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<A NAME="CHILD_LINKS"><STRONG>Subsections</STRONG></A>

<UL>
<LI><A NAME="tex2html2232"
  HREF="#SECTION002610000000000000000">23.1 Creating DLL <TT>
<FONT COLOR="#0000ff">.so</FONT></TT> Files</A>
<LI><A NAME="tex2html2233"
  HREF="#SECTION002620000000000000000">23.2 DLL Versioning</A>
<LI><A NAME="tex2html2234"
  HREF="#SECTION002630000000000000000">23.3 Installing DLL <TT>
<FONT COLOR="#0000ff">.so</FONT></TT> Files</A>
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<H1><A NAME="SECTION002600000000000000000">
23. Shared Libraries</A>
</H1>

<P>
<A NAME="chap:sharedlib"></A>
<P>
This chapter follows directly from our construction of <I>static</I> <TT>
<FONT COLOR="#0000ff">.a</FONT></TT>
libraries in Chapter <A HREF="node25.html#chap:trivintroc">22</A>. It discusses creation and installation
of <I>Dynamically Linked Libraries</I> (DLLs). Here I show you both so that you
have a good technical overview of how DLLs work on U<SMALL>NIX</SMALL>. You
can then promptly forget everything except <TT>
<FONT COLOR="#0000ff">ldconfig</FONT></TT> and
<TT>
<FONT COLOR="#0000ff">LD_LIBRARY_PATH</FONT></TT> discussed below.

<P>
The <TT>
<FONT COLOR="#0000ff">.a</FONT></TT> library file is good for creating functions that
many programs can include. This practice is called <I>code reuse</I>. But note
how the <TT>
<FONT COLOR="#0000ff">.a</FONT></TT> file is <I>linked into</I> (included) in the executable
<TT>
<FONT COLOR="#0000ff">mytest</FONT></TT> in Chapter <A HREF="node25.html#chap:trivintroc">22</A>. <TT>
<FONT COLOR="#0000ff">mytest</FONT></TT> is enlarged by the size of
<TT>
<FONT COLOR="#0000ff">libsimple_math.a</FONT></TT>. When hundreds of programs use
the same <TT>
<FONT COLOR="#0000ff">.a</FONT></TT> file, that code is effectively duplicated all
over the file system. Such inefficiency was deemed unacceptable
long before L<SMALL>INUX</SMALL>, so library files were invented that only link
with the program when it runs--a process known as
<I>dynamic</I> linking. Instead of <TT>
<FONT COLOR="#0000ff">.a</FONT></TT> files, similar <TT>
<FONT COLOR="#0000ff">.so</FONT></TT>
(<TT>
<FONT COLOR="#0000ff">s</FONT></TT><I>hared</I> <TT>
<FONT COLOR="#0000ff">o</FONT></TT><I>bject</I>) files live in <TT>
<FONT COLOR="#0000ff">/lib/</FONT></TT>
and <TT>
<FONT COLOR="#0000ff">/usr/lib/</FONT></TT> and are automatically linked to a program when it
runs.

<P>

<H1><A NAME="SECTION002610000000000000000">
23.1 Creating DLL <TT>
<FONT COLOR="#0000ff">.so</FONT></TT> Files</A>
</H1>

<P>
<A NAME="sec:sharedobj"></A>
<P>
Creating a DLL requires several changes to the <TT>
<FONT COLOR="#0000ff">Makefile</FONT></TT>
on page <A HREF="node25.html#page:libmakefile"><IMG  ALIGN="BOTTOM" BORDER="1" ALT="[*]" SRC="crossref.png"></A>:

<P>
<A NAME="page:sharemakefile"></A>
<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>5</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>10</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>15</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<font size="-1"><code>20</code></font><code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>OBJS&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;=&nbsp;simple_math_sqrt.o&nbsp;simple_math_pow.o</code><br>
<code>LIBNAME&nbsp;&nbsp;&nbsp;=&nbsp;simple_math</code><br>
<code>SONAME&nbsp;&nbsp;&nbsp;&nbsp;=&nbsp;libsimple_math.so.1.0.0</code><br>
<code>SOVERSION&nbsp;=&nbsp;libsimple_math.so.1.0</code><br>
<code>CFLAGS&nbsp;&nbsp;&nbsp;&nbsp;=&nbsp;-Wall</code><br>
<code>&nbsp;</code><br>
<code>all:&nbsp;&nbsp;&nbsp;&nbsp;lib$(LIBNAME).so&nbsp;mytest</code><br>
<code>&nbsp;</code><br>
<code>mytest:&nbsp;lib$(LIBNAME).so&nbsp;mytest.o</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;gcc&nbsp;$(CFLAGS)&nbsp;-o&nbsp;$@&nbsp;mytest.o&nbsp;-L.&nbsp;-l${LIBNAME}</code><br>
<code>&nbsp;</code><br>
<code>lib$(LIBNAME).so:&nbsp;$(OBJS)</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;gcc&nbsp;-shared&nbsp;$(CFLAGS)&nbsp;$(OBJS)&nbsp;-lc&nbsp;-Wl,-soname&nbsp;-Wl,$(SOVERSION)&nbsp;\</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;-o&nbsp;$(SONAME)&nbsp;&#038;&#038;&nbsp;\</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;ln&nbsp;-sf&nbsp;$(SONAME)&nbsp;$(SOVERSION)&nbsp;&#038;&#038;&nbsp;\</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;ln&nbsp;-sf&nbsp;$(SONAME)&nbsp;lib$(LIBNAME).so</code><br>
<code>&nbsp;</code><br>
<code>.c.o:</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;gcc&nbsp;-fPIC&nbsp;-DPIC&nbsp;$(CFLAGS)&nbsp;-c&nbsp;-o&nbsp;$*.o&nbsp;$&#060;</code><br>
<code>&nbsp;</code><br>
<code>clean:</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;rm&nbsp;-f&nbsp;*.o&nbsp;*.a&nbsp;*.so&nbsp;mytest</code><br>
</FONT></TD></TR></TABLE><P>

