Re: Creating debs using a cross-build envronment?

+++ Neil Williams [2012-02-23 13:42 +0000]:
> On Thu, 23 Feb 2012 14:16:42 +0100
> Svante Signell <[email protected]> wrote:
> 
> > On Thu, 2012-02-23 at 13:03 +0000, Neil Williams wrote:
> > > You don't need cross-build tools for that. Your doing a native build,
> > > so just use the normal Debian packaging tools (dpkg-buildpackage) and
> > > build for hurd-i386.
> > 
> > I'm doing that natively for most packages already. Some packages are
> > problematic though, so maybe a cross-build can be of use.
> 
> How? When a package will not build natively, it won't cross build
> either.

I don't think that's necessarily true - it depends on the details
(implementation language, significance of architecture differences in
tool ouput).

> > > > b) And to enable support for gnat in GNU/Hurd, either as a deb or
> > > > binaries created by the cross-build environment and copied to the target
> > > > for native .deb builds there.
> > > 
> > > Forget about linux-i386/hurd-i386 cross-building and just change the
> > > package in such a way that it can build a minimal .deb which provides
> > > enough functionality without the complicating dependencies, such that
> > > you can then build the next stage. Cross-building is not the answer
> > > here and even if you are convinced it is, Emdebian cannot provide the
> > > toolchains to do it.
> > 
> > I think cross-building is the _only_ answer here since gnat needs gnat,
> > see my previous mail.
> 
> No, we've been here before in Emdebian. When foo needs foo, foo needs
> to be modified to build mini-foo which doesn't need extra stuff, and
> provides just enough functionality to allow mini-foo to be installed
> (natively) and then support the building of another foo before you maybe
> go on and build the full foo.

THis only works if mini-foo is in a language you already have a
compiler for. So it works for perl, which has a core written in C. It
doesn't work for mono, which has its core written in mono, for
example.

> perl does this, compilers do this. The issue with cross-building is
> that you cannot run the binaries compiled as cross binaries on the
> system which is doing the build. So the best solution here is to build
> natively, building only a small subset of the functionality.

You can't build a language natively if you haven't yet bootstrapped
that language on the target architecture. Sometimes
cross-bootstrapping is the only way to get started. (if the version of
tha language built on another architecure (the BUILD arch) is
sufficiently compatible to be used to compile/intepret the language on
the HOST arch. Usually it is because there has to be _some_ way of
bootstrapping the language.

Several languages have this property: haskell, mono, and I think java
too.

I know nothing about ada, which is the question here. But so far as I
can tell it doesn't have a core C implmentation so you have to have an
ada compiler in order to build it. A cross-gnat compiler seems like
the only general way to get such a thing,  (on x86 linux->hurd it may
be possible to bodge something with the existing i386-linux-gnu ada
compiler, assuming there is some mechanism for running linux binaries
on the hurd?)

> Identifying the subset is the hard part.
> 
> Even if you do a cross-build, you're still going to need to build this
> mini-foo subset natively in order to execute it during the cross-build
> of the rest of the package.

No. other way round. You may have to cross-build mini-foo in order to
be able to natively build either foo or more probably mini-foo
natively. 

Wookey
-- 
Principal hats:  Linaro, Emdebian, Wookware, Balloonboard, ARM
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