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03.09.2008 at 10:29AM PDT, ID: 23226789
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9.0

Daemon behavior (detaching from tty)

Zones: C Programming Language, Linux, Linux Programming
This question relates to programming for Linux.  I understand that all programs which run under a bash terminal will exit when the terminal closes, unless they are run in the background by appending an '&' symbol at the command line.

But I assume most serious daemons are programmed to detach from the terminal automatically, i.e. this behavior is coded into the application itself.  When I look at the init.d scripts for many popular daemons, such as Apache for example, I do not see a '&' symbol after the start-stop-daemon command in the init.d script, so I assume these daemons are coded to detach from the terminal.

So I tried to write my application so that it automatically detaches from the terminal.  According to the linux man page, this can be done using ioctl.  As a small test, I tried:

      int fd = open("/dev/tty", O_RDWR);
      if (fd >= 0) {
            int ret = ioctl(fd, TIOCNOTTY, 0);
            if (ret == -1) perror("ioctl");
            close(fd);
      }
      else perror("open");

      // pretend we are a daemon...
      for (;;) sleep(10);

This seems to work, because when I view all running processes, a question mark appears under TTY.  But this still doesn't produce the behavior I would expect from a daemon.  When I execute the program from the bash prompt, it still hangs in the foreground, rather than returning me to the bash prompt.  Why does it do this, when it is no longer associated with the current terminal?
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Question Stats
Zone: Programming
Question Asked By: chsalvia
Solution Provided By: ravenpl
Participating Experts: 5
Solution Grade: A
Views: 31
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03.09.2008 at 11:04AM PDT, ID: 21082092

Rank: Sage

ravenpl:

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03.09.2008 at 11:14AM PDT, ID: 21082123

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omarfarid:

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03.09.2008 at 11:18AM PDT, ID: 21082132

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omarfarid:

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03.09.2008 at 11:33AM PDT, ID: 21082187

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omarfarid:

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03.09.2008 at 12:05PM PDT, ID: 21082277

Rank: Guru

evilrix:

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03.09.2008 at 06:05PM PDT, ID: 21083480

Rank: Wizard

duncan_roe:

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03.09.2008 at 08:16PM PDT, ID: 21083956

Rank: Guru

Nopius:

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03.09.2008 at 11:04AM PDT, ID: 21082092

Rank: Sage

ravenpl:
Accepted Solution
 
03.09.2008 at 11:14AM PDT, ID: 21082123

Rank: Guru

omarfarid:
Please see:

http://www.hmug.org/man/2/setsid.php

You need to refer to a very good book by W. Richard Stevens "Unix Network Programming" 1st edition or his other book "Advanced Programming in the Unix Environment"

 
03.09.2008 at 11:18AM PDT, ID: 21082132

Rank: Guru

omarfarid:
Assisted Solution
 
03.09.2008 at 11:33AM PDT, ID: 21082187

Rank: Guru

omarfarid:
 
03.09.2008 at 12:05PM PDT, ID: 21082277

Rank: Guru

evilrix:
You can use fork() to start a child process running as a daemon. The parent then quits, leaving the child process running in the background. Very short example below.

Unix Daemon Server Programming
http://www.enderunix.org/documents/eng/daemon.php
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#include <unistd.h>
#include <stdio.h>
 
int main()
{
        pid_t pid = fork();
 
        if(pid < 0)
        {
                // Oops, something wicked this way comes!
                puts("Error forking");
                perror(NULL);
        }
        else
        if(0 == pid)
        {
                setsid(); // Detatch from parent
                for(;;) sleep(0); // Keep child alive to prove it's daemonised
        }
        else
        {
                // Parent waits for child
                printf("Daemon started (%d)\n", pid);
        }
 
        return 0;
}
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Assisted Solution
 
03.09.2008 at 06:05PM PDT, ID: 21083480

Rank: Wizard

duncan_roe:
There is actually a function to do all the above - see "man -s 3 daemon"
 
03.09.2008 at 08:16PM PDT, ID: 21083956

Rank: Guru

Nopius:
All the posts above about using fork() are correct and I agree with all the experts.
But your question is not that simple as it seems :-)

> When I execute the program from the bash prompt, it still hangs in the foreground, rather than returning me to the bash prompt.  Why does it do this, when it is no longer associated with the current terminal?

Closing controlling terminal does not mean exiting from the process. Also there is no way to close login shell's tty from any child process.

You have closed the controlling terminal of your current process, but not of your parent shell (it still owns it) and shell knows nothing about your manipulation with tty device. Also your program never returns control to the parent shell and shell still waits until your program exits. That's a native behavior of your _shell_ to run any program and wait while it exits until & is used. And that's why all this these tricks with fork() (BTW ampersand in a shell also maps to the separate fork() syscall).

But I know many advanced sysadmins and programmers who think that 'fork' is the only way to create constantly running process in background. But that's not the absolute true :-)

That's not necessary to use fork(). You _may_ create a background process that never exits without using fork(), but you loose your login shell in that case and you should connect again (but now the program will be running in a background). Since we can't close the tty of parent one easy way to 'detach' is to kill() the parent. See two sample C programs in attachment. One with fork() the other with killing parent. I used this sample programs when leading Linux programming training some time ago, specially to clarify why fork() is not the only, but the only good way to daemonize.
When testing, daemon_bad kills your parent shell, so be prepared to disconnect and get new login: prompt, when running on local machine (the daemon still remains running). When running from ssh session, we have more than 1 parent to kill to completely close the session, so just close the session window and it still will be running.






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/* demonstrate easy daemonization */
/* (c) 2005-2008, by Nopius */
#include <unistd.h>
#include <signal.h>
#include <stdlib.h>
int
main()
{
  struct sigaction act;
  /* some info about me */
  printf("Pid: %d, Ppid: %d, Pgrp: %d, Sid: %d\n", getpid(), getppid(), 
    getpgrp(), getsid(0));
 
  act.sa_handler=SIG_IGN;
  sigaction(SIGHUP, &act, NULL); /* ignore termilal disconnect */
 
  kill(getppid(), SIGKILL); /* kill the parrent */
  do 
  {
      printf("I'm a daemon\n");
      sleep(5);
    } while(1);
 
}
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daemon-fork.txt (555 bytes) (File Type Details) 
Daemonization with fork() call
 
 
daemon-bad.txt (514 bytes) (File Type Details) 
BAD daemonization without fork()
 
Assisted Solution
 
 
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