Re: Building with float support

[Rick Mann <rmann at latencyzero dot com>]

On Jan 5, 2009, at 11:26:36, Jeff Johnston wrote:

> Rick Mann wrote:
> > Is there anything to read about building newlib with floating-point  
> > support (particularly in snprintf()).
> >
> > I did this:
> >
> > ./configure --target=arm-elf --prefix=/usr/local/arm-elf-float
> >
> > and modified the resulting Makefile to include these lines:
> >
> > CFLAGS_FOR_TARGET = -DFLOATING_POINT -g -O2
> > CXXFLAGS_FOR_TARGET = -DFLOATING_POINT -g -O2
> >
> > Then I did "make install" and placed the results from arm-elf-float  
> > into my existing arm-elf-gcc/newlib installation. After recompiling  
> > my project, I'm still getting behavior that indicates there's no  
> > floating point support (garbage printed for %f printf).
> >
> > Did I miss a setting or do something wrong?
>
> Floating point I/O support is on by default.  You can configure to  
> turn it off but this is not being done for arm (or any other  
> platform at this time).
>
> One thing to check is whether you have forgotten to include stdio.h  
> in your test case.  The compiler will default the prototype for  
> printf and get floating-point arguments wrong.  If that is not the  
> case, you might want to check that the newlib floating-point  
> conversion routines are working in this particular instance by  
> calling dtoa (<stdio.h>) .  If you look in libc/include/machine/ 
> ieeefp.h, you will see that arm has a few variants with regards to  
> floating-point format so make sure that the flags are being set  
> correctly according to how the float argument is passed.

If it's on by default, how does one turn it off? I ask, just to  
understand newlib a bit better.

The other thing I don't understand is why the compiler would get  
floating-point arguments wrong without seeing a declaration for  
printf(). I mean, isn't it going to just look like (char* f, ...)?

In any case, all this points to me not properly replacing my existing  
newlib properly. Any suggestions on how to find exactly where the  
build is finding newlib? I'm pretty sure that's the one in the  
compiler's directory, but I've built the arm toolchain many times, so  
maybe it's getting confused.

Thanks!

--
Rick