PAM_CONV(3)                     Linux-PAM Manual                     PAM_CONV(3)

       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 pointers.

       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 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.

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

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

       •   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(8)

Linux-PAM Manual                   11/10/2020                        PAM_CONV(3)