Why / when to use `intptr_t` for type-casting in C?

Question

I have a question regarding using intptr_t vs. long int. I've observed that incrementing memory addresses (e.g. via manual pointer arithmetic) differs by data type. For instance incrementing a char pointer adds 1 to the memory address, whereas incrementing an int pointer adds 4, 8 for a double, 16 for a long double, etc...

At first I did something like this:

char myChar, *pChar;
float myFloat, *pFloat;

pChar = &myChar;
pFloat = &myFloat;

printf( "pChar:  %d\n", ( int )pChar );
printf( "pFloat: %d\n", ( int )pFloat );

pChar++;
pFloat++;

printf( "and then after incrementing,:\n\n" );
printf( "pChar:  %d\n", (int)pChar );
printf( "pFloat:    %d\n", (int)pFloat );

which compiled and executed just fine, but XCode gave me warnings for my typecasting: "Cast from pointer to integer of different size."

After some googling and binging (is the latter a word yet?), I saw some people recommend using intptr_t:

#include <stdint.h>
...

printf( "pChar:  %ld\n", ( intptr_t )pChar );
printf( "pFloat: %ld\n", ( intptr_t )pFloat );

which indeed resolves the errors. So, I thought, from now on, I should use intptr_t for typecasting pointers... But then after some fidgeting, I found that I could solve the problem by just replacing int with long int:

printf( "pChar:  %ld\n", ( long int )pChar );
printf( "pFloat: %ld\n", ( long int )pFloat );

So my question is, why is intptr_t useful, and when should it used? It seems superfluous in this instance. Clearly, the memory addresses for myChar and myFloat were just too big to fit in an int... so typecasting them to long ints solved the problem.

Is it that sometimes memory addresses are too big for long int as well? Now that I think about it, I guess that's possible if you have > 4GB of RAM, in which case memory addresses could exceed 2³² - 1 (max value for unsigned long ints...) but C was created long before that was imaginable, right? Or were they that prescient?

Thanks!

Answer 1

Here's the thing: on some platforms, int is the right size, but on others, long is the right size. How do you know which one is the one you should use? You don't. One might be right, but the standard makes no guarantees about which one it would be (if it is either). So the standard provides a type that is defined to be the correct size, regardless of what platform you're on. Where before you had to write:

#ifdef PLATFORM_A
  typedef long intptr;
#else
  typedef int intptr;
#endif

Now you just write:

#include <stdint.h>

And it covers so many more cases. Imagine specializing the snippet above for every single platform your code runs on.