I don't think I fully understand realloc, can someone explain to me why a realloc in my code sometimes fails but sometimes doesn't

Question

I'm writing a very simple list datatype in C (its horrendously inefficient, but it works) for this larger program but depending on how I use the code, I sometimes get segmentation faults but if my input data is slightly different the code fine.

The offending section is this:

struct Vec {
        int size;
        Node* ptr;
};

void push_to_vec(struct Vec* root, Node to_push) {
        root -> size ++;
        root -> ptr = realloc(root -> ptr, root -> size);
        root -> ptr[root -> size - 1] = to_push;
}

I tried debugging this code using print stamens and then with lldb, and it seems like the realloc call in the push_to_vec function is the line that fails.

Any help would be appreciated and thanks in advance.

Answer 1

You're not allocating enough space.

The second parameter to realloc is the size in bytes to allocate, not the number of array elements. As a result, you're writing past the end of allocated memory invoking undefined behavior

You need to multiply the number of elements by the element size.

root->ptr = realloc(root->ptr, root->size * sizeof(*root->ptr));