Question

I’m trying to understand what this code does (and if it’s even allowed):

int * A;
int * B;
A = (int *)malloc( size_t somenumber );
B = A;

// bunch of stuff using B, B++, etc.

Everything I’ve read always shows equating things to pointers either using the reference operator (&) or the derefernce operator (*).

What does this sort of equating do?

And, when I ultimately free(A) what happens to B?

Answer 1

Pictures are always good when it comes to pointer confusion:

int * A; // create a pointer to an int named "A"
int * B; // create a pointer to an int named "B"
A = (int *)malloc( size_t somenumber ); // Allocate A some memory, now B is an 
                                        // uninitialized pointer; A is initialized,
                                        // but just to uninitialized memory

Conceptually:

B = A; // Assign B to the value of A (The uninitialized memory)

free(A);

So after all that I think you can see what’s happening. B is being assigned the value of A, which is the allocated and uninitialized memory chunk. So now you just have two pointers pointing to the same area.

As to the free() question, as you can see when you call free(A); you’re left with both A and B pointing to the same area, there’s just nothing allocated to your program there anymore. This is why when calling free() it’s good to set your pointer to NULL.

Now way back to your initial question. If you wanted to check of two pointers were ==:

int * A; // create a pointer to an int named "A"
int * B; // create a pointer to an int named "B"
A = (int *)malloc( size_t somenumber ); // Allocate A some memory, now B is an 
                                        // uninitialized pointer; A is initialized,
                                        // but just to uninitialized memory

if(B == A){
   // The pointers are pointing to the same thing!
}
if(*B == *A){
   // The values these pointers are pointing to is the same!
}

UPDATE
So to answer your updated questions, We need to change the definition of B.

int *A;  // A is a pointer to an int
int **B; // B is a pointer to a pointer to an int
B = &A;  // B is now pointing to A

So to illustrate that:

For B=*A:

int *A;
int B;
A = malloc(sizeof(int));
*A = 5;
B = *A;

This is a deference of A. So you’re just taking whatever A is pointing to and assigning it to B, in this case 5