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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="boost_dll.f_a_q_"></a><a class="link" href="f_a_q_.html" title="F.A.Q.">F.A.Q.</a>
</h2></div></div></div>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
<span class="bold"><strong>Question:</strong></span> Is Boost.DLL thread-safe?
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
<span class="bold"><strong>Answer:</strong></span> Some platforms allow concurrent
calls to <code class="computeroutput"><span class="identifier">dlopen</span></code> like
functions. For those platforms Boost.DLL is safe in the manner as
all the C++ Standard Library containers are: it is safe to use different
instances of shared_library from different threads even if all the
instances loaded the same library. On other platforms it is not safe
to concurrently call any of the functions from Boost.DLL (even a
<code class="computeroutput"><span class="identifier">shared_library</span><span class="special">::</span><span class="identifier">location</span><span class="special">()</span></code>
call triggers a race condition). See <a class="link" href="limitations.html#boost_dll.limitations.multithread" title="Thread safe library loading (Fails on FreeBSD, MacOS, iOS and some other)">Limitations,
Thread safe library loading</a>.
</li></ul></div>
</li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
<span class="bold"><strong>Question:</strong></span> Why on Linux symbols from one
plugin are seen in another? Can't get symbol with same name from right
plugin!
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
<span class="bold"><strong>Answer:</strong></span> You've run into the symbol
shadowing problem. Compile your plugins with "-fvisibility=hidden"
flag and take a look to the Tutorial section.
</li></ul></div>
</li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
<span class="bold"><strong>Question:</strong></span> How Unicode (Windows) is handled?
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
<span class="bold"><strong>Answer:</strong></span> Boost.DLL supports Unicode,
so that you could provide Unicode paths to it.
</li></ul></div>
</li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
<span class="bold"><strong>Question:</strong></span> Can I open an executable file?
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
<span class="bold"><strong>Answer:</strong></span> Yes, you can. Symbols need
be exported using in the executable using <code class="computeroutput"><span class="identifier">BOOST_SYMBOL_EXPORT</span></code>
or <code class="computeroutput"><span class="identifier">BOOST_DLL_ALIAS</span></code>.
You can call <code class="computeroutput"><span class="identifier">shared_library</span><span class="special">(</span><span class="identifier">program_location</span><span class="special">())</span></code> to load yourself. Refer to the
Tutorial section for more info. You can also query executables, just
provide a path to the executable to <code class="computeroutput"><span class="identifier">library_info</span></code>
class.
</li></ul></div>
</li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
<span class="bold"><strong>Question:</strong></span> What if I specify wrong type
in <code class="computeroutput"><span class="identifier">shared_library</span><span class="special">::</span><span class="identifier">get</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>
or <code class="computeroutput"><span class="identifier">import</span><span class="special"><</span><span class="identifier">T</span><span class="special">></span></code>?
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
<span class="bold"><strong>Answer:</strong></span> Usually you'll end up with
<code class="computeroutput"><span class="identifier">Segmentation</span> <span class="identifier">Fault</span></code>.
However it is safe to make types more strict, for example making
<code class="computeroutput"><span class="keyword">const</span> <span class="keyword">int</span></code>
from an <code class="computeroutput"><span class="keyword">int</span></code> will not
harm.
</li></ul></div>
</li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
<span class="bold"><strong>Question:</strong></span> Does your library guarantee
ABI stability of methods?
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
<span class="bold"><strong>Answer:</strong></span> Library only guarantees
that alias names created using the <code class="computeroutput"><span class="identifier">BOOST_DLL_ALIAS</span></code>
macros will not change with the change of compiler or platform. You
must take care of functions ABI and API stability by your own.
</li></ul></div>
</li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
<span class="bold"><strong>Question:</strong></span> Are there any function signature
restrictions for the exported/imported functions?
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
<span class="bold"><strong>Answer:</strong></span> No. You may import/export
functions with any signature and any return parameter.
</li></ul></div>
</li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
<span class="bold"><strong>Question:</strong></span> I have 2 plugins that use same
shared library. Would be the shared library loaded twice?
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
<span class="bold"><strong>Answer:</strong></span> No. Pugins will share the
shared library instance.
</li></ul></div>
</li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
<span class="bold"><strong>Question:</strong></span> I have 2 plugins each of them
must work with it's own version of <code class="computeroutput"><span class="identifier">libsome_library</span></code>
shared library. How to achieve that?
