The C Coding Standard

Whenever I read a programming book I do notes, test the examples and try to solve the problems. I was reading the book Hacking: The Art of Exploitation, and just in the second chapter I wasn’t able to run a example. I checked the code and I didn’t made any mistake. The code was quiet simple:

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

int main() {
   int i;
   char char_array[5] = {'a', 'b', 'c', 'd', 'e'};
   unsigned int hacky_nonpointer;
   hacky_nonpointer = (unsigned int) char_array;

   for(i=0; i < 5; i++) {      
        printf("[hacky_nonpointer] points to %p, which contains the char '%c'\n",
            hacky_nonpointer, *((char *) hacky_nonpointer));
            hacky_nonpointer = hacky_nonpointer + sizeof(char);
   }
}

Basically the program iterate over the char array using a unsigned int pointer just to explain how typecasting works. Nonetheless, this code taken from the examples produced a segmentation fault. As you may observed, the code has no mistakes. However, this code is Non-compliant. A compliant code is a code that follows a standard.

The SEI CERT C Coding Standard documented in this wiki, has a section related to converting a pointer to integer or integer to pointer.

Even though, it makes sense to use integers and pointers interchangeably in C both are 32 bit length, according to the C standard, using integers as a pointer or viceversa may have undesired consequences. Read more here.

Any valid pointer to void can be converted to or from intptr_t or uintptr_t with no value change. Also, a pointer to void may be converted to or from a pointer to any type and back again, and the result must remain the same. Thus, direct conversion of a char pointer to a uintptr_t is allowed as long as the implementation supports the uintptr_t type.

In the example taken from the book, we were in the same case. There was a cast of a pointer to char to unsigned int, that must be changed to uintptr_t in to be consequent with the standard.

Code compliant version of the above code is:

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

int main() {
   int i;
   char char_array[5] = {'a', 'b', 'c', 'd', 'e'};
   uintptr_t hacky_nonpointer;
   hacky_nonpointer = (uintptr_t) char_array;

   for(i=0; i < 5; i++) { // Iterate through the int array with the int_pointer.
      printf("[hacky_nonpointer] points to %p, which contains the char '%c'\n",
        hacky_nonpointer, *((char *) hacky_nonpointer));
        hacky_nonpointer = hacky_nonpointer + sizeof(char);
   }
}

Now the output makes sense:

[hacky_nonpointer] points to 0x7ffcfaf9e433, which contains the char 'a'
[hacky_nonpointer] points to 0x7ffcfaf9e434, which contains the char 'b'
[hacky_nonpointer] points to 0x7ffcfaf9e435, which contains the char 'c'
[hacky_nonpointer] points to 0x7ffcfaf9e436, which contains the char 'd'
[hacky_nonpointer] points to 0x7ffcfaf9e437, which contains the char 'e'