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're 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);
}