toys/other/oneit.c

/* oneit.c - tiny init replacement to launch a single child process.
 *
 * Copyright 2005, 2007 by Rob Landley <rob@landley.net>.

USE_ONEIT(NEWTOY(oneit, "^<1c:p", TOYFLAG_SBIN))

config ONEIT
  bool "oneit"
  default y
  help
    usage: oneit [-p] [-c /dev/tty0] command [...]

    A simple init program that runs a single supplied command line with a
    controlling tty (so CTRL-C can kill it).

    -p	Power off instead of rebooting when command exits.
    -c	Which console device to use.

    The oneit command runs the supplied command line as a child process
    (because PID 1 has signals blocked), attached to /dev/tty0, in its
    own session. Then oneit reaps zombies until the child exits, at
    which point it reboots (or with -p, powers off) the system.
*/

#define FOR_oneit
#include "toys.h"
#include <sys/reboot.h>

GLOBALS(
  char *console;
)

// The minimum amount of work necessary to get ctrl-c and such to work is:
//
// - Fork a child (PID 1 is special: can't exit, has various signals blocked).
// - Do a setsid() (so we have our own session).
// - In the child, attach stdio to /dev/tty0 (/dev/console is special)
// - Exec the rest of the command line.
//
// PID 1 then reaps zombies until the child process it spawned exits, at which
// point it calls sync() and reboot().  I could stick a kill -1 in there.


void oneit_main(void)
{
  int i;
  pid_t pid;

  // Create a new child process.
  pid = vfork();
  if (pid) {

    // pid 1 just reaps zombies until it gets its child, then halts the system.
    while (pid != wait(&i));
    sync();

    // PID 1 can't call reboot() because it kills the task that calls it,
    // which causes the kernel to panic before the actual reboot happens.
    if (!vfork()) reboot((toys.optflags & FLAG_p) ? RB_POWER_OFF : RB_AUTOBOOT);
    sleep(5);
    _exit(1);
  }

  // Redirect stdio to /dev/tty0, with new session ID, so ctrl-c works.
  setsid();
  for (i=0; i<3; i++) {
    close(i);
    xopen(TT.console ? TT.console : "/dev/tty0", O_RDWR);
  }

  // Can't xexec() here, because we vforked so we don't want to error_exit().
  toy_exec(toys.optargs);
  execvp(*toys.optargs, toys.optargs);
  _exit(127);
}