Minimal example to use PAM

auth.c

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

#include <security/pam_appl.h>

int login_conv(int num_msg, const struct pam_message **msg,
               struct pam_response **resp, void *appdata_ptr)
{
    char *str = NULL;
    *resp = calloc(sizeof(struct pam_response), num_msg);

    for (size_t i = 0; i < num_msg; i++)
        switch (msg[i]->msg_style)
        {
        case PAM_PROMPT_ECHO_ON:
        case PAM_PROMPT_ECHO_OFF:
            resp[i]->resp = getpass(msg[i]->msg);
            break;

        case PAM_ERROR_MSG:
        case PAM_TEXT_INFO:
            fprintf(stderr, "%s\n", msg[i]->msg);
        }

    return (PAM_SUCCESS);
}

int main(int argc, char const *argv[])
{
    int status = EXIT_SUCCESS;
    char *user = NULL;
    struct pam_conv conv = {.conv = login_conv, .appdata_ptr = NULL};
    pam_handle_t *pamh = {NULL};

    if (argc != 2)
        user = getpwuid(getuid())->pw_name;
    else
        user = (char *)argv[1];

    if (pam_start("login", user, &conv, &pamh))
        err(EXIT_FAILURE, "pam_start()");

    if (pam_authenticate(pamh, 0))
    {
        fprintf(stderr, "Authentication failure!\n");
        status = EXIT_FAILURE;
    }
    else
        printf("Success!\n");

    pam_end(pamh, status);
    return (status);
}

Compile:

cc  auth.c -o auth -lpam