Detecting arch of a fat binary using the preprocessor

arch-test.c

#include <stdio.h>

int main () {
  if (sizeof(int*) == 4) {
    printf("ia32\n");
  }
  if (sizeof(int*) == 8) {
    printf("x64\n");
  }
  return 0;
}

test.sh

$ gcc -o arch-test arch-test.c -arch i386 -arch x86_64
$ ./arch-test
x64
$ arch -32 ./arch-test
ia32
$ arch -64 ./arch-test
x64