Re: RFC: implement daemon() in the kernel

["H. Peter Anvin" <hpa@zytor.com>]

Simon Richter wrote:
>  - daemonize a process
>
> There is a function called daemon() that does this; its behaviour is
> roughly defined by (modulo error handling)
>
> int daemon(int nochdir, int noclose)
> {
> 	if(!nochdir)
> 		chdir("/");
>
> 	if(!noclose)
> 	{
> 		int fd = open("/dev/null", O_RDWR);
> 		dup2(fd, 0);
> 		dup2(fd, 1);
> 		dup2(fd, 2);
> 		close(fd);
> 	}
>
> 	if(fork() > 0)

... that should be if (fork() == 0) ...

> 		_exit(0);

	setsid();
> }


> Since it calls _exit() right after fork() returns (so daemon() never
> returns to the calling process except in case of an error) it would be
> possible to implement this on MMUless machines if the last two lines
> could happen in the kernel.

You could do this quite easily with clone() and a small assembly wrapper.

The assembly wrapper needs to do the last two lines without touching the 
stack in the parent.  That is usually quite trivial, even on 
register-starved architectures; for example, on i386 it would look like 
(ignoring vsyscalls for the moment, which are only an optimization anyway).

__detach_from_parent:
	pushl	%ebx
	movl	$__NR_clone, %eax
	movl	$CLONE_VM|SIGCHLD, %ebx
	xorl	%ecx, %ecx
	int	$0x80
	cmpl	$-4096, %eax
	ja	1f
	andl	%eax, %eax
	je	2f
	# Parent process, must _exit(0)
	xorl	%ebx, %ebx
	movl	$__NR_exit, %eax
	int	$0x80
	# _exit() should never return
	hlt
1:	# Error on fork(), set errno and return -1
	negl	%eax
	movl	%eax, errno		# Or TLS equivalent
	orl	$-1, %eax
2:	# Child process jumps here with %eax == 0 already
	popl	%ebx
	ret

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