Re: about function attributes for functions returning a pointer

["Uwe Kleine-König" <u.kleine-koenig@pengutronix.de>]

Hello Ian,

On Mon, Oct 04, 2010 at 11:23:45AM -0700, Ian Lance Taylor wrote:
> Uwe Kleine-König <u.kleine-koenig@pengutronix.de> writes:
> 
> > in the linux kernel I defined a function as follows:
> >
> > 	static struct platform_device *__init __maybe_unused imx_add_imx_dma(void)
> > 	{
> > 		...
> > 	}
> >
> > and the only used was #ifdefed out.
> >
> > With the following defines:
> >
> > 	#define __section(S) __attribute__ ((__section__(#S)))
> > 	#define __cold __attribute__((__cold__))
> > 	#define notrace __attribute__((no_instrument_function))
> > 	#define __init	__section(.init.text) __cold notrace
> > 	#define __maybe_unused                  __attribute__((unused))
> >
> > this still generated the "defined but unused" warning.
> >
> > Then after changing the definition to
> >
> > 	static struct platform_device __init __maybe_unused *imx_add_imx_dma(void)
> >
> > (i.e. move the * after the attribute stuff) the warning is gone.  In
> > both cases (and when the function was used) it is put in the
> > ".init.text" section though.  That is in the first case __init worked,
> > but __maybe_unused did not.  Is this intended?  Do I something wrong?
> > What is the most correct position for function attributes for functions
> > returning a pointer?
> >
> > (I'm using gcc 4.3.2 for arm, OSELAS.Toolchain-1.99.3.6 here.  Could not
> > reproduce with Debian's gcc 4.4.5 for x86 using a minimal example.)
> 
> This message is not appropriate for the mailing list gcc@gcc.gnu.org.
> It would be appropriate for gcc-help@gcc.gnu.org.  Please take any
> followups to gcc-help.  Thanks.
oops, ok, sorry.

> The syntax for attributes is documented at
> http://gcc.gnu.org/onlinedocs/gcc-4.5.1/gcc/Attribute-Syntax.html .
This is heavy reading.  And though I'd claim not to be a novice C
programmer I don't understand it.

For me

	static struct platform_device *__init __maybe_unused imx_add_imx_dma(void)

makes most sense, because the * belongs to struct platform_device and
it's imx_add_imx_dma() that is unused and should live in .init.text, not
*imx_add_imx_dma().

> I think the differences you are seeing are because some attributes can
> apply to types and some can only apply to declarations.  Moving the
> location of the __attribute__ affects which type it applies to.  In
> particular __attribute__ ((unused)) may be used with a type, but
> __attribute__ ((section (...))) may only be used with a declaration.
As far as I got it both section() and unused are variable/function
attributes and not type attributes.  So I think this explanation doesn't
match, does it?

Best regards
Uwe

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