3.2. What is expected of an application
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## 3.2. What is expected of an application

### 3.2.1. The 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;
    };

#### 3.2.1.1. 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])

#### 3.2.1.2. 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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