PAM Authentication for a Legacy Application

I have a legacy app that receives a username/password request asynchronously over the wire. Since I already have the username and password stored as variables, what would be the best way to authenticate with PAM on Linux (Debian 6)?

I've tried writing my own conversation function, but I'm not sure of the best way of getting the password into it. I've considered storing it in appdata and referencing that from the pam_conv struct, but there's almost no documentation on how to do that.

Is there a simpler way to authenticate users without the overkill of a conversation function? I'm unable to use pam_set_data successfully either, and I'm not sure that's even appropriate.

Here's what I'm doing:

user = guiMessage->username;
pass = guiMessage->password;

pam_handle_t* pamh = NULL;
int           pam_ret;
struct pam_conv conv = {
  my_conv,
  NULL
};

pam_start("nxs_login", user, &conv, &pamh);
pam_ret = pam_authenticate(pamh, 0);

if (pam_ret == PAM_SUCCESS)
  permissions = 0xff;

pam_end(pamh, pam_ret);

And initial attempts at the conversation function resulted in (password is hard-coded for testing):

int 
my_conv(int num_msg, const struct pam_message **msg, struct pam_response **resp, void *data)
{
  struct pam_response *aresp;

  if (num_msg <= 0 || num_msg > PAM_MAX_NUM_MSG)
    return (PAM_CONV_ERR);
  if ((aresp = (pam_response*)calloc(num_msg, sizeof *aresp)) == NULL)
    return (PAM_BUF_ERR);
  aresp[0].resp_retcode = 0;
  aresp[0].resp = strdup("mypassword");

  *resp = aresp;
  return (PAM_SUCCESS);
}

Any help would be appreciated. Thank you!