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35<div class="titlepage"><div><div><h2 class="title" style="clear: both">
36<a name="bbv2.tutorial.linkage"></a>Static and shared libaries</h2></div></div></div>
37<p>Libraries can be either
38    <span class="emphasis"><em>static</em></span>, which means they are included in executable
39    files that use them, or <span class="emphasis"><em>shared</em></span> (a.k.a.
40    <span class="emphasis"><em>dynamic</em></span>), which are only referred to from executables,
41    and must be available at run time. Boost.Build can create and use both kinds.
42    </p>
43<p>The kind of library produced from a <code class="computeroutput">lib</code> target is
44    determined by the value of the <code class="varname">link</code> feature. Default
45    value is <code class="literal">shared</code>, and to build static library, the value
46    should be <code class="literal">static</code>. You can either requiest static build
47    on the command line:
48    </p>
49<pre class="screen">
50bjam link=static
51    </pre>
52<p>
53    or in the library's requirements:   
54    </p>
55<pre class="programlisting">
56lib l : l.cpp : &lt;link&gt;static ;
57    </pre>
58<p>
59      We can also use the <code class="varname">&lt;link&gt;</code> property
60      to express linking requirements on a per-target basis. 
61      For example, if a particular executable can be correctly built
62      only with the static version of a library, we can qualify the
63      executable's <a href="../reference/definitions.html#bbv2.reference.targets.references">target
64      reference</a> to the library as follows:
65
66</p>
67<pre class="programlisting">
68exe important : main.cpp helpers/&lt;link&gt;static ;</pre>
69<p>
70
71    No matter what arguments are specified on the <span><strong class="command">bjam</strong></span>
72    command-line, <code class="filename">important</code> will only be linked with
73    the static version of <code class="filename">helpers</code>.
74    </p>
75<p>
76        Specifying properties in target references is especially useful if you
77        use a library defined in some other project (one you can't
78        change) but you still want static (or dynamic) linking to that library
79        in all cases. If that library is used by many targets,
80        you <span class="emphasis"><em>could</em></span> use target references
81        everywhere:
82
83        </p>
84<pre class="programlisting">
85exe e1 : e1.cpp /other_project//bar/&lt;link&gt;static ;
86exe e10 : e10.cpp /other_project//bar/&lt;link&gt;static ;</pre>
87<p>
88
89        but that's far from being convenient. A better approach is
90        to introduce a level of indirection.  Create a local
91        <span class="type">alias</span> target that refers to the static (or
92        dynamic) version of <code class="filename">foo</code>:
93
94        </p>
95<pre class="programlisting">
96alias foo : /other_project//bar/&lt;link&gt;static ;
97exe e1 : e1.cpp foo ;
98exe e10 : e10.cpp foo ;</pre>
99<p>
100
101        The <a href="../advanced/builtins/targets.html#bbv2.builtins.alias" title="Alias"><code class="computeroutput">alias</code></a> 
102        rule is specifically used to rename a reference to a target and possibly
103        change the properties.
104       
105        </p>
106<div class="tip" style="margin-left: 0.5in; margin-right: 0.5in;">
107<h3 class="title">Tip</h3>
108<p>
109          When one library uses another, you put the second library is
110          the source list of the first. For example:
111          </p>
112<pre class="programlisting">
113lib utils : utils.cpp /boost/filesystem//fs ;   
114lib core : core.cpp utils ;
115exe app : app.cpp core ;</pre>
116<p>
117          This works no matter what kind of linking is used. When
118          <code class="filename">core</code> is built as a shared library, it is linked
119          directly into <code class="filename">utils</code>. Static libraries can't
120          link to other libraries, so when <code class="filename">core</code> is built
121          as a static library, its dependency on <code class="filename">utils</code> is passed along to
122          <code class="filename">core</code>'s dependents, causing
123          <code class="filename">app</code> to be linked with both
124          <code class="filename">core</code> and <code class="filename">utils</code>."
125        </p>
126</div>
127<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
128<h3 class="title">Note</h3>
129<p>(Note for non-UNIX system). Typically, shared libraries must be
130        installed to a directory in the dynamic linker's search
131        path. Otherwise, applications that use shared libraries can't be
132        started. On Windows, the dynamic linker's search path is given by the
133        <code class="envar">PATH</code> environment variable. This restriction is lifted
134        when you use Boost.Build testing facilities&#8212;the
135        <code class="envar">PATH</code> variable will be automatically adjusted before
136        running executable.
137        </p>
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