Difference between %p and %x

Functions belonging to the printf function family have the type specifiers "%p" and "%x".

• "x" and "X" serve to output a hexadecimal number. "x" stands for lower case letters (abcdef) while "X" for capital letters (ABCDEF).
• "p" serves to output a pointer. It may differ depending upon the compiler and platform.

One specifier is often used instead of another on 32-bit systems, but it is a mistake. Here is an example:

int a = 10;
int *b = &a;
printf("%p\n",b);
printf("%X\n",b);

On a Win32 system, the following result will be printed:

0018FF20
18FF20

As you may see, the output results for "%p" and "%X" are rather similar. This similarity leads to inaccuracy in the code and this, in its turn, results in errors occurring when you port a program to a 64-bit platform. Most often it is "%X" that is used instead of "%p" to output the value of a pointer and it results in printing a wrong value if the object is situated outside the four less significant Gbytes of the address space. Let us consider the corresponding 64-bit version of this program:

size_t Gb = 1024*1024*1024;
char *a = (char *)malloc(2 * Gb * sizeof(char));
char *b = (char *)malloc(2 * Gb * sizeof(char));
printf("use %%X: a=%X\n", a);
printf("use %%X: b=%X\n", b);
printf("use %%p: a=%p\n", a);
printf("use %%p: b=%p\n", b); 
use %X: a=80000040
use %X: b=40010040
use %p: a=0000000080000040
use %p: b=0000000140010040

The pointer value "b" is printed incorrectly when using "%X". To make sure that such errors are quite real, see the post "Atavisms in large systems 2".
Here is one more example. Although it looks strange, the code given here in an abridged form was used in a real application in the UNDO/REDO subsystem:

// Here the pointers were saved in the form of a string
int *p1, *p2;
....
char str[128];
sprintf(str, "%X %X", p1, p2);
// In another function this string was processed
// in this way:
void foo(char *str)
{
  int *p1, *p2;
  sscanf(str, "%X %X", &p1, &p2);
  // The result is incorrect values of pointers p1 and p2.
  ...
}

Manipulation with the pointers using "%X" resulted in an incorrect program behavior on a 64-bit system. Note that such errors might occur very rarely. To diagnose these and other similar flaws it is good to use the analyzer Viva64.