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Copyright (c) 2001, 2002, 2003 Steven Knight
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-->
<para>
No build tools is perfect.
Here are some &SCons; issues that
do not yet have solutions.
</para>
<section>
<title>Interaction with SC-config</title>
<para>
The SC-config tool will be used in the &SCons; installation
process to generate an appropriate default construction environment
so that building most software works "out of the box" on the
installed platform. The SC-config tool will find reasonable default
compilers (C, C++, Fortran), linkers/loaders, library archive tools,
etc. for specification in the default &SCons; construction
environment.
</para>
</section>
<section>
<title>Interaction with test infrastructures</title>
<para>
&SCons; can be configured to use SC-test (or some other test tool)
to provide controlled, automated testing of software. The &Link;
method could link a <filename>test</filename> subdirectory to a build
subdirectory:
</para>
<programlisting>
Link('test', 'build')
SConscript('test/SConscript')</programlisting>
<para>
Any test cases checked in with the source code will be linked
into the <filename>test</filename> subdirectory and executed. If
&SConscript; files and test cases are written with this in mind, then
invoking:
</para>
<programlisting>
% sccons test</programlisting>
<para>
Would run all the automated test cases that depend on any changed
software.
</para>
<!--
YYY integrate with SC-test to provide sanity check on new tools
"discovery testing" of new tools
results recorded in a central database
central database can be somewhere else on web
-->
</section>
<section>
<title>Java dependencies</title>
<para>
Java dependencies are difficult for an external dependency-based
construction tool to accomodate. Determining Java class dependencies
is more complicated than the simple pattern-matching of C or C++
<literal>#include</literal> files. From the point of view of an
external build tool, the Java compiler behaves "unpredictably"
because it may create or update multiple output class files and
directories as a result of its internal class dependencies.
</para>
<para>
An obvious &SCons; implementation would be to have the &Scanner;
object parse output from <command>Java -depend -verbose</command> to
calculate dependencies, but this has the distinct disadvantage of
requiring two separate compiler invocations, thereby slowing down
builds.
</para>
</section>
<section>
<title>Limitations of digital signature calculation</title>
<para>
In practice, calculating digital signatures of a file's contents is a
more robust mechanism than time stamps for determining what needs
building. However:
</para>
<orderedlist numeration="arabic">
<listitem>
<para>
Developers used to the time stamp model of &Make; can initially
find digital signatures counter-intuitive. The assumption that:
<programlisting>
% touch file.c</programlisting>
will cause a rebuild of <filename>file</filename> is strong...
</para>
</listitem>
<listitem>
<para>
Abstracting dependency calculation into a single digital signature
loses a little information: It is no longer possible to tell
(without laborious additional calculation) which input file dependency
caused a rebuild of a given target file. A feature that could
report, "I'm rebuilding file X because it's out-of-date with respect
to file Y," would be good, but an digital-signature implementation of
such a feature is non-obvious.
</para>
</listitem>
</orderedlist>
</section>
<section>
<title>Remote execution</title>
<para>
The ability to use multiple build systems through remote execution
of tools would be good. This should be implementable through the
&Job; class. Construction environments would need modification
to specify build systems.
</para>
</section>
<section>
<title>Conditional builds</title>
<para>
The ability to check run-time conditions as suggested on the
sc-discuss mailing list ("build X only if: the machine is idle /
the file system has Y megabytes free space") would also be good,
but is not part of the current design.
</para>
</section>
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