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From: Simon Clubley <clubley@remove_me.eisner.decus.org-Earth.UFP>
Newsgroups: comp.lang.ada
Subject: Re: How do I write directly to a memory address?
Date: Sun, 6 Feb 2011 19:05:58 +0000 (UTC)
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Date: 2011-02-06T19:05:58+00:00
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On 2011-02-06, Niklas Holsti <niklas.holsti@tidorum.invalid> wrote:
> 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."
>

That's good to know.

However, I've used C compilers in the past when it wasn't true so I
developed a style of testing the not null condition first in a if
statement before going into the while condition.

I suspect I am not the first person whose current coding style reflects
lessons learnt from using older compilers while those problems have
subsequently been fixed in later standards. :-)

Simon.

-- 
Simon Clubley, clubley@remove_me.eisner.decus.org-Earth.UFP
Microsoft: Bringing you 1980s technology to a 21st century world