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<?xml version="1.0"?>
<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN"
"http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd">
<chapter xml:id="sharing">
<title>Sharing PKCS#11 modules</title>
<section xml:id="sharing-problem">
<title>Multiple consumers of PKCS#11 in a process</title>
<para>As more and more applications and libraries use PKCS#11 we run
into a very basic problem. The PKCS#11 modules cannot be initialized and
finalized properly without coordination between the various consumers.
</para>
<para>An example: An application might use GnuTLS for
TLS connections, and use libgcr for display of certificates. Both of
these want to load (and initialze) the same PKCS#11 modules. There are
many places where this situation occurs, including large applications
like Evolution which due to their dependencies end up using both NSS and
GnuTLS.</para>
<para>Consumer A loads a PKCS#11 module and uses the module's
C_Initialize function to initialize it, which works as expected.
When consumer B initializes the module (also using C_Initialize),
the error code <literal>CKR_CRYPTOKI_ALREADY_INITIALIZED</literal>
is correctly returned. This is normal PKCS#11 specification
defined behavior for when a module is initalized twice in the
same process. If consumer B is aware of this situation they may
choose to ignore this error code.</para>
<para>However when the consumer A is done with its use of the
PKCS#11 module it finalizes the module using the module's
C_Finalize function. This is expected of a well behaved PKCS#11
consumer. This then causes errors and/or crashes for consumer B,
which cannot know that the module has now been finalized out
from underneath it.</para>
<para>It is necessary for the two consumers to coordinate their
initialization and finalization in some fashion. In
<literal>p11-kit</literal> we provide this coordination in a
loosely coupled, backwards compatible, and flexible way.</para>
</section>
<section xml:id="sharing-managed">
<title>Managed modules</title>
<para><literal>p11-kit</literal> wraps PKCS#11 modules to manage
them and customize their functionality so that they are able
to be shared between multiple callers in the same process.</para>
<para>Each caller that uses the
<link linkend="p11-kit-modules-load"><function>p11_kit_modules_load()</function></link>
or <link linkend="p11-kit-module-load"><function>p11_kit_module_load()</function></link>
function gets independent wrapped PKCS#11 module(s). This is unless a caller
or module configuration specifies that a module should be used in an
unmanaged fashion.</para>
<para>When modules are managed, the following aspects are wrapped and
coordinated:</para>
<itemizedlist>
<listitem>
<para>Calls to <literal>C_Initialize</literal> and
<literal>C_Finalize</literal> can be called by multiple
callers.</para>
<para>The first time that the managed module
<literal>C_Initialize</literal> is called, the PKCS#11 module's actual
<literal>C_Initialize</literal> function is called. Subsequent calls by
other callers will cause <literal>p11-kit</literal> to increment an
internal initialization count, rather than calling
<literal>C_Initialize</literal> again.</para>
<para>Multiple callers can call the managed
<literal>C_Initialize</literal> function concurrently from different
threads and <literal>p11-kit</literal> will guarantee that this managed
in a thread-safe manner.</para>
</listitem>
<listitem>
<para>When the managed module <literal>C_Finalize</literal> is used
to finalize a module, each time it is called it decrements the internal
initialization count for that module. When the internal initialization
count reaches zero, the module's actual <literal>C_Finalize</literal>
function is called.</para>
<para>Multiple callers can call the managed <literal>C_Finalize</literal>
function concurrently from different threads and <literal>p11-kit</literal>
will guarantee that this managed in a thread-safe manner.</para>
</listitem>
</itemizedlist>
</section>
</chapter>
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