# pam_conv

## 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.
    


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