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It also kepttrack of multiply loaded modules so that a module wasn't unloadedprematurely. libltdl already does all of this though, and it iswasteful to duplicate all of that work. This system uses<CODE>lt_dlforeach</CODE> and <CODE>lt_dlgetinfo</CODE> to access libltdls recordsof loaded modules, and save on duplication. These two functions aredescribed fully insection `Libltdl interface' in <CITE>The Libtool Manual</CITE>. </P><P><TABLE width=100%><tr><td> </td><td class=example bgcolor=#6688aa><br><pre></pre></td></tr></table></P><P>This function asks libltdl to call the function<CODE>unload_ltmodule</CODE> for each of the modules it has loaded, along withsome details of the module it wants to unload. The tricky part of thecallback function below is recalculating the entry point addresses forthe module to be unloaded and then removing all matching addresses fromthe appropriate internal structures. Otherwise, the balance of thiscallback is involved in informing the calling <CODE>lt_dlforeach</CODE> loopof whether a matching module has been found and handled:</P><P><TABLE width=100%><tr><td> </td><td class=example bgcolor=#6688aa><br><pre></pre></td></tr></table></P><P>The <CODE>userdata_address_compare</CODE> helper function at the end is usedto compare the address of recalculated entry points against the alreadyregistered functions and handlers to find which items need to beunregistered.</P><P>There is also a matching header file to export the module interface, sothat the code for loadable modules can make use of it:</P><P><TABLE width=100%><tr><td> </td><td class=example bgcolor=#6688aa><br><pre></pre></td></tr></table></P><P>This header also includes some of the other Sic headers, so that in mostcases, the source code for a module need only <SAMP>`#include<sic/module.h>'</SAMP>.</P><P>To make the module loading interface useful, I have added built-ins for<SAMP>`load'</SAMP> and <SAMP>`unload'</SAMP>. Naturally, these must be compiled intothe bare <CODE>sic</CODE> executable, so that it is able to load additionalmodules:</P><P><TABLE width=100%><tr><td> </td><td class=example bgcolor=#6688aa><br><pre></pre></td></tr></table></P><P>These new built-in commands are simply wrappers around the module loadingcode in <TT>`module.c'</TT>.</P><P>As with <SAMP>`dlopen'</SAMP>, you can use libltdl to <SAMP>`lt_dlopen'</SAMP> themain executable, and then lookup <EM>its</EM> symbols. I have simplifiedthe initialisation of Sic by replacing the <CODE>sic_init</CODE> function in<TT>`src/sic.c'</TT> by `loading' the executable itself as a module. Thisworks because I was careful to use the same format in<TT>`sic_builtin.c'</TT> and <TT>`sic_syntax.c'</TT> as would be required for a genuine loadable module, like so:</P><P><TABLE width=100%><tr><td> </td><td class=example bgcolor=#6688aa><br><pre></pre></td></tr></table></P><P><A NAME="A Loadable Module"></A></TR></TABLE><BR> <FONT SIZE="-1">This document was generatedby <I>Gary V. Vaughan</I> on <I>September, 12 2004</I>using <A HREF="http://www.mathematik.uni-kl.de/~obachman/Texi2html"><I>texi2html</I></A></BODY></HTML>⌨️ 快捷键说明
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