linux-kernel - Re: [PATCH] [ACPI] utilities/: Compliment va_start() with va

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Date:	Tue, 27 Nov 2007 03:10:50 +0100
From:	Richard Knutsson <ricknu-0@...dent.ltu.se>
To:	"Moore, Robert" <robert.moore@...el.com>
CC:	Len Brown <lenb@...nel.org>, linux-kernel@...r.kernel.org,
	linux-acpi@...r.kernel.org
Subject: Re: [PATCH] [ACPI] utilities/: Compliment va_start() with va_end().

Moore, Robert wrote:
> Yes, it's official ANSI C, so I agree with the portability. I'm probably
> asking more about the history of the thing.
>
>   
"the history of the thing"? Sorry, you lost me there. I know there were 
a pre-ANSI
version of va_start() & co., but they seemed quite messy. When it comes 
to va_end()
and maintainers, they often seem positive to this. I guess the 
occasional lack off
va_end() is usually an oversight.
>   
>> -----Original Message-----
>> From: Richard Knutsson [mailto:ricknu-0@...dent.ltu.se]
>> Sent: Monday, November 26, 2007 4:16 PM
>> To: Moore, Robert
>> Cc: Len Brown; linux-kernel@...r.kernel.org; linux-acpi@...r.kernel.org
>> Subject: Re: [PATCH] [ACPI] utilities/: Compliment va_start() with
>> va_end().
>>
>> Moore, Robert wrote:
>>     
>>> This is an interesting one to me.
>>>
>>> From various documentation:
>>>
>>> After all arguments have been retrieved, va_end resets the pointer to
>>> NULL.
>>>
>>> va_end
>>> Each invocation of va_start must be matched by a corresponding
>>> invocation of va_end in the same function. After the call va_end(ap)
>>>       
> the
>   
>>> variable ap is undefined. Multiple transversals of the list, each
>>> bracketed by va_start and va_end are possible. va_end may be a macro
>>>       
> or
>   
>>> a function.
>>>
>>> Now, I'm all for defensive programming, but I don't really see the
>>>       
> point
>   
>>> of va_end when the list will be only traversed once.
>>>
>>>
>>>       
>> First off, I think it is a good idea to follow the documentation, which
>> stated:
>> "va_end
>> Each invocation of va_start must be matched by a corresponding
>> invocation of va_end in the same function."
>>
>> Then if it is not really needed, does it take up extra cycles?
>> "In practice, with most C compilers, calling |va_end| does nothing
>> and you do not really need to call it.  This is always true in the GNU
>>     
> C
>   
>> compiler."[1]
>>
>> Portability:
>> "But you might as well call |va_end| just in case your
>> program is someday compiled with a peculiar compiler."[2]
>> This argument is not as likely thou, but who knows? (Since I guess
>>     
> Intel's
>   
>> compiler is included in the 'most C compilers')
>>
>>     
>>> We don't set all local pointers to NULL at function exit, what is the
>>> point of doing it here?
>>>
>>>       
>> I think it is a good thing if the code follows the documentation, both
>> for the person who tries
>> to understand the code (to see when the 'args' is no longer needed and
>> not getting confused
>> by the absent of va_end(), after all, IMHO we should write the code how
>> we want things to
>> work and let the compiler do the optimizations (it usually does a
>>     
> better
>   
>> job at it then we do))
>> and to automated searches (that is how I found this one).
>>     
>>> I suppose some implementation could allocate memory at va_start, but
>>>       
> in
>   
>>> practice, does this happen?
>>>
>>>       
>> Not sure what you mean.
>>     
>>> Bob
>>>
>>>
>>>       
>> cu
>> Richard Knutsson
>>
>> [1]
>> http://www.cs.utah.edu/dept/old/texinfo/glibc-manual-0.02/library_28.ht
>>     
> ml
>   
>> [2] The rest of [1]'s line.
>>     
> -
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