busybox 1.1.3 - fork on nommu

[Rob Landley]

On Wednesday 25 October 2006 9:03 pm, Denis Vlasenko wrote:
> Someone was lazy and didn't code the check.
> Send patch to fix it.
> 
> Well, I am unfortunately do not have nommu system here
> (I would love an qemu-usable image of one. hint).

I never got one to run under qemu, but the uClinux guys did walk me through 
setting up ARMulator:
http://busybox.net/lists/busybox/2006-July/023392.html 

> I see that we have some NOMMU friendly helpers:
> 
> * vfork_daemon_rexec() - a daemon() equivalent for NOMMU?

I kept meaning to replace that, never got around to it.  This is one of the 
things a global argc/argv allows you to do, jump through fewer hoops 
supplying it again when doing daemonize.

The problem is that vfork() blocks the parent until the child calls exec() or 
_exit().  Even though all the parent wants to do is exit(), it can't unblock 
to do so until the child execs something.  So we have to pull some variant of 
the /proc/self/exe trick.  (One of the functions I already have in toybox is 
an early attempt at find_in_path(), which finds an executable in your $PATH.  
Kind of surprising that libc parses colon seprated paths in execvp() and 
such, but doesn't have an orthogonal function for it...)

It's possible that most of what it's doing with the fork thing can also be 
done with things like setsid(), I'll have to check.  The one thing you can't 
do without a fork is move your parent process: only our parent exiting will 
reliably give us init as our parent process.  But if our parent process 
doesn't reap zombies, how much do we really care?

Note you can also do pretty much all of daemonize() through a command shell, 
ala:

(command > /dev/null 2> /dev/null </dev/null &)&

And yes, that backgrounds it twice intentionally...

> * spawn() - vfork()+exec()

I wrote that to be nommu friendly, yes.  Haven't stringently tested it.

> but none is applicable to the piece of code you are talking of.
> 
> The more I look into it, init.c will need some surgery if you
> want to make it work without forking (not fork+exec, just fork).

It was on my todo list. :)

Rob
-- 
"Perfection is reached, not when there is no longer anything to add, but
when there is no longer anything to take away." - Antoine de Saint-Exupery