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<title>Chapter 27. Frequently Asked Questions</title>
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Accessing environment variables
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<div class="titlepage"><div><div><h2 class="title">
<a name="bbv2.faq"></a>Chapter 27. Frequently Asked Questions</h2></div></div></div>
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<p><b>Table of Contents</b></p>
<dl>
<dt><span class="section"><a href="faq.html#id2861593">
I'm getting "Duplicate name of actual target" error. What
does it mean?
</a></span></dt>
<dt><span class="section"><a href="faq/envar.html">
Accessing environment variables
</a></span></dt>
<dt><span class="section"><a href="faq/s03.html">
How to control properties order?
</a></span></dt>
<dt><span class="section"><a href="faq/s04.html">
How to control the library order on Unix?
</a></span></dt>
<dt><span class="section"><a href="faq/external.html">Can I get output of external program as a variable in a Jamfile?
</a></span></dt>
<dt><span class="section"><a href="faq/s06.html">How to get the project-root location?
</a></span></dt>
<dt><span class="section"><a href="faq/s07.html">How to change compilation flags for one file?
</a></span></dt>
<dt><span class="section"><a href="faq/dll-path.html">Why are the <code class="computeroutput">dll-path</code> and
<code class="computeroutput">hardcode-dll-paths</code> properties useful?
</a></span></dt>
<dt><span class="section"><a href="recipies/site-config.html">Targets in site-config.jam</a></span></dt>
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<div class="section" lang="en">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="id2861593"></a>
I'm getting "Duplicate name of actual target" error. What
does it mean?
</h2></div></div></div>
<p>
The most likely case is that you're trying to
compile the same file twice, with almost the same,
but differing properties. For example:
</p>
<pre class="programlisting">
exe a : a.cpp : <include>/usr/local/include ;
exe b : a.cpp ;
</pre>
<p>
The above snippet requires two different compilations
of 'a.cpp', which differ only in 'include' property.
Since the 'include' property is free, Boost.Build
can't generate two objects files into different directories.
On the other hand, it's dangerous to compile the file only
once -- maybe you really want to compile with different
includes.
</p>
<p>
To solve this issue, you need to decide if file should
be compiled once or twice.</p>
<div class="orderedlist"><ol type="1">
<li>
<p>Two compile file only once, make sure that properties
are the same:
</p>
<pre class="programlisting">
exe a : a.cpp : <include>/usr/local/include ;
exe b : a.cpp : <include>/usr/local/include ;
</pre>
</li>
<li>
<p>
If changing the properties is not desirable, for example
if 'a' and 'b' target have other sources which need
specific properties, separate 'a.cpp' into it's own target:
</p>
<pre class="programlisting">
obj a_obj : a.cpp : <include>/usr/local/include ;
exe a : a_obj ;
</pre>
</li>
<li>
<p>
To compile file twice, you can make the object file local
to the main target:
</p>
<pre class="programlisting">
exe a : [ obj a_obj : a.cpp ] : <include>/usr/local/include ;
exe b : [ obj a_obj : a.cpp ] ;
</pre>
</li>
</ol></div>
<p>
A good question is why Boost.Build can't use some of the above
approaches automatically. The problem is that such magic would
require additional implementation complexities and would only
help in half of the cases, while in other half we'd be silently
doing the wrong thing. It's simpler and safe to ask user to
clarify his intention in such cases.
</p>
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