Re: [Dev-C++] Question about pointers (beginner)

[<or...@vp...>]

Christian Schmidt wrote:
> Given x = 123 and ptr_int = &x after that *ptr_int = 456 has been executed.
> What is the value of x?

If I implemented this, this is how I would do it:

#include <stdio.h>

main()
{
   int x, *ptr_int;

   x = 123;         /* Given x = 123 */
   ptr_int = &x;    /* and ptr_int =&x */
   *ptr_int = 456;  /* after that *ptr_int = 456 has been executed */
   printf("%d",x);  /* What is the value of x? */
   /* The answer is: 123 */
}

> This is how I see it:
> 
> main ( )
> {
> int x, *ptr_int;
> 
> *ptr_int = 456; /*This pointer is a null pointer at this moment right? */
> 
> x =123;
> ptr_int = &x; /* As x is just set to 123 and the statement *ptr_int = 456;
> was executed before I would think *ptr_int would be 123 */
> 
> }

This is a *wrong* implementation of the example. The book does not state 
this. It states what I wrote above.

And the reason why it does not segfault is only luck. *ptr_int is not 
NULL at the beginning! Variables are NOT initialised, it is up to you to 
do that. *ptr_int just points to somewhere, and you only had luck, that 
using it didn't cause any trouble. Try setting ptr_int to NULL before 
you dereference it (*ptr_int). It will surely segfault then.
You should always keep it in mind, that variables are NEVER initialised 
to a default value! You must have to do that!

> But the books says the value of *ptr_int at the end is 456. How come??

The book is correct.

--
Greetings,
   Balázs