Example of getty program

stegetty.c

#define _GNU_SOURCE

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <getopt.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/ioctl.h>

static int do_dup(int fd, int newfd) {
	int dupped_fd = dup3(fd, newfd, 0);
	if(dupped_fd < 0 && errno != EINVAL) {
		return -1;
	}

	return !(dupped_fd == newfd);
}

static int get_the_tty(const char* tty) {
	int fd_tty = open(tty, O_RDWR);

	if(fd_tty < 0) {
		goto __get_the_tty_ret0;
	}

	close(STDIN_FILENO);
	close(STDOUT_FILENO);
	close(STDERR_FILENO);

	if(do_dup(fd_tty, STDIN_FILENO) != 0) {
		goto __get_the_tty_ret1;
	}

	if(do_dup(fd_tty, STDOUT_FILENO) != 0) {
		goto __get_the_tty_ret1;
	}

	if(do_dup(fd_tty, STDERR_FILENO) != 0) {
		goto __get_the_tty_ret1;
	}

	if(fd_tty > STDERR_FILENO) {
		close(fd_tty);
	}

	if(!isatty(STDIN_FILENO)) {
		perror("isatty");
		goto __get_the_tty_ret0;
	}

	if(ioctl(STDIN_FILENO, TIOCSCTTY, 0) != 0) {
		perror("ioctl");
		goto __get_the_tty_ret0;
	}

	return STDIN_FILENO;

__get_the_tty_ret1:
	close(fd_tty);
__get_the_tty_ret0:
	return -1;
}


int main(int argc, char** argv) {
	char *tty;
	int opt;

	while((opt = getopt(argc, argv, "t:")) != -1) {
		switch(opt) {
			case 't':
				tty = optarg;
				break;
		}
	}

	if(setsid() < 0 && errno != EPERM) {
		exit(11);
	}

	int tty_fd = get_the_tty(tty);
	if(tty_fd < 0) {
		exit(22);
	}

	if(tcsetpgrp(tty_fd, getpgrp()) != 0) {
		exit(33);
	}

	printf("stegetty on %s\n", strrchr(tty, '/') + 1);

	execl("/bin/login", "/bin/login", "--", NULL);
	perror("execl");

	exit(44);
}