blob: f9b59c26321906d9b7039fc977ebf80cbe8ca6b4 [file] [log] [blame]
/* vi: set sw=4 ts=4: */
* Copyright (c) 2007 Denys Vlasenko <>
* Licensed under GPLv2, see file LICENSE in this source tree.
#include "libbb.h"
//kbuild:lib-$(CONFIG_CTTYHACK) += cttyhack.o
//config:config CTTYHACK
//config: bool "cttyhack"
//config: default y
//config: help
//config: One common problem reported on the mailing list is the "can't
//config: access tty; job control turned off" error message, which typically
//config: appears when one tries to use a shell with stdin/stdout on
//config: /dev/console.
//config: This device is special - it cannot be a controlling tty.
//config: The proper solution is to use the correct device instead of
//config: /dev/console.
//config: cttyhack provides a "quick and dirty" solution to this problem.
//config: It analyzes stdin with various ioctls, trying to determine whether
//config: it is a /dev/ttyN or /dev/ttySN (virtual terminal or serial line).
//config: On Linux it also checks sysfs for a pointer to the active console.
//config: If cttyhack is able to find the real console device, it closes
//config: stdin/out/err and reopens that device.
//config: Then it executes the given program. Opening the device will make
//config: that device a controlling tty. This may require cttyhack
//config: to be a session leader.
//config: Example for /etc/inittab (for busybox init):
//config: ::respawn:/bin/cttyhack /bin/sh
//config: Starting an interactive shell from boot shell script:
//config: setsid cttyhack sh
//config: Giving controlling tty to shell running with PID 1:
//config: # exec cttyhack sh
//config: Without cttyhack, you need to know exact tty name,
//config: and do something like this:
//config: # exec setsid sh -c 'exec sh </dev/tty1 >/dev/tty1 2>&1'
//config: Starting getty on a controlling tty from a shell script:
//config: # getty 115200 $(cttyhack)
//usage:#define cttyhack_trivial_usage
//usage: "[PROG ARGS]"
//usage:#define cttyhack_full_usage "\n\n"
//usage: "Give PROG a controlling tty if possible."
//usage: "\nExample for /etc/inittab (for busybox init):"
//usage: "\n ::respawn:/bin/cttyhack /bin/sh"
//usage: "\nGiving controlling tty to shell running with PID 1:"
//usage: "\n $ exec cttyhack sh"
//usage: "\nStarting interactive shell from boot shell script:"
//usage: "\n setsid cttyhack sh"
#if !defined(__linux__) && !defined(TIOCGSERIAL) && !ENABLE_WERROR
# warning cttyhack will not be able to detect a controlling tty on this system
/* From <linux/vt.h> */
struct vt_stat {
unsigned short v_active; /* active vt */
unsigned short v_signal; /* signal to send */
unsigned short v_state; /* vt bitmask */
enum { VT_GETSTATE = 0x5603 }; /* get global vt state info */
/* From <linux/serial.h> */
struct serial_struct {
int type;
int line;
unsigned int port;
int irq;
int flags;
int xmit_fifo_size;
int custom_divisor;
int baud_base;
unsigned short close_delay;
char io_type;
char reserved_char[1];
int hub6;
unsigned short closing_wait; /* time to wait before closing */
unsigned short closing_wait2; /* no longer used... */
unsigned char *iomem_base;
unsigned short iomem_reg_shift;
unsigned int port_high;
unsigned long iomap_base; /* cookie passed into ioremap */
int reserved[1];
int cttyhack_main(int argc, char **argv) MAIN_EXTERNALLY_VISIBLE;
int cttyhack_main(int argc UNUSED_PARAM, char **argv)
int fd;
char console[sizeof(int)*3 + 16];
union {
struct vt_stat vt;
struct serial_struct sr;
char paranoia[sizeof(struct serial_struct) * 3];
} u;
strcpy(console, "/dev/tty");
fd = open(console, O_RDWR);
if (fd < 0) {
/* We don't have ctty (or don't have "/dev/tty" node...) */
do {
#ifdef __linux__
/* Note that this method does not use _stdin_.
* Thus, "cttyhack </dev/something" can't be used.
* However, this method is more reliable than
* TIOCGSERIAL check, which assumes that all
* serial lines follow /dev/ttySn convention -
* which is not always the case.
* Therefore, we use this method first:
int s = open_read_close("/sys/class/tty/console/active",
console + 5, sizeof(console) - 5);
if (s > 0) {
char *last;
/* Found active console via sysfs (Linux 2.6.38+).
* It looks like "[tty0 ]ttyS0\n" so zap the newline:
console[4 + s] = '\0';
/* If there are multiple consoles,
* take the last one:
last = strrchr(console + 5, ' ');
if (last)
overlapping_strcpy(console + 5, last + 1);
if (ioctl(0, VT_GETSTATE, &u.vt) == 0) {
/* this is linux virtual tty */
sprintf(console + 8, "S%u" + 1, (int)u.vt.v_active);
if (ioctl(0, TIOCGSERIAL, & == 0) {
/* this is a serial console; assuming it is named /dev/ttySn */
sprintf(console + 8, "S%u", (int);
/* nope, could not find it */
console[0] = '\0';
} while (0);
if (!argv[0]) {
if (!console[0])
if (fd < 0) {
fd = open_or_warn(console, O_RDWR);
if (fd < 0)
goto ret;
//bb_error_msg("switching to '%s'", console);
dup2(fd, 0);
dup2(fd, 1);
dup2(fd, 2);
while (fd > 2)
/* Some other session may have it as ctty,
* try to steal it from them:
ioctl(0, TIOCSCTTY, 1);