Re: Question on SIGSTOP

From: Barry Margolin (barmar_at_alum.mit.edu)
Date: 03/31/05 

Date: Wed, 30 Mar 2005 20:35:43 -0500

In article <1112212957.827926.3290@o13g2000cwo.googlegroups.com>,
 "Minti" <imanpreet@gmail.com> wrote:

> Richard Kettlewell wrote:
> > "Minti" <imanpreet@gmail.com> writes:
> > > What is the the behaviour of "overloading" the SIGSTOP? When I try
> > > to provide a handler for it using signal, I get SIG_ERR.
> >
> > You cannot provide a handler for SIGSTOP.
> >
> > > However bash, csh and other s****s seem to be able to provide
> > > handlers for SIGSTOP for when I press "Ctrl+Z" or when I do "kill
> > > -SIGSTOP process_id" I get a "STOPPED message".
> >
> > The message comes from the parent s****, not the suspended process.
> > It is the subprocess, not the s****, that receives the stop signal.
>
>
> I would have understood this if it were to print "STOPPED" message only
> for "Ctrl+Z", for it is only the s**** that sends the signal. But what
> I am confused about is that why does this happen when I _specifically_
> send SIGSTOP using kill. S****, be it parent or subprocess has no way
> of providing a handler for SIGSTOP, so how does it get to know about
> SIGSTOP signal.

When a child process is stopped, the parent process is sent a SIGCHLD
signal. This signal can be caught, and the shell's handler then calls
one of the wait() variants, discovers that the child was stopped, and
prints the "STOPPED" message. 

-- 
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***

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