mc sets process group differently from shell; forgets to close fd# 3

[Pavel Tsekov]

On Fri, 20 Apr 2007, Denis Vlasenko wrote:

> If you compile and run the following program
> under shell and under mc...

[...]

> you will see the following:
>
> bash-3.2# ./z
> pid=8183
> ppid=8181
> tty_pgrp=8183    <========== child is in its own process group
> task_pgrp=8183   <==========
> fd# 0: '/dev/pts/4'
> fd# 1: '/dev/pts/4'
> fd# 2: '/dev/pts/4'
>
> bash-3.2# echo $$ ; exec mc
> 8181
>
> # ./z
> pid=8212
> ppid=8181
> tty_pgrp=8181    <========== child is in mc's process group
> task_pgrp=8181   <========== child is in mc's process group
> fd# 0: '/dev/pts/4'
> fd# 1: '/dev/pts/4'
> fd# 2: '/dev/pts/4'
> fd# 3: '/dev/tty'  <=== ??!
>
> Note that there are two differences, one minor and one pretty big.
>
> Small one: child process is not given its own porcess group when run
> under mc, while under sh it is. It means that if child will create its

Yes - because, you have a shell which supports job control.

> own process group by itself and then die, under shell it works but
> under mc mc will auto-background itself on child's death,
> because tty's pgrp will not match mc's. Nasty.

I am confused a bit by the statement above. The foreground process
group (tty's pgrp) is controlled by the process which has the
controlling terminal - the shell itself. Neither the child nor MC
can manipulate the foreground process group. Please, explain in
details what you mean. If can provides a simple testcase to reproduce
the problem I'd be happy to investigate.

> Big one (actually, I think it's a bug): stray fd# opened
> to controlling terminal.

/dev/tty is opened by the S-Lang library and, yes, it should
be closed when the new process executes.