Fedora and Cross Compiling

[Ralf Corsepius]

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. 

One would have find a way to map "native BuildRequires" into
* build-requires on "build-tools" on the host 
(e.g. to translate BR: gcc => <target>-gcc) 
* cross-host's target-link-library build-requires 
(E.g. to translate BR: libncurses-devel => <target>-libncurses-devel).

> The issue is that for cross, it has to go and check that yacc is
> installed in the *host* rpmdb, and that libblah-devel is installed in
> the *arch chroot* rpmdb.   The reason is that yacc is going to get
> executed on the host as part of the build action, but libblah-devel is
> going to get linked against in the target arch chroot world.
> 
> It's therefore not useful if you have a host version of libblah-devel
> installed or you have a target arch version of yacc installed in the
> arch chroot.
Right.

> At the moment there is no way to differentiate between these two kind of
>  Build requirements, I propose a full solution is
> 
> BuildRequires: yacc.host libblah-devel
> 
> and rpmbuild is enhanced to check in the right rpmdb accordingly.
Hmm, ...

> A cheap and dirty solution is to ignore BuildRequires completely when
> the target arch != host arch and you will find out if there is a problem
> from build errors...
Wrt. Fedora->Fedora there probably is simpler "hack": 
Treat BuildRequires as identical on target and on host 
=> install them both.

>  hm I guess ./configure might be too smart if there
> are missing libs though and just turn stuff off without errors... hm...
Yes, such cases exist. 

One classical case is 
configure --build=<host> --host=<target>
and not having a <host>-><target>-gcc installed.

The standard autoconf checks will then fall back to using the native
tools. Depending on a package's details the result will either be
building for the wrong architecture, or a package bombing out somewhere
midst building.

> > What I am doing is aiming at cross-building target-binaries, not target
> > packages/rpms.
> 
> I assume this was a misunderstanding of what I was describing above (or
> perhaps I can say the fruit of my not explaining it well enough).
Seems so :)

Ralf