Declaring all variables as pointers vs. using address operators in C

Question

The following two blocks of code do the same thing:

void foo(int *num) {...}

int main(void)
{
    int num = 5;
    foo(&num);

    ...
}

void foo(int *num) {...}

int main(void)
{
    int *num;
    *num = 5;
    foo(num);

    ...
}

In both cases foo expects a pointer-to-int argument. In the first I declare an int and use the address operator (&) to pass it into foo. In the second I declare a pointer variable and directly pass it into foo.

Is there a reason to use either one, other than a stylistic preference? I prefer to declare all of my variables as pointers, as in the second, but I see the first a lot more frequently.

Answer 1

While the first code snippet gets the address of a variable and passes that to foo, the second dereferences an uninitialized pointer. What you instead could have done was:

int *num = malloc(sizeof(int));
*num = 5;

foo(num);

free(num);

Or:

int num = 5;
int *p = #
foo(p);

The main thing to take away from this is that the first form is the simplest. A valid pointer variable must be pointing to memory within your program's control, and that requires more work than just int *num; *num = 5;