Compiler warnings and notes but dropping & does not work - c

Question

In the below code snippet I'm getting a gcc compiler warning: "passing argument 1 of initArr from incompatible pointer type initArr(&stack,2);"
and a note: "expected 'struct Arr *' but argument is of type 'struct Arr**'"
which makes perfect sense to me.
As suggested by many a SO post, dropping the ampersand from the call to initArr(&stack, 2) however, results in another warning: "'stack' is used unitialized in this function: init(stack, 2);" and an immediate segfault error when ran.
What am I doing incorrectly?
I also tried using struct Arr in the malloc sizeof call and as expected, no change.
#include<stdio.h>
#include <stdlib.h>
#define TYPE int
struct Arr {
TYPE * data; // Pointer to the data array.
int size; // Number of elements in the array.
int capacity; // Capacity of the array.
};
void initArr(struct Arr * da, int capacity) {
da->data = malloc(sizeof(TYPE) * capacity);
da->size = 0;
da->capacity = capacity;
}
int main() {
struct Arr *stack;
initArr(&stack, 2);
return 0;
}

Answer 1

As haccks answer says,
you need to initialize your pointer, so it points to a real location
pass the pointer (rather than the address of the pointer variable)
int main()
{
struct Arr realArray; // allocate memory for the Arr (on the CPU's stack)
struct Arr *stack; // pointer variable (undefined)
stack = &realArray; // make variable "stack" point to realArray
initArr(stack, 2); // pass the pointer to realArray
}

Answer 2

stack is a pointer and it must be pointing to some valid memory location. Since stack is passed to the function and isn't initialized, you are getting a warning. In the function you are trying to access some random memory location which can cause the program crash or any erroneous behavior of the program, aka undefined behavior.

Answer 3

Change this:
struct Arr *stack;
To this:
struct Arr stack;

Answer 4

First look at the declaration of your initArr function:
void initArr(struct Arr * da, int capacity)
Your first argument is of type 'pointer to a struct'
Now look at your invocation of this function:
struct Arr *stack;
initArr(&stack, 2);
We see that stack is of type 'pointer to a struct', and in the function you are passing the address of stack (.i.e. the type is a pointer to a pointer to a struct). Which is what the gcc error is about, the note actually gives more information about the type that the function expects and the type that you are providing
There are two ways to fix this
leave the declaration of initArr as it is, and then change the function
invocation from initArr(&stack, 2) to initArr(stack, 2). In this case you will need to make sure that you have allocated memory for the structure prior to passing it to the function;
int main()
{
struct Arr* da = NULL;
da = malloc(sizeof(struct Arr));
initArr(da, 2);
}
or equivalently,
int main
{
struct Arr stack;
initArr(&stack, 2);
}
I would probably go for one of the above solutions.
Modify the declaration of initArr to initArray(struct Arr** da, int capacity). You will still need to make sure memory is allocated as before. However, if you change initArr to:
void initArr(struct Arr** da, int capacity)
{
*da = malloc(sizeof(struct Arr));
(*da)->data = malloc(sizeof(TYPE) * capacity);
(*da)->size = 0;
(*da)->capacity = capacity;
}
In this case I am changing the semantics of your initArr function so that I am initializing the structure all in one place. The code in your main function would remain unchanged.
N.B. For clarity, error checking has been omitted in the above code snippets, please don't do this in real code. Always check the return value from malloc to ensure that memory has been allocated.