Re: How do I write directly to a memory address?

[Simon Clubley <clubley@remove_me.eisner.decus.org-Earth.UFP>]

On 2011-02-04, Ludovic Brenta <ludovic@ludovic-brenta.org> wrote:
> Ludovic Brenta <ludo...@ludovic-brenta.org> wrote:
>> Syntax Issues wrote on comp.lang.ada:
>>> ...
>>> unsigned int print(char *message, unsigned int line)
>>> {
>>> � � � � char *vidmem = (char *) 0xb8000;
>>> � � � � unsigned int i= 0;
>>>
>>> � � � � i=(line*80*2);
>>>
>>> � � � � while(*message!=0) // 24h
>
> Did I mention that *every* time I look at C code I see a bug?
> Sometimes, looking twice at the same code reveals two bugs! Here you
> do not check that message != NULL; the condition should be:
>
>            while (message && *message != 0)
>

That still isn't good enough unless you can guarantee that the compiler
generated code will not evaluate the second condition if the first condition
is false.

The reason is that this code will attempt to examine the contents of address
0 when message is null and on some operating systems page 0 is unmapped in
order to catch these kinds of bugs.

Simon.

-- 
Simon Clubley, clubley@remove_me.eisner.decus.org-Earth.UFP
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