Could I ever want to access the address zero?

Question

The constant 0 is used as the null pointer in C and C++. But as in the question \"Pointer to a specific fixed address\" there seems to be some possible use of assigning fixe

Answer 1

It may surprise many people, but in the core C language there is no such thing as a special null pointer. You are totally free to read and write to address 0 if it's physically possible.

The code below does not even compile, as NULL is not defined:

int main(int argc, char *argv[])
{
    void *p = NULL;
    return 0;
}

OTOH, the code below compiles, and you can read and write address 0, if the hardware/OS allows:

int main(int argc, char *argv[])
{
    int *p = 0;
    *p = 42;
    int x = *p; /* let's assume C99 */
}

Please note, I did not include anything in the above examples. If we start including stuff from the standard C library, NULL becomes magically defined. As far as I remember it comes from string.h.

NULL is still not a core C feature, it's a CONVENTION of many C library functions to indicate the invalidity of pointers. The C library on the given platform will define NULL to a memory location which is not accessible anyway. Let's try it on a Linux PC:

#include 
int main(int argc, char *argv[])
{
        int *p = NULL;
        printf("NULL is address %p\n", p);
        printf("Contents of address NULL is %d\n", *p);
        return 0;
}

The result is:

NULL is address 0x0
Segmentation fault (core dumped)

So our C library defines NULL to address zero, which it turns out is inaccessible. But it was not the C compiler, of not even the C-library function printf() that handled the zero address specially. They all happily tried to work with it normally. It was the OS that detected a segmentation fault, when printf tried to read from address zero.