On systems that support shared library version numbers, I believe that these should be used even when building with --layout=system.<br><br>At present, when you build Boost with --layout=system, you end up with libraries called 
<span style="font-family: courier new,monospace;">libboost_xyz-mt.so</span> that have this same value set at their ELF SONAME (this is the shared library version number embedded into the library itself).<br><br>The problem with these libraries is that you can never have multiple versions of them installed side-by-side in the same place. Go too far down this road and you end up with Windows-style &#39;DLL hell&#39;.
<br><br>If instead the BOOST_VERSION is appended to the target name as it is with --layout=versioned, you end up with SONAME&#39;d libraries that can live side-by-side with earlier/later versions of Boost.<br><br>For example, by adding the version number to the libraries, you end up with a lib directory that looks like:
<br><br><span style="font-family: courier new,monospace;"></span><span style="font-family: courier new,monospace;">lrwxrwxrwx 1 cepstein&nbsp;&nbsp; 33 Aug&nbsp;&nbsp;2 13:44 libboost_date_time-mt-d.so -&gt; libboost_date_time-mt-d.so.1.35.0
</span><br style="font-family: courier new,monospace;"><span style="font-family: courier new,monospace;">-rwxrwxr-x 1 cepstein 388K Aug&nbsp;&nbsp;2 13:44 libboost_date_time-mt-d.so.1.35.0</span><br style="font-family: courier new,monospace;">
<span style="font-family: courier new,monospace;">lrwxrwxrwx 1 cepstein&nbsp;&nbsp; 31 Aug&nbsp;&nbsp;2 13:44 libboost_date_time-mt.so -&gt; libboost_date_time-mt.so.1.35.0</span><br style="font-family: courier new,monospace;"><span style="font-family: courier new,monospace;">
-rwxrwxr-x 1 cepstein&nbsp;&nbsp;52K Aug&nbsp;&nbsp;2 13:44 libboost_date_time-mt.so.1.35.0<br>...<br></span><br>Now when you link applications using -lboost_date_time-mt, the linker records the dependency on the library&#39;s SONAME, which is 
<span style="font-family: courier new,monospace;">libboost_date_time_mt.so.1.35.0</span>.&nbsp; No other name will satisfy this runtime dependency.&nbsp; As a result, you can install the (hypothetical) Boost 1.36.0 libs into the same directory and old applications will continue to pick up the *.so.1.35.0 libs correctly, while new applications built when the symlink points to the new library versions will pick up *.so.1.36.0.
<br><br>This is how Boost.Build v1 handled --layout=system and I believe this is the most correct and desirable behavior for platforms that support versioned shared libraries.&nbsp; Attached is my simple patch against the latest SVN trunk to make this work.
<br><br>-- <br>Caleb Epstein<br>