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<H2><A NAME="SECTION000520000000000000000">&nbsp;</A>
<BR>
4.2 Callbacks 
</H2>

<P>
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 global CPython interpreter lock to do a potentially blocking
operation, and the operation calls a callback. Then we must take the GIL back,
since calling Python APIs without holding it is not allowed.

<P>
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.

<P>
The other problem is solved using thread local variables.  Whenever the GIL is
released before calling into an OpenSSL API, the PyThreadState pointer returned
by <tt class="cfunction">PyEval_SaveState</tt> is stored in a global thread local variable
(using Python's own TLS API, <tt class="cfunction">PyThread_set_key_value</tt>).  When it is
necessary to re-acquire the GIL, either after the OpenSSL API returns or in a C
callback invoked by that OpenSSL API, the value of the thread local variable is
retrieved (<tt class="cfunction">PyThread_get_key_value</tt>) and used to re-acquire the GIL.
This allows Python threads to execute while OpenSSL APIs are running and allows
use of any particular pyOpenSSL object from any Python thread, since there is
no per-thread state associated with any of these objects and since OpenSSL is
threadsafe (as long as properly initialized, as pyOpenSSL initializes it).

<P>

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<span class="release-info">Release 0.10.</span>
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