<P>
The <TT>
<FONT COLOR="#0000ff">-shared</FONT></TT> option to <TT>
<FONT COLOR="#0000ff">gcc</FONT></TT> builds our shared
library. The <TT>
<FONT COLOR="#0000ff">-W</FONT></TT> options are linker options that set the version
number of the library that linking programs will load at runtime. The
<TT>
<FONT COLOR="#0000ff">-fPIC -DPIC</FONT></TT> means to generate
<I>position-independent code</I>,
that is, code suitable for dynamic linking.

<P>
After running <TT>
<FONT COLOR="#0000ff">make</FONT></TT> we have

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red size="-2">
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue size="-2">
<code>lrwxrwxrwx&nbsp;&nbsp;&nbsp;&nbsp;1&nbsp;root&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;root&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;23&nbsp;Sep&nbsp;17&nbsp;22:02&nbsp;libsimple_math.so&nbsp;-&#062;&nbsp;libsimple_math.so.1.0.0</code><br>
<code>lrwxrwxrwx&nbsp;&nbsp;&nbsp;&nbsp;1&nbsp;root&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;root&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;23&nbsp;Sep&nbsp;17&nbsp;22:02&nbsp;libsimple_math.so.1.0&nbsp;-&#062;&nbsp;libsimple_math.so.1.0.0</code><br>
<code>-rwxr-xr-x&nbsp;&nbsp;&nbsp;&nbsp;1&nbsp;root&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;root&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;6046&nbsp;Sep&nbsp;17&nbsp;22:02&nbsp;libsimple_math.so.1.0.0</code><br>
<code>-rwxr-xr-x&nbsp;&nbsp;&nbsp;&nbsp;1&nbsp;root&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;root&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;13677&nbsp;Sep&nbsp;17&nbsp;22:02&nbsp;mytest</code><br>
</FONT></TD></TR></TABLE><P>

<P>

<H1><A NAME="SECTION002620000000000000000">
23.2 DLL Versioning</A>
</H1>

<P>
You may observe that our three <TT>
<FONT COLOR="#0000ff">.so</FONT></TT> files are
similar to the many files in <TT>
<FONT COLOR="#0000ff">/lib/</FONT></TT> and <TT>
<FONT COLOR="#0000ff">/usr/lib/</FONT></TT>. This
complicated system of linking and symlinking is part of the process
of <I>library versioning</I>. Although generating a DLL is out of the
scope of most system admin tasks, library versioning is important to
understand.

<P>
DLLs have a problem. Consider a DLL that is outdated or buggy:
simply overwriting the DLL file with an updated file will affect all the
applications that use it. If these applications rely on certain
behavior of the DLL code, then they will probably crash with the
fresh DLL. U<SMALL>NIX</SMALL> has elegantly solved this problem by allowing
multiple versions of DLLs to be present simultaneously. The
programs themselves have their required version number built into
them. Try

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>ldd&nbsp;mytest</code><br>
</FONT></TD></TR></TABLE><P>
which will show the DLL files that <TT>
<FONT COLOR="#0000ff">mytest</FONT></TT> is scheduled to link
with:

