C. Sample PAM Conversation Function

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.

#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);
}