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
<span class="bold"><strong>Answer:</strong></span> Avoid such situations by
statically linking in the libsome_library into each plugin and loading
plugins with <code class="computeroutput"><span class="identifier">load_mode</span><span class="special">::</span><span class="identifier">rtld_deepbind</span></code>.
</li></ul></div>
</li></ul></div>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
<span class="bold"><strong>Question:</strong></span> How to load a shared object
from memory??
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
<span class="bold"><strong>Answer:</strong></span> All existing OS avoid loading
shared libraries directly from userspace memory, so you'll find no
syscall for such case. Currently Boost.DLL provides no means for
honest loading shared objects from memory. This requires reimplementing
dynamic linker logic in userspace for all the platforms, which is
a huge amount of work and very error-prone. However working patches
are welcomed!
</li></ul></div>
</li></ul></div>
<p>
Workaround would be to write plugin into a temporary file in RAM and load plugin
from it:
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">filesystem</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">dll</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span>
<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">;</span>
<span class="identifier">dll</span><span class="special">:</span><span class="identifier">shared_library</span> <span class="identifier">load_from_memory</span><span class="special">(</span><span class="keyword">unsigned</span> <span class="keyword">char</span><span class="special">*</span> <span class="identifier">data</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">size_t</span> <span class="identifier">size</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">filesystem</span><span class="special">::</span><span class="identifier">path</span><span class="special">&</span> <span class="identifier">tmp_plugin_path</span> <span class="special">=</span> <span class="identifier">filesystem</span><span class="special">::</span><span class="identifier">unique_path</span><span class="special">()</span> <span class="special">/</span> <span class="string">"libplugin.so"</span><span class="special">)</span> <span class="special">{</span>
<span class="keyword">const</span> <span class="identifier">filesystem</span><span class="special">::</span><span class="identifier">path</span> <span class="identifier">plugin_location</span> <span class="special">=</span> <span class="identifier">filesystem</span><span class="special">::</span><span class="identifier">temp_directory_path</span><span class="special">()</span> <span class="special">/</span> <span class="identifier">tmp_plugin_path</span><span class="special">;</span>
<span class="identifier">filesystem</span><span class="special">::</span><span class="identifier">create_directories</span><span class="special">(</span><span class="identifier">plugin_location</span><span class="special">.</span><span class="identifier">parent_path</span><span class="special">());</span>
<span class="identifier">filesystem</span><span class="special">::</span><span class="identifier">ofstream</span> <span class="identifier">ofs</span><span class="special">(</span><span class="identifier">plugin_location</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">ios</span><span class="special">::</span><span class="identifier">out</span><span class="special">|</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ios</span><span class="special">::</span><span class="identifier">bin</span><span class="special">|</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">ios</span><span class="special">::</span><span class="identifier">trunc</span><span class="special">);</span>
<span class="identifier">ofs</span><span class="special">.</span><span class="identifier">write</span><span class="special">(</span><span class="identifier">data</span><span class="special">,</span> <span class="identifier">size</span><span class="special">);</span>
<span class="keyword">return</span> <span class="identifier">dll</span><span class="special">::</span><span class="identifier">shared_library</span><span class="special">(</span><span class="identifier">plugin_location</span><span class="special">);</span>
<span class="special">}</span>
</pre>
<p>
</p>
<p>
But there's no guarantee that <code class="computeroutput"><span class="identifier">filesystem</span><span class="special">::</span><span class="identifier">temp_directory_path</span><span class="special">()</span></code> will actually write to RAM, that's very platform
dependent.
</p>
<pre class="programlisting"></pre>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; "><li class="listitem">
<span class="bold"><strong>Question:</strong></span> I have found a bug, how do I
notify?
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: circle; "><li class="listitem">
<span class="bold"><strong>Answer:</strong></span> Create an <a href="https://github.com/apolukhin/Boost.DLL/issues" target="_top">issue
at GitHub</a> with a detailed description.
</li></ul></div>
</li></ul></div>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
<td align="left"></td>
<td align="right"><div class="copyright-footer">Copyright © 2014 Renato Tegon Forti, Antony Polukhin<br>Copyright © 2015 Antony Polukhin<br>Copyright © 2016 Antony Polukhin, Klemens Morgenstern<p>
Distributed under the Boost Software License, Version 1.0. (See accompanying
file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
</p>
</div></td>
</tr></table>
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