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>libsimple_math.so.1.0&nbsp;=&#062;&nbsp;./libsimple_math.so.1.0&nbsp;(0x40018000)</code><br>
<code>libc.so.6&nbsp;=&#062;&nbsp;/lib/libc.so.6&nbsp;(0x40022000)</code><br>
<code>/lib/ld-linux.so.2&nbsp;=&#062;&nbsp;/lib/ld-linux.so.2&nbsp;(0x40000000)</code><br>
</FONT></TD></TR></TABLE><P>
At the moment, we are interested in <TT>
<FONT COLOR="#0000ff">libsimple_math.so.1.0</FONT></TT>.
Note how it matches the <TT>
<FONT COLOR="#0000ff">SOVERSION</FONT></TT> variable in the
<TT>
<FONT COLOR="#0000ff">Makefile</FONT></TT>. Note also how we have chosen our symlinks.
We are effectively allowing <TT>
<FONT COLOR="#0000ff">mytest</FONT></TT> to link with any future
<TT>
<FONT COLOR="#0000ff">libsimple_math.so.1.0.</FONT></TT><I>?</I> (were our <TT>
<FONT COLOR="#0000ff">simple_math</FONT></TT> library
to be upgraded to a new version) purely because of the way we have chosen
our symlinks. However, it will not link with any
library <TT>
<FONT COLOR="#0000ff">libsimple_math.so.1.1.</FONT></TT><I>?</I>, for example. As developers of
<TT>
<FONT COLOR="#0000ff">libsimple_math</FONT></TT>, we are deciding that libraries of a different
<I>minor</I> <FONT COLOR="#ffa500">[For this example we are considering libraries to be
named
<TT>
<FONT COLOR="#0000ff">lib</FONT></TT><I>name</I><TT>
<FONT COLOR="#0000ff">.so.</FONT></TT><I>major</I><TT>
<FONT COLOR="#0000ff">.</FONT></TT><I>minor</I><TT>
<FONT COLOR="#0000ff">.</FONT></TT><I>patch</I>]</FONT>version number will be incompatible, whereas libraries of a different
<I>patch</I> level will not be incompatible.

<P>
We could also change <TT>
<FONT COLOR="#0000ff">SOVERSION</FONT></TT> to
<TT>
<FONT COLOR="#0000ff">libsimple_math.so.1</FONT></TT>. This would effectively be saying that
future libraries of different minor version numbers are compatible; only
a change in the major version number would dictate incompatibility.

<P>

<H1><A NAME="SECTION002630000000000000000">
23.3 Installing DLL <TT>
<FONT COLOR="#0000ff">.so</FONT></TT> Files</A>
</H1>

<P>
If you run <TT>
<FONT COLOR="#0000ff">./mytest</FONT></TT>, you will be greeted with an <TT>
<FONT COLOR="#0000ff">error
while loading shared libraries</FONT></TT> message. The reason is that the
dynamic linker does not search the current directory for <TT>
<FONT COLOR="#0000ff">.so</FONT></TT>
files. To run your program, you will have to install your library:

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>mkdir&nbsp;-p&nbsp;/usr/local/lib</code><br>
<code>install&nbsp;-m&nbsp;0755&nbsp;libsimple_math.so&nbsp;libsimple_math.so.1.0&nbsp;\</code><br>
<code>&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;libsimple_math.so.1.0.0&nbsp;/usr/local/lib</code><br>
</FONT></TD></TR></TABLE><P>
Then, edit the <TT>
<FONT COLOR="#0000ff">/etc/ld.so.conf</FONT></TT> file and add a line

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>/usr/local/lib</code><br>
</FONT></TD></TR></TABLE><P>
Then, reconfigure your libraries with

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>ldconfig</code><br>
</FONT></TD></TR></TABLE><P>
Finally, run your program with

<P><TABLE nowrap="1" width="100%" border="0" cellspacing="0" cellpadding="0">
<TR>
<TD valign="top" class="source" width="2%"><FONT color=red>
<code>&nbsp;</code><br>
<code>&nbsp;</code><br>
</FONT></TD><TD valign="top" class="source" bgcolor="#FFE0C0"><FONT color=blue>
<code>export&nbsp;LD_LIBRARY_PATH="$LD_LIBRARY_PATH:/usr/local/lib"</code><br>
<code>./mytest</code><br>
</FONT></TD></TR></TABLE><P>

<P>
<A NAME="page:ldlibpathvar"></A><TT>
<FONT COLOR="#0000ff">ldconfig</FONT></TT> configures all libraries on the system.
It recreates appropriate symlinks (as we did) and rebuilds a lookup
cache. The library directories it considers are <TT>
<FONT COLOR="#0000ff">/lib</FONT></TT>,
<TT>
<FONT COLOR="#0000ff">/usr/lib</FONT></TT>, and those listed in <TT>
<FONT COLOR="#0000ff">/etc/ld.so.config</FONT></TT>.
The <TT>
<FONT COLOR="#0000ff">ldconfig</FONT></TT> command should be run automatically when the
system boots and manually whenever libraries are installed or upgraded.

<P>
The <TT>
<FONT COLOR="#0000ff">LD_LIBRARY_PATH</FONT></TT> environment variable is
relevant to every executable on the system and similar to the 
<TT>
<FONT COLOR="#0000ff">PATH</FONT></TT> environment variable. <TT>
<FONT COLOR="#0000ff">LD_LIBRARY_PATH</FONT></TT>
dictates what directories should be searched for
library files. Here, we appended <TT>
<FONT COLOR="#0000ff">/usr/local/lib</FONT></TT> to the
search path in case it was missing. Note that even with
<TT>
<FONT COLOR="#0000ff">LD_LIBRARY_PATH</FONT></TT> unset, <TT>
<FONT COLOR="#0000ff">/lib</FONT></TT> and <TT>
<FONT COLOR="#0000ff">/usr/lib</FONT></TT>
will always be searched.

<P>
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