Re: assigning to const int via pointer

[Ian Lance Taylor <ian at airs dot com>]

Joe Steeve <joe_steeve@gmx.net> writes:

> The following code assigns a value to a `const int` via a
> pointer.,
> 
> #include <stdio.h>
> 
> int 
> main()
> {
>   const int x=5;
>   int *ptr;
>   ptr = &x;
>   *ptr = 10;
>   printf("%d",x);
> }
> 
> The code gives `10` for the following compilation
> 
>     $gcc -o test test.c
> 
> It gives `5` when using optimisations switches.,
> 
>     $gcc -o test -O2 test.c
> 
> Feature or bug or any explanation for this? 

When you declare that the variable is const, you are declaring that
the value does not change.  When you do change it, you are using
undefined behaviour.  When the compiler sees undefined behaviour, it
does not behave predictably.

ISO C99 6.7.3: "If an attempt is made to modify an object defined with
a const-qualified type through use of an lvalue with
non-const-qualified type, the behavior is undefined."

Ian