Conditional compilation depending on CONFIG_FOOBAR

[Javier Martinez Canillas]

On Mon, Dec 12, 2011 at 8:59 AM, Srivatsa Bhat <bhat.srivatsa at gmail.com> wrote:
>
>
> On Mon, Dec 12, 2011 at 9:17 AM, contemplating zombie
> <contemplatingzombie at gmail.com> wrote:
>>
>> Just to clarify more, if I put my code in something like:
>> #ifdef CONFIG_FOOBAR
>> my code
>> #endif
>>
>> Then my module's Makefile should check if this parameter is defined in the
>> ".config" file. How should I modify my Makefile to do that?
>>
>>
>> On Sun, Dec 11, 2011 at 10:44 PM, contemplating zombie
>> <contemplatingzombie at gmail.com> wrote:
>>>
>>> Hi,
>>>
>>> I want to conditionally compile some code in module if CONFIG_FOOBAR is
>>> defined.
>>> How can I add this check to my module's makefile?
>>>
>>> -Thanks.
>>
>>
>
> If I understand correctly, you want some part of your module to be compiled
> only
> when CONFIG_FOOBAR is defined. You already mentioned the solution yourself!:
> Just put that code within #ifdef CONFIG_FOOBAR and #endif
> That would check the .config file and see if CONFIG_FOOBAR is defined; and
> only if it
> is defined, that code piece gets compiled, else it will simply get stripped
> off.
>

Now if the question is _how_ to define the configuration symbol
FOOBAR. Then you have to add a new configuration option to a Kconfig
file (probably the one that exists in the directory where you have the
file that is using it).

For example, suppose that you have a char device driver located in
drivers/char/yourdev.c and yourdev uses a FOOBAR option to do
something. Then you have to add something like this in
drivers/char/Kconfig

config FOOBAR
        bool "decide to use foobar or not"
         help
         Say Y here if you want to enable the foobar option.

When you execute make menuconfig or other make targets that generate
the compilation options menu. You will see your option under "Device
drivers" -> "Character devices"

The mconf binary will write in .config the value that you chose for
FOOBAR and will append CONFIG_

So if you enable FOOBAR, you will have something like:

CONFIG_FOOBAR=y

Then this symbols are used to create a generated autoconf.h file in
include/generated/autoconf.h

every compilation command in the kernel includes this file

gcc yourdev.c -I include/generated/autoconf.h

For a detailed description of the kernel build system look at
Documentation/kbuild/

Hope it helps,

-- 
Javier Martínez Canillas
(+34) 682 39 81 69
Barcelona, Spain