Fedora and Cross Compiling

[David Smith]

Ralf Corsepius wrote:
> On Fri, 2007-06-15 at 08:25 +0100, Andy Green wrote:
>> Ralf Corsepius wrote:
> 
>>>>>>   When cross 
>>>>>> building you need to pull in a mixture of native (host-arch) and cross 
>>>>>> (target arch) packages to meet the build dependencies.  This is part of 
>>>>>> where the "fun" is.
>>>>> Well, I question the "need", but consider this kind of implementation to
>>>>> be "an option".
>>>> It can be necessary though, for example if the package build wants yacc
>>>> or bison or whatever to build, with cross building they are going to be
>>>> host yacc and bison despite you are building for a different target arch
>>> If I understand you correctly the problem you are referring to is 
>>> separating "tools being used at build-time on the host" from 
>>> "tools being used at run-time on the target".
>>> (Classic situation: building inserts hard-coded directories/paths
>>> referring to files on the build-host into target-files)
>> No the issue I (and Brendan I believe) am talking about is,
>> "BuildRequires... required where?".  Consider
>>
>> BuildRequires: yacc libblah-devel
> Understood.
> 
>> rpmbuild should go and confirm that the packages mentioned here are
>> installed before doing the build.
> Hmm, difficult. 

The problem is actually a bit worse than that.  bison and flex are 
actually required for the host *and* target.  You need the host version 
for the actual bison/flex executable and you need the target version for 
the static library both typically include.

I used to be part of Brendan's group.  Here's how we solved this problem 
in the past (they might have changed things since I last looked).

Assume a package that originally has the following line for native compiles:

BuildPrereq: autoconf, bison, flex, libblah-devel, sed

For cross compiles we change it to look like this:

%if "%{_arch}" == %{_build_arch}
BuildPrereq: autoconf, bison, flex, libblah-devel, sed
%else
BuildPrereq: bison, flex, libblah-devel
%endif%

Note that we leave things so that native compiles still work.  We then 
check native dependencies (_arch == _build_arch) against our mock 
chroot's native rpm database, then check cross dependencies (_arch != 
_build_arch) against the mock chroot's target rpm database.

Target versions of autoconf and sed aren't needed since nothing links 
against anything in those packages (those packages are only run 
natively).  The only problem with this scheme is that we end up with an 
extra native dependency of libblah-devel (which really isn't needed 
since nothing links against it in the cross compile).  But the benefit 
of unaffected native compiles outweighed the extra dependency.

-- 
David Smith
dsmith at redhat.com
Red Hat
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