Simple example of how `getent ---service <service>` retrieves the information for specific service only.

auth.c

#include <sys/types.h>
#include <grp.h>
#include <pwd.h>
#include <nss.h>
#include <stdio.h>

/* Possibly a memory leak according to documentation */
#define SETUP_DB(db, service) if (service != NULL) { __nss_configure_lookup(db, service); }

void print_passwd(struct passwd *entry) {
    printf("Name: %s\n", entry->pw_name);
    /* entry->pw_passwd is likely always 'x' */
    printf("UID: %lu\n", entry->pw_uid);
    printf("GID: %lu\n", entry->pw_gid);
    printf("Info: %s\n", entry->pw_gecos);
    printf("HOME: %s\n", entry->pw_dir);
    printf("SHELL: %s\n", entry->pw_shell);
}

#define MAX_GROUPS 128

void print_group_name(gid_t gid) {
    struct group *entry = getgrgid(gid);

    printf("%s", entry ? entry->gr_name : "(NULL)");
}

/* Works for files and winbind */
void print_passwd_groups(struct passwd *entry) {
    int count = 0;
    int ngroups = MAX_GROUPS;
    gid_t groups[MAX_GROUPS];

    printf("Group: ");
    print_group_name(entry->pw_gid);
    printf("\n");

    count = getgrouplist(entry->pw_name, entry->pw_gid, groups, &ngroups);

    printf("Groups:");
    for (int i = 0; i < count; i++) {
        printf(" ");
        print_group_name(groups[i]);
    }
    printf("\n");
}

#define DB_PASSWD "passwd"

void handle_passwd(char* service) {
    struct passwd *entry = NULL;

    setpwent();
    while ((entry = getpwent()) != NULL) {
        print_passwd(entry);
        print_passwd_groups(entry);
        printf("\n");
    }
    endpwent();
}

void print_group(struct group *entry) {
    char **members = NULL;
    char *member = NULL;

    printf("Name: %s\n", entry->gr_name);
    /* entry->gr_passwd is likely always 'x' */
    printf("GID: %lu\n", entry->gr_gid);
    printf("Members:");

    /* Doesn't work for winbind */
    members = entry->gr_mem;
    for (member = *members; member != NULL; member = *(++members)) {
        printf(" %s", member);
    }
    printf("\n");
}

#define DB_GROUP "group"

void handle_group(char* service) {
    struct group *entry = NULL;

    setgrent();
    while ((entry = getgrent()) != NULL) {
        print_group(entry);
        printf("\n");
    }
    endgrent();
}

int main(int argc, char** argv) {
    /* files, winbind and the like */
    char* service = (argc > 1) ? argv[1] : NULL;

    SETUP_DB(DB_PASSWD, service);
    SETUP_DB(DB_GROUP, service);

    printf("List of users:\n\n");
    handle_passwd(service);

    printf("List of groups:\n\n");
    handle_group(service);

    return 0;
}