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<para>
There are two occasions when &SCons; will,
by default, remove target files.
The first is when &SCons; determines that
an target file needs to be rebuilt
and removes the existing version of the target
before executing
The second is when &SCons; is invoked with the
<literal>-c</literal> option to "clean"
a tree of its built targets.
These behaviours can be suppressed with the
&Precious; and &NoClean; functions, respectively.
</para>
<section>
<title>Preventing target removal during build: the &Precious; Function</title>
<para>
By default, &SCons; removes targets before building them.
Sometimes, however, this is not what you want.
For example, you may want to update a library incrementally,
not by having it deleted and then rebuilt from all
of the constituent object files.
In such cases, you can use the
&Precious; method to prevent
&SCons; from removing the target before it is built:
</para>
<scons_example name="precious-ex1">
<file name="SConstruct" printme="1">
env = Environment(RANLIBCOM='')
lib = env.Library('foo', ['f1.c', 'f2.c', 'f3.c'])
env.Precious(lib)
</file>
<file name="f1.c">
int f1() { }
</file>
<file name="f2.c">
int f2() { }
</file>
<file name="f3.c">
int f3() { }
</file>
</scons_example>
<para>
Although the output doesn't look any different,
&SCons; does not, in fact,
delete the target library before rebuilding it:
</para>
<scons_output example="precious-ex1">
<scons_output_command>scons -Q</scons_output_command>
</scons_output>
<para>
&SCons; will, however, still delete files marked as &Precious;
when the <literal>-c</literal> option is used.
</para>
</section>
<section>
<title>Preventing target removal during clean: the &NoClean; Function</title>
<para>
By default, &SCons; removes all built targets when invoked
with the <literal>-c</literal> option to clean a source tree
of built tragets.
Sometimes, however, this is not what you want.
For example, you may want to remove only intermediate generated files
(such as object files),
but leave the final targets
(the libraries)
untouched.
In such cases, you can use the &NoClean; method to prevent &SCons;
from removing a target during a clean:
</para>
<scons_example name="noclean-ex1">
<file name="SConstruct" printme="1">
env = Environment(RANLIBCOM='')
lib = env.Library('foo', ['f1.c', 'f2.c', 'f3.c'])
env.NoClean(lib)
</file>
<file name="f1.c">
int f1() { }
</file>
<file name="f2.c">
int f2() { }
</file>
<file name="f3.c">
int f3() { }
</file>
</scons_example>
<para>
Notice that the <filename>libfoo.a</filename>
is not listed as a removed file:
</para>
<scons_output example="noclean-ex1">
<scons_output_command>scons -Q</scons_output_command>
<scons_output_command>scons -c</scons_output_command>
</scons_output>
</section>
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