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+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE refentry PUBLIC "-//OASIS//DTD DocBook XML V4.1.2//EN"
+ "http://www.oasis-open.org/docbook/xml/4.1.2/docbookx.dtd">
+<refentry id='pam_conv'>
+ <refmeta>
+ <refentrytitle>pam_conv</refentrytitle>
+ <manvolnum>3</manvolnum>
+ <refmiscinfo class='setdesc'>Linux-PAM Manual</refmiscinfo>
+ </refmeta>
+
+ <refnamediv id="pam_conv-name">
+ <refname>pam_conv</refname>
+ <refpurpose>PAM conversation function</refpurpose>
+ </refnamediv>
+
+<!-- body begins here -->
+
+ <refsynopsisdiv>
+ <funcsynopsis id="pam_conv-synopsis">
+ <funcsynopsisinfo>#include &lt;security/pam_appl.h&gt;</funcsynopsisinfo>
+ </funcsynopsis>
+ <programlisting>
+struct pam_message {
+ int msg_style;
+ const char *msg;
+};
+
+struct pam_response {
+ char *resp;
+ int resp_retcode;
+};
+
+struct pam_conv {
+ int (*conv)(int num_msg, const struct pam_message **msg,
+ struct pam_response **resp, void *appdata_ptr);
+ void *appdata_ptr;
+};
+ </programlisting>
+ </refsynopsisdiv>
+
+ <refsect1 id='pam_conv-description'>
+ <title>DESCRIPTION</title>
+ <para>
+ The PAM library uses an application-defined callback to allow
+ a direct communication between a loaded module and the application.
+ This callback is specified by the
+ <emphasis>struct pam_conv</emphasis> passed to
+ <citerefentry>
+ <refentrytitle>pam_start</refentrytitle><manvolnum>3</manvolnum>
+ </citerefentry>
+ at the start of the transaction.
+ </para>
+ <para>
+ When a module calls the referenced conv() function, the argument
+ <emphasis>appdata_ptr</emphasis> is set to the second element of
+ this structure.
+ </para>
+ <para>
+ The other arguments of a call to conv() concern the information
+ exchanged by module and application. That is to say,
+ <emphasis>num_msg</emphasis> holds the length of the array of
+ pointers, <emphasis>msg</emphasis>. After a successful return, the
+ pointer <emphasis>resp</emphasis> points to an array of pam_response
+ structures, holding the application supplied text. The
+ <emphasis>resp_retcode</emphasis> member of this struct is unused and
+ should be set to zero. It is the caller's responsibility to release
+ both, this array and the responses themselves, using
+ <citerefentry>
+ <refentrytitle>free</refentrytitle><manvolnum>3</manvolnum>
+ </citerefentry>. Note, <emphasis>*resp</emphasis> is a
+ <emphasis>struct pam_response</emphasis> array and not an array of
+ pointers.
+ </para>
+ <para>
+ The number of responses is always equal to the
+ <emphasis>num_msg</emphasis> conversation function argument.
+ This does require that the response array is
+ <citerefentry>
+ <refentrytitle>free</refentrytitle><manvolnum>3</manvolnum>
+ </citerefentry>'d after
+ every call to the conversation function. The index of the
+ responses corresponds directly to the prompt index in the
+ pam_message array.
+ </para>
+ <para>
+ On failure, the conversation function should release any resources
+ it has allocated, and return one of the predefined PAM error codes.
+ </para>
+ <para>
+ Each message can have one of four types, specified by the
+ <emphasis>msg_style</emphasis> member of
+ <emphasis>struct pam_message</emphasis>:
+ </para>
+ <variablelist>
+ <varlistentry>
+ <term>PAM_PROMPT_ECHO_OFF</term>
+ <listitem>
+ <para>
+ Obtain a string without echoing any text.
+ </para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>PAM_PROMPT_ECHO_ON</term>
+ <listitem>
+ <para>
+ Obtain a string whilst echoing text.
+ </para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>PAM_ERROR_MSG</term>
+ <listitem>
+ <para>
+ Display an error message.
+ </para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>PAM_TEXT_INFO</term>
+ <listitem>
+ <para>
+ Display some text.
+ </para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ <para>
+ The point of having an array of messages is that it becomes possible
+ to pass a number of things to the application in a single call from
+ the module. It can also be convenient for the application that related
+ things come at once: a windows based application can then present a
+ single form with many messages/prompts on at once.
+ </para>
+ <para>
+ In passing, it is worth noting that there is a discrepancy between
+ the way Linux-PAM handles the const struct pam_message **msg
+ conversation function argument and the way that Solaris' PAM
+ (and derivatives, known to include HP/UX, are there others?) does.
+ Linux-PAM interprets the msg argument as entirely equivalent to the
+ following prototype
+ const struct pam_message *msg[] (which, in spirit, is consistent with
+ the commonly used prototypes for argv argument to the familiar main()
+ function: char **argv; and char *argv[]). Said another way Linux-PAM
+ interprets the msg argument as a pointer to an array of num_msg read
+ only 'struct pam_message' pointers. Solaris' PAM implementation
+ interprets this argument as a pointer to a pointer to an array of
+ num_msg pam_message structures. Fortunately, perhaps, for most
+ module/application developers when num_msg has a value of one these
+ two definitions are entirely equivalent. Unfortunately, casually
+ raising this number to two has led to unanticipated compatibility
+ problems.
+ </para>
+ <para>
+ For what its worth the two known module writer work-arounds for trying
+ to maintain source level compatibility with both PAM implementations
+ are:
+ </para>
+ <itemizedlist>
+ <listitem>
+ <para>
+ never call the conversation function with num_msg greater than one.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ set up msg as doubly referenced so both types of conversation
+ function can find the messages. That is, make
+ </para>
+ <programlisting>
+ msg[n] = &amp; (( *msg )[n])
+ </programlisting>
+ </listitem>
+ </itemizedlist>
+ </refsect1>
+
+ <refsect1 id="pam_conv-return_values">
+ <title>RETURN VALUES</title>
+ <variablelist>
+ <varlistentry>
+ <term>PAM_BUF_ERR</term>
+ <listitem>
+ <para>
+ Memory buffer error.
+ </para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>PAM_CONV_ERR</term>
+ <listitem>
+ <para>
+ Conversation failure. The application should not set
+ <emphasis>*resp</emphasis>.
+ </para>
+ </listitem>
+ </varlistentry>
+ <varlistentry>
+ <term>PAM_SUCCESS</term>
+ <listitem>
+ <para>
+ Success.
+ </para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ </refsect1>
+
+ <refsect1 id='pam_conv-see_also'>
+ <title>SEE ALSO</title>
+ <para>
+ <citerefentry>
+ <refentrytitle>pam_start</refentrytitle><manvolnum>3</manvolnum>
+ </citerefentry>,
+ <citerefentry>
+ <refentrytitle>pam_set_item</refentrytitle><manvolnum>3</manvolnum>
+ </citerefentry>,
+ <citerefentry>
+ <refentrytitle>pam_get_item</refentrytitle><manvolnum>3</manvolnum>
+ </citerefentry>,
+ <citerefentry>
+ <refentrytitle>pam_strerror</refentrytitle><manvolnum>3</manvolnum>
+ </citerefentry>,
+ <citerefentry>
+ <refentrytitle>pam</refentrytitle><manvolnum>8</manvolnum>
+ </citerefentry>
+ </para>
+ </refsect1>
+</refentry>