I am confused as to what ANSI specification says about changing a variable declared const can be legally modified through its address. Unfortunately I do not have access to C90 specification but got conflicting pointers:
The keyword const doesn't turn a variable into a constant! A symbol with the const qualifier merely means that the symbol cannot be used for assignment. This makes the value re ad -onl y through that symbol; it does not prevent the value from being modified through some other means internal (or even external) to the program. It is pretty much useful only for qualifying a pointer parameter, to indicate that this function will not change the data that argument points to, but other functions may. (Expert C Programming: Deep C Secrets: Peter van der Linden)
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. If an attempt is made to refer to an object defined with a volatile-qualified type through use of an lvalue with non-volatile-qualified type, the behavior is undefined. (http://flash-gordon.me.uk/ansi.c.txt)
I have seen the latter in C99 specification (n1256.pdf).
Can anyone clarify as to which of the above two views is true please?
Edit: The Expect C Programming actually gives an example to demonstrate the ability to change a const variable using pointer.
It's similar in C90(C89) as C99.
C89 §3.5.3 Type qualifiers
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. If an attempt is made to refer to an object defined with a volatile-qualified type through use of an lvalue with non-volatile-qualified type, the behavior is undefined.
Undefined behavior doesn't mean that C prohibits it at all, just the behavior is, well, not defined. So actually the two of your statements are both true.