Thread: casting int and float pointers

I'm just learning, and I've been playing with a copy of the GNU C Tutorial for fun. One of the questions asks "Write a statement which converts a pointer to an integer into a pointer to a double type." I think what the question is getting at is something like "my_double_ptr = (double *) my_int_ptr;".

But printing the values with printf didn't give me what I expected. So I tried to do something simpler first, and this is what I wrote just to see what would happen:

Code:

int main () 
{
  float my_float = 12.3456;
  float *my_float_ptr;
  int *my_int_ptr;

  my_float_ptr = &my_float;
  my_int_ptr = (int *) my_float_ptr;
  printf("My_float_ptr is %p\n", my_float_ptr);
  printf("My_int_ptr is %p\n", my_int_ptr);
  /* printf("My_float_ptr points to float %f\n", *my_float_ptr);
  printf("My_float_ptr points to int %d\n", *my_float_ptr); */
  printf("My_int_ptr points to float %f\n", *my_int_ptr);
  printf("My_int_ptr points to int %d\n", *my_int_ptr);

  return 0;
}

When I run it with commented lines included, I get the output:
My_float_ptr is 0x22efc4
My_int_ptr is 0x22efc4
My_float_ptr points to float 12.345600
My_float_ptr points to int -2147483648
My_int_ptr points to float 12.345598
My_int_ptr points to int 1095075732

But when I run it as shown, with comments commented, what I get is:
My_float_ptr is 0x22efc4
My_int_ptr is 0x22efc4
My_int_ptr points to float 0.000000
My_int_ptr points to int 1095075732

I can understand if ints and floats are represented differently in storage, then 12.3456 may print out as some screwy int. But what confuses me is why it outputs "My_int_ptr points to float 0.000000" in one case and "My_int_ptr points to float 12.345598" in the other, when all I did was remove a few printf lines.

Does anyone have any insight into this, or can anyone direct me to some documentation on the web where this might be explained?

Also, gcc -dumpmachine gives me "i686-pc-cygwin" and gcc -dumpversion gives me "3.3.3".

Let me check to make sure I get it:
Since there's a float stored at my_int_ptr, *my_int_ptr is not guaranteed to make any sense. So really of the numbers, the only line I can count on displaying in an expected way is printf("My_float_ptr points to float %f\n", *my_float_ptr);.
Is that the right understanding?

If so, then I have a follow-up question. As far as floats and ints go, if one can't derefence a pointer of one type when a number of the other type is stored at that address, then would there ever be any practical reason to cast one of these types of pointers to the other? All you'd have after the cast is the address, and no way to tell what's stored there without casting it back to the original type, right?

The reason I ask is because the original exercise that prompted this asks "Write a statement which converts a pointer to an integer into a pointer to a double type." I'm not sure why anyone would want to do this, other than to complete this exercise...