Why POSIX daemonization is difficult

---

Here's how Stevens thinks you should demonize:

void daemonize(const char *cmd) {
	if ((pid = fork()) < 0)
		err(1, "fork fail");
	else if (pid != 0) /* parent */
		exit(0)
	
	/* child A */
	setsid()
 
	if ((pid = fork()) < 0)
		err(2, "fork fail");
	else if (pid != 0) /* child A * /
		exit(0)
	
	/* child B (grandchild) */
	do_daemon_stuff();
}
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

Most people have two questions about this code:

1. Why setsid()
   
   
   
   is it in the first child?

   
   
   
   
   

2. Why do fork()
   
   
   
   it again?

   
   
   
   
   

Why is setsid () in the first child?

According to Stevens, setsid()



it does three important things:

1. The process becomes the session leader of the new session, which contains only the calling process. (PID = SID)

   
   
   
   
   

2. . (PID = SID = PGID)

   
   
   
   
   

3. . setsid()
   
   
   
   , .

   
   
   
   
   

, setsid()



, (PID = PGID), fork()



, , ( ).

setsid()



, . , , .

fork() ?

:

1. , init
   
   
   
   , . Unix , fork()
   
   
   
   . , , , init
   
   
   
   , , waitpid()
   
   
   
   , fork()
   
   
   
   . fork()



waitpid()
   
   
   
   , . , , , - init
   
   
   
   , , init
   
   
   
   SIGCLD, , init



waitpid()
   
   
   
   .

   
   
   
   
   

2. System V fork()
   
   
   
   - . System V: AIX, Solaris HP-UX, fork()
   
   
   
   , .

   
   
   
   
   

?

, , wait/waitpid()



, ( - !). wait/waitpid()



pid



, . .

, - . , busybox - 2005 , - , init



. , , waitpid



, .

:

1. 
   
   
   
   

2. ,

   
   
   
   
   

3. , wait/waitpid
   
   
   
   

   
   
   
   
   

, , , :

• kernel reaper , SIGCHLD (OpenBSD).

  
  
  
  
  

• exit()
  
  
  
  , SIGCHLD, , (XNU/OSX)

  
  
  
  
  

, . SIGCHLD, fork



setsid



. SIGCHLD, , wait



.

static void signal_handler(int sig) {
	int stat;
	wait(&stat);
}


void main(void) {
    struct sigaction sigact;
    sigact.sa_handler = signal_handler;
    sigemptyset(&sigact.sa_mask);
    sigact.sa_flags = 0;
    sigaction(SIGCHLD, &sigact, (struct sigaction *)NULL);
 
	if ((pid = fork()) < 0)
		err(1, "fork fail");
	else if (pid != 0)
		do_parent_stuff();
	else {
		setsid()
		do_child_stuff();
	}
}
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

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