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])