The following is a minimal implementation of pam_unix(8), offering only authentication services. It should build and run with most PAM implementations, but takes advantage of OpenPAM extensions if available: note the use of pam_get_authtok(3), which enormously simplifies prompting the user for a password.

/*-
 * 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.
 *
 * $P4: //depot/projects/openpam/modules/pam_unix/pam_unix.c#3 $
 * $FreeBSD: release/10.2.0/en_US.ISO8859-1/articles/pam/pam_unix.c 38826 2012-05-17 19:12:14Z hrs $
 */

#include <sys/param.h>

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

#include <security/pam_modules.h>
#include <security/pam_appl.h>

#ifndef _OPENPAM
static char password_prompt[] = "Password:";
#endif

#ifndef PAM_EXTERN
#define PAM_EXTERN
#endif

PAM_EXTERN int
pam_sm_authenticate(pam_handle_t *pamh, int flags,
	int argc, const char *argv[])
{
#ifndef _OPENPAM
	struct pam_conv *conv;
	struct pam_message msg;
	const struct pam_message *msgp;
	struct pam_response *resp;
#endif
	struct passwd *pwd;
	const char *user;
	char *crypt_password, *password;
	int pam_err, retry;

	/* identify user */
	if ((pam_err = pam_get_user(pamh, &user, NULL)) != PAM_SUCCESS)
		return (pam_err);
	if ((pwd = getpwnam(user)) == NULL)
		return (PAM_USER_UNKNOWN);

	/* get password */
#ifndef _OPENPAM
	pam_err = pam_get_item(pamh, PAM_CONV, (const void **)&conv);
	if (pam_err != PAM_SUCCESS)
		return (PAM_SYSTEM_ERR);
	msg.msg_style = PAM_PROMPT_ECHO_OFF;
	msg.msg = password_prompt;
	msgp = &msg;
#endif
	for (retry = 0; retry < 3; ++retry) {
#ifdef _OPENPAM
		pam_err = pam_get_authtok(pamh, PAM_AUTHTOK,
		    (const char **)&password, NULL);
#else
		resp = NULL;
		pam_err = (*conv->conv)(1, &msgp, &resp, conv->appdata_ptr);
		if (resp != NULL) {
			if (pam_err == PAM_SUCCESS)
				password = resp->resp;
			else
				free(resp->resp);
			free(resp);
		}
#endif
		if (pam_err == PAM_SUCCESS)
			break;
	}
	if (pam_err == PAM_CONV_ERR)
		return (pam_err);
	if (pam_err != PAM_SUCCESS)
		return (PAM_AUTH_ERR);

	/* compare passwords */
	if ((!pwd->pw_passwd[0] && (flags & PAM_DISALLOW_NULL_AUTHTOK)) ||
	    (crypt_password = crypt(password, pwd->pw_passwd)) == NULL ||
	    strcmp(crypt_password, pwd->pw_passwd) != 0)
		pam_err = PAM_AUTH_ERR;
	else
		pam_err = PAM_SUCCESS;
#ifndef _OPENPAM
	free(password);
#endif
	return (pam_err);
}

PAM_EXTERN int
pam_sm_setcred(pam_handle_t *pamh, int flags,
	int argc, const char *argv[])
{

	return (PAM_SUCCESS);
}

PAM_EXTERN int
pam_sm_acct_mgmt(pam_handle_t *pamh, int flags,
	int argc, const char *argv[])
{

	return (PAM_SUCCESS);
}

PAM_EXTERN int
pam_sm_open_session(pam_handle_t *pamh, int flags,
	int argc, const char *argv[])
{

	return (PAM_SUCCESS);
}

PAM_EXTERN int
pam_sm_close_session(pam_handle_t *pamh, int flags,
	int argc, const char *argv[])
{

	return (PAM_SUCCESS);
}

PAM_EXTERN int
pam_sm_chauthtok(pam_handle_t *pamh, int flags,
	int argc, const char *argv[])
{

	return (PAM_SERVICE_ERR);
}

#ifdef PAM_MODULE_ENTRY
PAM_MODULE_ENTRY("pam_unix");
#endif