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<title>How to cross-compile skalibs</title>
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<p>
<a href="index.html">skalibs</a><br />
<a href="//skarnet.org/software/">Software</a><br />
<a href="//skarnet.org/">skarnet.org</a>
</p>
<h1> How to cross-compile skalibs </h1>
<p>
There are three issues in the world of cross-compilation:
</p>
<ul>
<li> support for parallel builds: the source directory must be read-only
and each build process must have its own build tree. </li>
<li> support for compiler options: to specify the target architecture,
the header and library files that should be used, etc.
<li> build-time tests performed on the build architecture are invalid
if the target architecture is different. </li>
</ul>
<h2> Support for parallel builds </h2>
<p>
skalibs does not support out-of-tree builds at the moment,
but since the source code tree is small, it's not costly to duplicate it
to perform parallel builds. If the source code tree grows significantly
larger, out-of-tree build support will be considered.
</p>
<h3> Support for staging </h3>
<p>
The skalibs Makefile, as well as every other
<a href="//skarnet.org/software/">skarnet.org package</a>'s
Makefile, supports the DESTDIR convention for staging. To install
skalibs under a <em>stage</em> fake root:
</p>
<ul>
<li> Give the options you need to <tt>./configure</tt>.
The arguments to the <tt>--prefix</tt>, <tt>--libdir</tt> and similar
options must be the valid paths for <strong>run-time</strong>. </li>
<li> To install the software, run <tt>make install
DESTDIR=<em>stage</em></tt>. </li>
</ul>
<h2> Support for build-time options </h2>
<p>
skalibs now uses a standard <tt>./configure && make && make install</tt>
process, and cross-compiling build-time options can be given on the
<tt>./configure</tt> command line.
</p>
<h2> Bypassing the build-time tests </h2>
<p>
This is the hardest part of cross-compilation, and very few build systems
get it right. (GNU autotools does not, which is one of the reasons why
skarnet.org packages do not use autotools.)
</p>
<p>
Native build procedures usually perform build-time tests: they compile
executables and run them (on the build platform, which is the same as
the target platform) to check for features and system quirks. skalibs
does exactly that: the <tt>./configure</tt> step performs tests on the
build platform and stores the system-dependent results in a directory
that it calls the <em>sysdeps</em> for this platform.
</p>
<p>
But in a
cross-compilation environment, build-time tests are invalid, since the build
platform and the target platform differ.
There is only one way to cross-compile portable code without resorting
to build-time autodetection:
<strong>you must provide by hand the sysdeps for your target
architecture</strong>, via the --with-sysdeps option to configure.
</p>
<p>
The easiest way to get the correct sysdeps for a target achitecture is
to natively run skalibs' <tt>./configure</tt> script on that target,
and steal the produced sysdeps files, which are normally written to the
<tt>./sysdeps.cfg</tt> directory.
Doing this is easy with a virtual machine, qemu for instance.
You could also (politely) ask for precompiled sysdeps on the
skaware mailing-list, if you cannot find them anywhere on the Internet.
</p>
<h2> Credits </h2>
<p>
<a href="http://www.kegel.com/">Dan Kegel</a> brought up the need for a
clean cross-compilation system.
</p>
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