The conversation function presented below is a greatly simplified version of OpenPAM's openpam_ttyconv(3). It is fully functional, and should give the reader a good idea of how a conversation function should behave, but it is far too simple for real-world use. Even if you are not using OpenPAM, feel free to download the source code and adapt openpam_ttyconv(3) to your uses; we believe it to be as robust as a tty-oriented conversation function can reasonably get.

/*-
 * Copyright (c) 2002 Networks Associates Technology, Inc.
 * All rights reserved.
 *
 * This software was developed for the FreeBSD Project by ThinkSec AS and
 * Network Associates Laboratories, the Security Research Division of
 * Network Associates, Inc.  under DARPA/SPAWAR contract N66001-01-C-8035
 * ("CBOSS"), as part of the DARPA CHATS research program.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. The name of the author may not be used to endorse or promote
 *    products derived from this software without specific prior written
 *    permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 * $FreeBSD: release/10.2.0/en_US.ISO8859-1/articles/pam/converse.c 38826 2012-05-17 19:12:14Z hrs $
 */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>

#include <security/pam_appl.h>

int
converse(int n, const struct pam_message **msg,
	struct pam_response **resp, void *data)
{
	struct pam_response *aresp;
	char buf[PAM_MAX_RESP_SIZE];
	int i;

	data = data;
	if (n <= 0 || n > PAM_MAX_NUM_MSG)
		return (PAM_CONV_ERR);
	if ((aresp = calloc(n, sizeof *aresp)) == NULL)
		return (PAM_BUF_ERR);
	for (i = 0; i < n; ++i) {
		aresp[i].resp_retcode = 0;
		aresp[i].resp = NULL;
		switch (msg[i]->msg_style) {
		case PAM_PROMPT_ECHO_OFF:
			aresp[i].resp = strdup(getpass(msg[i]->msg));
			if (aresp[i].resp == NULL)
				goto fail;
			break;
		case PAM_PROMPT_ECHO_ON:
			fputs(msg[i]->msg, stderr);
			if (fgets(buf, sizeof buf, stdin) == NULL)
				goto fail;
			aresp[i].resp = strdup(buf);
			if (aresp[i].resp == NULL)
				goto fail;
			break;
		case PAM_ERROR_MSG:
			fputs(msg[i]->msg, stderr);
			if (strlen(msg[i]->msg) > 0 &&
			    msg[i]->msg[strlen(msg[i]->msg) - 1] != '\n')
				fputc('\n', stderr);
			break;
		case PAM_TEXT_INFO:
			fputs(msg[i]->msg, stdout);
			if (strlen(msg[i]->msg) > 0 &&
			    msg[i]->msg[strlen(msg[i]->msg) - 1] != '\n')
				fputc('\n', stdout);
			break;
		default:
			goto fail;
		}
	}
	*resp = aresp;
	return (PAM_SUCCESS);
 fail:
        for (i = 0; i < n; ++i) {
                if (aresp[i].resp != NULL) {
                        memset(aresp[i].resp, 0, strlen(aresp[i].resp));
                        free(aresp[i].resp);
                }
        }
        memset(aresp, 0, n * sizeof *aresp);
	*resp = NULL;
	return (PAM_CONV_ERR);
}