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Message-ID: <7b9198260805241700o2bc38848m60d5853b63e6ff26@mail.gmail.com>
Date: Sun, 25 May 2008 01:00:51 +0100
From: "Tom Spink" <tspink@gmail.com>
To: "Steven Fuerst" <sfuerst@stanford.edu>
Subject: Re: [RFC PATCH] kconfig: introduce KCONFIG_* symbols for .c files
Cc: linux-kernel@vger.kernel.org
In-Reply-To: <200805241636.22242.sfuerst@stanford.edu>
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2008/5/25 Steven Fuerst <sfuerst@stanford.edu>:
>
>> On Sat, May 24, 2008 at 09:58:47PM +0100, Jeremy Fitzhardinge wrote:
>> >
>> > Would
>> >
>> > #define KCONFIG(x) (CONFIG_##x - 0)
>> >
>> > if (KCONFIG(PREEMPT)) {
>> > ...
>> > }
>> >
>> > work?
>>
>> $ cat test.c
>> #define KCONFIG(x) (CONFIG_##x - 0)
>>
>> int main()
>> {
>> if (KCONFIG(PREEMPT))
>> ;
>>
>> return 0;
>> }
>> $ gcc -O2 -Wall test.c
>> test.c: In function āmainā:
>> test.c:5: error: āCONFIG_PREEMPTā undeclared (first use in this function)
>> test.c:5: error: (Each undeclared identifier is reported only once
>> test.c:5: error: for each function it appears in.)
>> $ gcc --version
>> gcc (Debian 4.3.0-5) 4.3.1 20080523 (prerelease)
>>
>> > J
>>
>> cu
>> Adrian
>
> How about this?
>
> #include <stdio.h>
>
> #define CONFIG_FOO      1
> /* #define CONFIG_BAR   1 */
>
> #define STR2(x) #x
> #define STR(x) STR2(x)
>
> #define PASTE2(x, y) x##y
> #define PASTE(x, y) PASTE2(x, y)
>
> #define KCONFIG2(x, y) STR(PASTE(CONFIG_, x))[y]
> #define KCONFIG(x) (!((KCONFIG2(x, 0) == 'C') && \
>                                        (KCONFIG2(x, 1) == 'O') && \
>                                        (KCONFIG2(x, 2) == 'N') && \
>                                        (KCONFIG2(x, 3) == 'F') && \
>                                        (KCONFIG2(x, 4) == 'I') && \
>                                        (KCONFIG2(x, 5) == 'G') && \
>                                        (KCONFIG2(x, 6) == '_')))
>
> int main()
> {
>        if (KCONFIG(FOO)) printf("FOO\n");
>        if (KCONFIG(BAR)) printf("BAR\n");
>        return 0;
> }
>
>
> It of course assumes that the result of all defined macros you want to test
> for do not start with "CONFIG_", so this doesn't work generally.  However,
> nearly all of the time the arguement to KCONFIG() will be defined to be "1"
> an integer, or some other known string that doesn't begin with "CONFIG_", so
> this trick is fairly robust.

I've seen worse abuse of the preprocessor[1]. ;-)

> Steven

-- 
Tom Spink

[1] http://www.ioccc.org/