NAME

       pam_conv - PAM conversation function

SYNOPSIS

       #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;
       };

DESCRIPTION

       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:

       PAM_PROMPT_ECHO_OFF
           Obtain a string without echoing any text.

       PAM_PROMPT_ECHO_ON
           Obtain a string whilst echoing text.

       PAM_ERROR_MSG
           Display an error message.

       PAM_TEXT_INFO
           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])

RETURN VALUES

       PAM_BUF_ERR
           Memory buffer error.

       PAM_CONV_ERR
           Conversation failure. The application should not set *resp.

       PAM_SUCCESS
           Success.

SEE ALSO

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