to an Object structure (obj_ptr)and the index (idx) of the bucket. The code looks as follows:

Bucket *at_idx = *(obj_ptr->b_array + idx);

Here is the problem: Upon executing this code, the at_idx variable becomes a null pointer. GDB can confirm this for me.

(gdb) p at_idx
$1 = (Bucket *) 0x0

However, the thing that I am trying to assign to at_idx is not null. This is also something that GDB can confirm. Even weirder, the exact same expression is used in an if-statement a couple lines earlier where I check *(obj_ptr->b_array + idx) == NULL and that returns false.

I am out of ideas of what is going on here, but it will probably be something simple that I'm missing.

Any help will be appreciated!

I am sorry for the possibly vague context of the code, but this issue is happening in a university assignment. Because of university policy I am not allowed to share my full code.

@MaartenWeyns "Because of university policy I am not allowed to share my full code" - then provide a separate minimal reproducible example that demonstrates the same issue. — Remy Lebeau
– Remy Lebeau, Commented Dec 3, 2020 at 20:03

0___________ · Accepted Answer · 2020-12-03 20:11:16Z

1

typedef struct Object {
    Bucket *b_array[];
} Object;

First of all C does allow variable size arrays in empty structures.

typedef struct Object {
    int a;
    Bucket *b_array[];
} Object;

You need to allocate memory for the variable size array (and the structure itself as well).

Object *obj = malloc(sizeof(*obj) + NELEMENTS * sizeof(obj -> b_array[0]));

Then you may start to assign the values to the array.

answered Dec 3, 2020 at 20:11

0___________

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