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From: Niklas Holsti <niklas.holsti@tidorum.invalid>
Newsgroups: comp.lang.ada
Subject: Re: How do I write directly to a memory address?
Date: Sun, 06 Feb 2011 19:18:23 +0100
Organization: Tidorum Ltd
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Date: 2011-02-06T19:18:23+01:00
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Simon Clubley wrote:
> 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.

That is guaranteed in C. The C "&&" operator is short-circuiting and 
corresponds to the Ada "and then". The C99 standard says "If the first 
operand compares equal to 0, the second operand is not evaluated."

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
       .      @       .