14.12 Pointer Arithmetic at Low Level

The behavior of pointer arithmetic is theoretically defined only when the pointer values all point within one object allocated in memory. But the addition and subtraction operators can’t tell whether the pointer values are all within one object. They don’t know where objects start and end. So what do they really do?

Adding pointer p to integer i treats p as a memory address, which is in fact an integer—call it pint. It treats i as a number of elements of the type that p points to. These elements’ sizes add up to i * sizeof (*p). So the sum, as an integer, is pint + i * sizeof (*p). This value is reinterpreted as a pointer like p.

If the starting pointer value p and the result do not point at parts of the same object, the operation is not officially legitimate, and C code is not “supposed” to do it. But you can do it anyway, and it gives precisely the results described by the procedure above. In some special situations it can do something useful, but non-wizards should avoid it.

Here’s a function to offset a pointer value as if it pointed to an object of any given size, by explicitly performing that calculation:

#include <stdint.h>

void *
ptr_add (void *p, int i, int objsize)
{
  intptr_t p_address = (long) p;
  intptr_t totalsize = i * objsize;
  intptr_t new_address = p_address + totalsize;
  return (void *) new_address;
}

This does the same job as p + i with the proper pointer type for p. It uses the type intptr_t, which is defined in the header file stdint.h. (In practice, long long would always work, but it is cleaner to use intptr_t.)