[4649] in Kerberos
Re: kadmin - Can't get it to work!
daemon@ATHENA.MIT.EDU (Axel-Stiphane C. Smxrgrav)
Mon Feb 20 14:22:52 1995
To: kerberos@MIT.EDU
Date: 20 Feb 1995 19:45:37 +0100
From: axelstep@ifi.uio.no (Axel-Stiphane C. Smxrgrav)
Path: axelstep
Newsgroups: comp.protocols.kerberos
Followup-To:
References: <1995Feb17.001538.23965@emba.uvm.edu> <3i3osf$2eu@news.doit.wisc.edu> <3i5csh$oq1@necco.harvard.edu>
From: axelstep@ifi.uio.no (Axel-Stiphane C. Smxrgrav)
Organization: Dept. of Informatics, University of Oslo, Norway
Subject: Re: kadmin - Can't get it to work!
Keywords:
In article <3i5csh$oq1@necco.harvard.edu>, dm@das.harvard.edu (David Mazieres) writes:
> In article <3i3osf$2eu@news.doit.wisc.edu>,
> Bruce Orchard <orchard@eceserv0.ece.wisc.edu> wrote:
> >
> >I have kadmin working (but see below) on a Sun running Solaris 2.4.
> >...
> >
> >The problem is that when the client (kadmin) exits, the
> >server (kadmind) goes into a loop handling the SIGCHILD
> >signal. Thus if you start kadmind and run kadmin, kadmin
> >works once; then you have to kill kadmind and start over.
> >I think this just applies to SVR4/Solaris; it will probably
> >work OK on BSD/SunOS4.
>
> Yes, I had this problem under HP-UX, too. I fixed it by adding
> the following ifdef to function do_child in kadmin/server/adm_network.c:
> (first part is what was originally there)
>
> #ifndef __hpux
> signal(SIGCHLD, do_child);
> pid = wait(&status);
> #else /* __hpux */
> /*
> * HPUX will deliver an immediate signal if a child is pending,
> * resulting in infinite recursion!
> */
> pid = wait(&status);
> signal(SIGCHLD, do_child);
> #endif /* __hpux */
>
> I guess the problem affects more than just HP-UX, though.
No, that is right. This problem will probably affect all systemV-ish
OS'es, and is caused by differences in the semantics of
SIGCHLD/SIGCLD.
Under SystemV, when a signal occurs, the signal handler is reset to
SIG_DFL, and hence the signal handler must be reinstalled. This is
usually done in the signal handler as shown in the above example
(if-branch).
Normally a signal will occur only once for each event to be signalled.
However, SIGCHLD is different. Under SystemV, when reinstalled, the
system checks if there are any child processes to be waited for, and
if there are, the parent process receives a new occurence of SIGCHLD.
This results in the signal handler being called recursively over and
over again, until the process eventually trashes as the result of a
stack overflow.
Under BSD SIGCHLD will occur only once for each terminated process,
even if the process has not been waited for.
-ascs
