"Address of" (&) an array / address of being ignored be gcc?

Question

Hi, I am a teaching assistant of a introductory programming course, and some students made this type of error:

char name[20];
scanf("%s",&name);

which is not surprising as they are learning... What is surprising is that, besides gcc warning, the code works (at least this part). I have been trying to understand and I wrote the following code:

void foo(int *v1, int *v2) {
  if (v1 == v2)
    printf("Both pointers are the same\n");
  else
    printf("They are not the same\n");
}

int main() {
  int test[50];
  foo(&test, test);
  if (&test == test)
    printf("Both pointers are the same\n");
  else
    printf("They are not the same\n");
}

Compiling and executing:

$ gcc test.c -g
test.c: In function ‘main’:
test.c:12: warning: passing argument 1 of ‘foo’ from incompatible pointer type
test.c:13: warning: comparison of distinct pointer types lacks a cast
$ ./a.out 
Both pointers are the same
Both pointers are the same

Can anyone explain why they are not different?

I suspect it is because I cannot get the address of an array (as I cannot have & &x), but in this case the code should not compile.

Edit: I know that an array by itself is the same as the address of the first element, but this is not related to this problem, I think. For example:

int main() {
  int a[50];
  int * p = a;
  printf("%d %d %d\n", p == a, p == &a[0], &p[0] == a);
  printf("%d %d %d\n", p == &a, &p == a, &p == &a);
}

prints:

$ ./a.out 
1 1 1
1 0 0

I don't understand why the second line begins with 1.

Answer 1

Actually, they are different, they don't have the same type at least.

But in C, the address of the array is the same as the address of the first element in the array that's why "they are not different", basically, they point to the same thing.

Answer 2

I believe this is a gcc optimization. Think about it.

• &test points to the address of test
• test points to the first element of test or &test[0]
• [0] is the same(for the most part) as *

So according to this &test could be different than test but gcc optimizes this away because there is no purpose of having an extra level of indirection at that point.

Answer 3

If You define an array like

char name[20];

name is implicitly convertible to char*, but &name is of the type char (*)[20] (a pointer to an array of 20 characters). The addresses are the same.

Check the address of (&name + 1). It differs form &name by the sizeof(char [20]).

Answer 4

The name of an array, in most circumstances, evaluates to the address of its initial element. The two exceptions are when it is the operand of sizeof or the unary &.

The unary & gives the address of its argument. The address of an array is the same as the address of its initial element, so (void*)&array == (void*)array will always be true.

array, when converted to a pointer to its initial element, has the type T *. The type of &array is T (*)[n], where n is the number of elements in the array. Thus,

int* p = array;        // Works; p is a pointer to array[0]
int* q = &array;       // Doesn't work; &array has type int (*)[10]
int (*r)[10] = &array; // Works; r is a pointer to array

Answer 5

In your example, the array test is a block of 50 ints. So it looks like this:

| int | int | ... | int |

When you apply the unary & operator to an array, you get the address of the array. Just like when you apply it to anything else, really. So &test is a pointer that points to that block of 50 ints:

(&test) -----------> | int | int | ... | int |

A pointer that points to an array of 50 ints has type int (*)[50] - that's the type of &test.

When you just use the name test in any place where it's not the operand of either the sizeof or unary-& operators, it is evaluated to a pointer to its first element. So the test that you pass to foo() evaluates to a pointer to the test[0] element:

(test) -----------------\
                        v
(&test) -----------> | int | int | ... | int |

You can see that these both are pointing to the same address - although &test is pointing to the whole array, and test is pointing to the first element of the array (which only shows up in the different types that those values have).