Re: [PATCH 05/10] kbuild: sort the list of symbols exported by the kernel (__ksymtab)

[Mike Frysinger <vapier.adi@gmail.com>]

On Wed, Nov 4, 2009 at 05:00, Alan Jenkins wrote:
> Rusty Russell wrote:
>> On Tue, 3 Nov 2009 08:36:17 pm Alan Jenkins wrote:
>>> +/*
>>> + * We use CPP macros since they are more familiar than assembly macros.
>>> + * Note that CPP macros eat newlines, so each statement must be
>>> terminated
>>> + * by a semicolon.
>>> + */
>>> +
>>> +#ifdef CONFIG_HAVE_SYMBOL_PREFIX
>>> +#define __SYM(sym) _##sym
>>> +#else
>>> +#define __SYM(sym) sym
>>> +#endif
>>>
>>
>> Ideally, you would used MODULE_SYMBOL_PREFIX here, but of course it's a
>> string.  I don't think Kconfig can do arbitrary identifiers, so we can't
>> make CONFIG_SYMBOL_PREFIX empty or _.
>>
>> Perhaps clarify it to a bool CONFIG_HAVE_MODULE_UNDERSCORE_PREFIX then,
>> since that's what you're assuming here?
>>
>> Thanks,
>> Rusty.
>
> I made the same assumption in patch 4.  The arch defines
> CONFIG_HAVE_SYMBOL_PREFIX, which then causes init/Kconfig to define
> CONFIG_SYMBOL_PREFIX="_".
>
> Mike suggested that I hack kbuild instead, to do something like
>
> unquote = ...
> AFLAGS_.tmp_export-asm.o += -DSYMBOL_PREFIX=$(unquote CONFIG_SYMBOL_PREFIX)
>
> I'm experimenting with the idea, but I haven't managed to get it working
> yet.

presumably you're having trouble with the preprocessor using the
define name literally instead of expanding it.  you can address this
two ways:
 - force gcc to expand it wit a few more levels
 - define the macro on the command line

here is the first way:
$ cat test.c
#include <stdio.h>
foo() { puts("foo"); }
_foo() { puts("_foo"); }
__foo() { puts("__foo"); }
#define __SYM(x) ___SYM(PFX, x) /* queue PFX */
#define ___SYM(pfx,x) ____SYM(pfx, x) /* expand PFX */
#define ____SYM(pfx,x) pfx##x /* paste them together */
main() { __SYM(foo)(); }
$ gcc test.c -DPFX=_ && ./a.out
_foo

and here is the second:
$ cat test.c
#include <stdio.h>
foo() { puts("foo"); }
_foo() { puts("_foo"); }
__foo() { puts("__foo"); }
main() { __SYM(foo)(); }
$ gcc test.c -D'__SYM(x)=_##x' && ./a.out
_foo

HTH
-mike