[gentoo-dev] Header files and ABI_X86

[Nikos Chantziaras]

Now that Gentoo is much better in handling multilib libraries, but 
Gentoo is source-based, there's the question of which header files are 
used between different ABI builds.

As I understand it, only the headers from the default ABI are installed. 
  That means that building for abi_x86_32 on a amd64 system will use the 
headers installed by the abi_x86_64 build of the used library.

 From a developer's point of view, does that mean that we now have to 
keep Gentoo in mind when writing header files, or does Gentoo apply any 
kind of magic here to fix differences between header files for different 
archs?  For example, if I need to provide a PTR_SIZE macro in a header 
that contains the size of void pointers on this particular system, I 
would detect this with autoconf and then just use that, so the header 
file would end up simply containing:

     #define PTR_SIZE 8

on x86-64, and:

     #define PTR_SIZE 4

on x86.  The x86-64 header won't work properly when building on Gentoo 
for abi_x86_32.  As a developer, this is a no-no for me:

     #ifdef THIS_ARCH
         #define PTR_SIZE 8
     #elif defined THAT_ARCH
         #define PTR_SIZE 4
     ...

because detection should be done in autoconf, not in the header file.

So how does Gentoo solve that problem?