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+<H2><A NAME="SECTION000520000000000000000"> </A>
+<BR>
+4.2 Callbacks
+</H2>
+<P>
+<EM><EM><EM>There are a number of problems with callbacks. First of all, OpenSSL is written
+as a C library, it's not meant to have Python callbacks, so a way around that
+is needed. Another problem is thread support. A lot of the OpenSSL I/O
+functions can block if the socket is in blocking mode, and then you want other
+Python threads to be able to do other things. The real trouble is if you've
+released the thread lock to do a potentially blocking operation, and the
+operation calls a callback. Then we must take the thread lock back<A NAME="tex2html6"
+ HREF="#foot916"><SUP>5</SUP></A>.
+</EM></EM></EM>
+<P>
+<EM><EM><EM>There are two solutions to the first problem, both of which are necessary. The
+first solution to use is if the C callback allows ''userdata'' to be passed to
+it (an arbitrary pointer normally). This is great! We can set our Python
+function object as the real userdata and emulate userdata for the Python
+function in another way. The other solution can be used if an object with an
+''app_data'' system always is passed to the callback. For example, the SSL
+object in OpenSSL has app_data functions and in e.g. the verification
+callbacks, you can retrieve the related SSL object. What we do is to set our
+wrapper <tt class="class">Connection</tt> object as app_data for the SSL object, and we can
+easily find the Python callback.
+</EM></EM></EM>
+<P>
+<EM><EM><EM>The other problem is also partially solved by app_data. Since we're associating
+our wrapper objects with the ''real'' objects, we can easily access data from
+the <tt class="class">Connection</tt> object. The solution then is to simply include a
+<tt class="ctype">PyThreadState</tt> variable in the <tt class="class">Connection</tt> declaration, and write
+macros similar to <tt class="cfunction">Py_BEGIN_ALLOW_THREADS</tt> and
+<tt class="cfunction">Py_END_ALLOW_THREADS</tt> that allows specifying of the
+<tt class="ctype">PyThreadState</tt> variable to use. Now we can simply ''begin allow
+threads'' before a potentially blocking operation, and ''end allow threads''
+before calling a callback.
+</EM></EM></EM>
+<P>
+<BR><HR><H4>Footnotes</H4>
+<DL>
+<DT><A NAME="foot916">... back</A><A
+ href="callbacks.html#tex2html6"><SUP>5</SUP></A></DT>
+<DD>I'm
+not sure why this is necessary, but otherwise I get a segmentation violation on
+<tt class="cfunction">PyEval_CallObject</tt>
+
+</DD>
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