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Although this is likely to work on many architectures, it does not provide maximum portabilityis much less portable than it could be.

Compliant Solution

On machines where pointers can be represented as integers, the types intptr_t and uintptr_t are provided for that purpose.  They are only guaranteed to be able to receive void pointers.

#include <stdint.h>
enum { ALIGN_MASK = 3 };
/* ... */
char *ptr1;
char *ptr2;
/* ... */
if (((uintptr_t)(void *)ptr1 & ALIGN_BITSMASK) == ((uintptr_t)(void *)ptr2 & ALIGN_BITSMASK)) {
  /* ... */

The header inttypes.h can be used instead of stdint.h to get the integer types in a hosted environment.

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