*** pltcl.sgml	2007-10-01 08:29:06.667578247 -0400
--- pltcl.sgml.new	2007-10-01 08:24:41.736708719 -0400
***************
*** 70,75 ****
--- 70,87 ----
      <literal>createlang pltcl <replaceable>dbname</></literal> or
      <literal>createlang pltclu <replaceable>dbname</></literal>.
     </para>
+    <para>
+     Care should be taken when linking the pltcl shared object code against
+     TCL libraries earlier than the TCL 8.4 release.  The pre-8.4 versions of TCL must
+     be built <emphasis>without</emphasis> multithreading support, i.e. with TCL_THREADS 
+     undefined.  Otherwise, the first use of PL/TCL functions will cause the postgres
+     backend to become multithreaded, resulting in subsequent "unexpected state" errors.  
+     PL/TCL <emphasis>can</emphasis> be safely linked against multithreaded versions of the 
+     TCL library for TCL versions 8.4 and later.  In these cases, the pltcl source code uses 
+     capabilities introduced in TCL 8.4 to override and disable the multithreading behavior.
+     Note that pre-8.4 versions of TCL were rarely built with multithreading support in 
+     pre-compiled distributions, so this problem is rather rare.
+    </para>
    </sect1>
  
    <!-- **** PL/Tcl description **** -->