Question

I was trying to understand void pointer typecasting in C. I wrote a program to try and create a segmentation fault, but I am not getting it. Can someone please explain to me why

Code:

#include <stdio.h>
typedef unsigned long U32; 
int main()
{
  void *obj;
  U32 value = 25;
  U32* ptr;
  printf("*(ptr)                           : %lu\n", *(ptr));
  obj = &value;
  ptr = &value;
  ptr++;  
  printf("*(U32 *)(obj)                    : %lu\n", *(U32 *)(obj)); 
  printf("*((U32 *)(obj) + 1)              : %lu\n", *((U32 *)(obj) + 1));   
  printf("*(U32 *)((U32 *)(obj) + 1)       : %lu\n", *(U32 *)((U32 *)(obj) + 1)); 
  printf("*(ptr)                           : %lu\n", *(ptr));
  return 0;
} 

Output :

*(ptr)                           : 458998657
*(U32 *)(obj)                    : 25
*((U32 *)(obj) + 1)              : 3215085752
*(U32 *)((U32 *)(obj) + 1)       : 3215085752
*(ptr)                           : 3215085752

The way I see it only the 2nd printf is legal as all others are referring to some uninitialized random memory and should cause a seg fault

Answer 1

 U32* ptr;
  printf("*(ptr)                           : %lu\n", *(ptr));

ptr is uninitialized so it is undefined behavior to dereference it,

  obj = &value;
  ptr = &value;
  ptr++;  
  printf("*(U32 *)(obj)                    : %lu\n", *(U32 *)(obj)); 

Ok, as long as obj is pointing to a valid U32 (and U32 has to be an unsigned long as you’re using %lu as the printf format)

  printf("*((U32 *)(obj) + 1)              : %lu\n", *((U32 *)(obj) + 1));   

You’re trying to dereference a pointer that points one U32 past any valid location, so it’s undefined behavior.

  printf("*(U32 *)((U32 *)(obj) + 1)       : %lu\n", *(U32 *)((U32 *)(obj) + 1)); 

Same as above.

  printf("*(ptr)                           : %lu\n", *(ptr));

Since you incremented ptr , it points one U32 past anything valid, which leads to undefined behavior.

As your code does things that invokes undefined behavior, it’s really hard and often impossible to reason about what is going to happen. It’s undefined. Something unexpected could happen. Something bad could happen. Nothing bad could happen, and so on. Doing something in C that you should not do, does not mean it will segfault.