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       pam_conv - PAM conversation function


       #include <security/pam_appl.h>

       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;


       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 struct pam_conv passed to pam_start(3) at
       the start of the transaction.

       When a module calls the referenced conv() function, the argument
       appdata_ptr is set to the second element of this structure.

       The other arguments of a call to conv() concern the information
       exchanged by module and application. That is to say, num_msg holds the
       length of the array of pointers, msg. After a successful return, the
       pointer resp points to an array of pam_response structures, holding the
       application supplied text. The resp_retcode 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 free(3).
       Note, *resp is a struct pam_response array and not an array of

       The number of responses is always equal to the num_msg conversation
       function argument. This does require that the response array is
       free(3)'d after every call to the conversation function. The index of
       the responses corresponds directly to the prompt index in the
       pam_message array.

       On failure, the conversation function should release any resources it
       has allocated, and return one of the predefined PAM error codes.

       Each message can have one of four types, specified by the msg_style
       member of struct pam_message:

           Obtain a string without echoing any text.

           Obtain a string whilst echoing text.

           Display an error message.

           Display some text.

       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.

       In passing, it is worth noting that there is a descrepency between the
       way Linux-PAM handles the const struct pam_message **msg conversation
       function argument from the way that Solaris' PAM (and derivitives,
       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.

       For what its worth the two known module writer work-arounds for trying
       to maintain source level compatibility with both PAM implementations

       o   never call the conversation function with num_msg greater than one.

       o   set up msg as doubly referenced so both types of conversation
           function can find the messages. That is, make

                      msg[n] = & (( *msg )[n])


           Memory buffer error.

           Conversation failure. The application should not set *resp.



       pam_start(3), pam_set_item(3), pam_get_item(3), pam_strerror(3), pam(7)