Page 170 - C-Language

Pre-Standard History:

Prior to C89 during K&R-C times there was no type void* (nor header <stdlib.h>, nor prototypes,
and hence no int main(void) notation), so the pointer was cast to long unsigned int and printed
using the lx length modifier/conversion specifier.

The example below is just for informational purpose. Nowadays this is invalid code, which
very well might provoke the infamous Undefined Behaviour.

#include <stdio.h> /* optional in pre-standard C - for printf() */

int main()
{
int i;
int *p = &i;

printf("The address of i is 0x%lx.\n", (long unsigned) p);

return 0;
}

Printing the Difference of the Values of two Pointers to an Object

*1
Subtracting the values of two pointers to an object results in a signed integer . So it would be
printed using at least the d conversion specifier.

To make sure there is a type being wide enough to hold such a "pointer-difference", since C99
<stddef.h> defines the type ptrdiff_t. To print a ptrdiff_t use the t length modifier.

C99

#include <stdlib.h> /* for EXIT_SUCCESS */
#include <stdio.h> /* for printf() */
#include <stddef.h> /* for ptrdiff_t */

int main(void)
{
int a[2];
int * p1 = &a[0], * p2 = &a[1];
ptrdiff_t pd = p2 - p1;

printf("p1 = %p\n", (void*) p1);
printf("p2 = %p\n", (void*) p2);
printf("p2 - p1 = %td\n", pd);

return EXIT_SUCCESS;
}

The result might look like this:

p1 = 0x7fff6679f430
p2 = 0x7fff6679f434

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