SCons / doc / user / make.in

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=head1 Why Cons? Why not Make?

Cons is a B<make> replacement. In the following paragraphs, we look at a few
of the undesirable characteristics of make, and typical build environments
based on make, that motivated the development of Cons.

=head2 Build complexity

Traditional make-based systems of any size tend to become quite complex. The
original make utility and its derivatives have contributed to this tendency
in a number of ways. Make is not good at dealing with systems that are
spread over multiple directories. Various work-arounds are used to overcome
this difficulty; the usual choice is for make to invoke itself recursively
for each sub-directory of a build. This leads to complicated code, in which
it is often unclear how a variable is set, or what effect the setting of a
variable will have on the build as a whole. The make scripting language has
gradually been extended to provide more possibilities, but these have
largely served to clutter an already overextended language. Often, builds
are done in multiple passes in order to provide appropriate products from
one directory to another directory. This represents a further increase in
build complexity.


=head2 Build reproducibility

The bane of all makes has always been the correct handling of
dependencies. Most often, an attempt is made to do a reasonable job of
dependencies within a single directory, but no serious attempt is made to do
the job between directories. Even when dependencies are working correctly,
make's reliance on a simple time stamp comparison to determine whether a
file is out of date with respect to its dependents is not, in general,
adequate for determining when a file should be rederived. If an external
library, for example, is rebuilt and then ``snapped'' into place, the
timestamps on its newly created files may well be earlier than the last
local build, since it was built before it became visible.


=head2 Variant builds

Make provides only limited facilities for handling variant builds. With the
proliferation of hardware platforms and the need for debuggable
vs. optimized code, the ability to easily create these variants is
essential. More importantly, if variants are created, it is important to
either be able to separate the variants or to be able to reproduce the
original or variant at will. With make it is very difficult to separate the
builds into multiple build directories, separate from the source. And if
this technique isn't used, it's also virtually impossible to guarantee at
any given time which variant is present in the tree, without resorting to a
complete rebuild.


=head2 Repositories

Make provides only limited support for building software from code that
exists in a central repository directory structure.  The VPATH feature of
GNU make (and some other make implementations) is intended to provide this,
but doesn't work as expected: it changes the path of target file to the
VPATH name too early in its analysis, and therefore searches for all
dependencies in the VPATH directory.  To ensure correct development builds,
it is important to be able to create a file in a local build directory and
have any files in a code repository (a VPATH directory, in make terms) that
depend on the local file get rebuilt properly.  This isn't possible with
VPATH, without coding a lot of complex repository knowledge directly into
the makefiles.

-->

 <para>

   XXX

 </para>

 <section>
 <title>Differences Between &Make; and &SCons;</title>

   <para>

   XXX

   </para>

 </section>

 <section>
 <title>Advantages of &SCons; Over &Make;</title>

   <para>

   XXX

   </para>

 </section>
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