226 lines
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7.6 KiB
XML
226 lines
No EOL
7.6 KiB
XML
<refentry xmlns="http://docbook.org/ns/docbook" version="5.0" xml:id="pam_conv">
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<refmeta>
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<refentrytitle>pam_conv</refentrytitle>
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<manvolnum>3</manvolnum>
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<refmiscinfo class="source">Linux-PAM</refmiscinfo>
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<refmiscinfo class="manual">Linux-PAM Manual</refmiscinfo>
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</refmeta>
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<refnamediv xml:id="pam_conv-name">
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<refname>pam_conv</refname>
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<refpurpose>PAM conversation function</refpurpose>
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</refnamediv>
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<!-- body begins here -->
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<refsynopsisdiv>
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<funcsynopsis xml:id="pam_conv-synopsis">
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<funcsynopsisinfo>#include <security/pam_appl.h></funcsynopsisinfo>
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</funcsynopsis>
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<programlisting>
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struct pam_message {
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int msg_style;
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const char *msg;
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};
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struct pam_response {
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char *resp;
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int resp_retcode;
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};
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struct pam_conv {
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int (*conv)(int num_msg, const struct pam_message **msg,
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struct pam_response **resp, void *appdata_ptr);
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void *appdata_ptr;
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};
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</programlisting>
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</refsynopsisdiv>
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<refsect1 xml:id="pam_conv-description">
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<title>DESCRIPTION</title>
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<para>
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The PAM library uses an application-defined callback to allow
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a direct communication between a loaded module and the application.
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This callback is specified by the
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<emphasis>struct pam_conv</emphasis> passed to
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<citerefentry>
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<refentrytitle>pam_start</refentrytitle><manvolnum>3</manvolnum>
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</citerefentry>
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at the start of the transaction.
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</para>
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<para>
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When a module calls the referenced conv() function, the argument
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<emphasis>appdata_ptr</emphasis> is set to the second element of
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this structure.
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</para>
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<para>
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The other arguments of a call to conv() concern the information
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exchanged by module and application. That is to say,
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<emphasis>num_msg</emphasis> holds the length of the array of
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pointers, <emphasis>msg</emphasis>. After a successful return, the
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pointer <emphasis>resp</emphasis> points to an array of pam_response
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structures, holding the application supplied text. The
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<emphasis>resp_retcode</emphasis> member of this struct is unused and
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should be set to zero. It is the caller's responsibility to release
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both, this array and the responses themselves, using
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<citerefentry>
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<refentrytitle>free</refentrytitle><manvolnum>3</manvolnum>
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</citerefentry>. Note, <emphasis>*resp</emphasis> is a
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<emphasis>struct pam_response</emphasis> array and not an array of
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pointers.
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</para>
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<para>
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The number of responses is always equal to the
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<emphasis>num_msg</emphasis> conversation function argument.
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This does require that the response array is
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<citerefentry>
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<refentrytitle>free</refentrytitle><manvolnum>3</manvolnum>
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</citerefentry>'d after
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every call to the conversation function. The index of the
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responses corresponds directly to the prompt index in the
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pam_message array.
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</para>
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<para>
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On failure, the conversation function should release any resources
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it has allocated, and return one of the predefined PAM error codes.
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</para>
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<para>
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Each message can have one of four types, specified by the
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<emphasis>msg_style</emphasis> member of
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<emphasis>struct pam_message</emphasis>:
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</para>
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<variablelist>
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<varlistentry>
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<term>PAM_PROMPT_ECHO_OFF</term>
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<listitem>
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<para>
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Obtain a string without echoing any text.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>PAM_PROMPT_ECHO_ON</term>
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<listitem>
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<para>
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Obtain a string whilst echoing text.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>PAM_ERROR_MSG</term>
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<listitem>
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<para>
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Display an error message.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>PAM_TEXT_INFO</term>
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<listitem>
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<para>
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Display some text.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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<para>
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The point of having an array of messages is that it becomes possible
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to pass a number of things to the application in a single call from
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the module. It can also be convenient for the application that related
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things come at once: a windows based application can then present a
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single form with many messages/prompts on at once.
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</para>
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<para>
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In passing, it is worth noting that there is a discrepancy between
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the way Linux-PAM handles the const struct pam_message **msg
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conversation function argument and the way that Solaris' PAM
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(and derivatives, known to include HP/UX, are there others?) does.
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Linux-PAM interprets the msg argument as entirely equivalent to the
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following prototype
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const struct pam_message *msg[] (which, in spirit, is consistent with
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the commonly used prototypes for argv argument to the familiar main()
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function: char **argv; and char *argv[]). Said another way Linux-PAM
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interprets the msg argument as a pointer to an array of num_msg read
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only 'struct pam_message' pointers. Solaris' PAM implementation
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interprets this argument as a pointer to a pointer to an array of
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num_msg pam_message structures. Fortunately, perhaps, for most
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module/application developers when num_msg has a value of one these
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two definitions are entirely equivalent. Unfortunately, casually
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raising this number to two has led to unanticipated compatibility
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problems.
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</para>
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<para>
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For what its worth the two known module writer work-arounds for trying
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to maintain source level compatibility with both PAM implementations
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are:
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</para>
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<itemizedlist>
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<listitem>
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<para>
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never call the conversation function with num_msg greater than one.
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</para>
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</listitem>
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<listitem>
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<para>
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set up msg as doubly referenced so both types of conversation
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function can find the messages. That is, make
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</para>
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<programlisting>
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msg[n] = & (( *msg )[n])
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</programlisting>
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</listitem>
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</itemizedlist>
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</refsect1>
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<refsect1 xml:id="pam_conv-return_values">
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<title>RETURN VALUES</title>
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<variablelist>
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<varlistentry>
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<term>PAM_BUF_ERR</term>
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<listitem>
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<para>
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Memory buffer error.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>PAM_CONV_ERR</term>
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<listitem>
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<para>
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Conversation failure. The application should not set
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<emphasis>*resp</emphasis>.
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</para>
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</listitem>
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</varlistentry>
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<varlistentry>
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<term>PAM_SUCCESS</term>
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<listitem>
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<para>
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Success.
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</para>
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</listitem>
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</varlistentry>
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</variablelist>
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</refsect1>
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<refsect1 xml:id="pam_conv-see_also">
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<title>SEE ALSO</title>
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<para>
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<citerefentry>
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<refentrytitle>pam_start</refentrytitle><manvolnum>3</manvolnum>
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</citerefentry>,
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<citerefentry>
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<refentrytitle>pam_set_item</refentrytitle><manvolnum>3</manvolnum>
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</citerefentry>,
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<citerefentry>
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<refentrytitle>pam_get_item</refentrytitle><manvolnum>3</manvolnum>
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</citerefentry>,
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<citerefentry>
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<refentrytitle>pam_strerror</refentrytitle><manvolnum>3</manvolnum>
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</citerefentry>,
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<citerefentry>
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<refentrytitle>pam</refentrytitle><manvolnum>8</manvolnum>
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</citerefentry>
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</para>
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</refsect1>
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</refentry> |