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14.15 Pointer-Integer Conversion

On modern computers, an address is simply a number. It occupies the same space as some size of integer. In C, you can convert a pointer to the appropriate integer types and vice versa, without losing information. The appropriate integer types are uintptr_t (an unsigned type) and intptr_t (a signed type). Both are defined in stdint.h.

For instance,

#include <stdint.h>
#include <stdio.h>

print_pointer (void *ptr)
  uintptr_t converted = (uintptr_t) ptr;

  printf ("Pointer value is 0x%x\n",
          (unsigned int) converted);

The specification ‘%x’ in the template (the first argument) for printf means to represent this argument using hexadecimal notation. It’s cleaner to use uintptr_t, since hexadecimal printing treats the number as unsigned, but it won’t actually matter: all printf gets to see is the series of bits in the number.

Warning: Converting pointers to integers is risky—don’t do it unless it is really necessary.