Why it is ok to pass an uninitialized variable address to pointer paramter of a function

Question

let's say we have a C function like this,

void assign(int* refs)
{
 *refs = 123;
}

and call it in two ways

1.

int a;
assign(&a); // a is assigned a value.

int* a;
assign(a);// runtime error

I know that assign value to pointer that points to null is not ok since there is not correspond memory being allocated.
But
[1] why passing address of uninitialized variable works well?
[2] What does &a means when a hasn't been initialized.

Answer 1

The value is uninitialized, but the memory is allocated.Pointer points to memory, not to a value.

Answer 2

It's only a problem to read an uninitialized variable. Taking its address is fine since a variable's address is fixed for its lifetime.

In your first example you have an uninitialized variable of type int . You take its address (which is valid as above) and pass it to a function which subsequently dereferences that address to write the value of a .

In the second example, you have an uninitialized variable of type int * , but then you pass its value to the function. This is not valid because the value of the variable is indeterminate .