Subj : Re: threaded program and wait() call
To   : comp.programming.threads
From : tadek
Date : Sun Aug 07 2005 08:46 pm

Hi Patrick,
Thank you for your comments and suggestions.  I do agree that creating
'main' and 'worker' thread does not make much sense.  But I am
relatively new in the organization I work for and since I am porting
old and supposably reliable code from QNX to Linux platform I just
wanted to make it work.  Even before I've got your reply, over the
weekend I decided to get rid of main/worker thread anyway.  But before
I do it let me clarify few points which perhaps were not clear in my
original posting:

1) 'worker'thread spawns many children (not one child) and it serves
two purposes:
  - to monitor whether any of them exits and then to respown it
  - to allow orderly shudown of the application by killing all children
when 'main'thread receives SIGTERM signal (by calling kill -s SIGTERM
pid of this process)

2) according to man page of waitpid/wait: "The wait() function shall
suspend execution of the calling thread until status information for
one of the the terminated child processes of the the calling process is
available, or until delivery of a signal whose action is either to
execute a signal-catching function or to terminate the process".  The
signal handler which I installed before starting 'worker' thread
catches SIGTERM and then causese 'main'thread to call pthread_kill with
SIGHUP as a signal.  The signal handler catches SIGHUP, but it never
causes wait() to get it.

I haven't tried your suggestion (SIGCLD) (I'm away from my target
system today), but I don't see how would it help.  BTW shouldn't I use
SIGCHLD instead?

As far as LinuxThreads vs NPTL part: my question is how do I know which
one am I using?  It is very likely silly question, but all I know is
that I compile my source with -pthread option and I am at the 'mercy'
of the linker to select one for me.  I am using 3.3.3 version of gcc.

Cheers,
Tad Dragowski

Patrick TJ McPhee wrote:
> In article <1123280898.049380.245390@g44g2000cwa.googlegroups.com>,
> tadek <tdragowski@tantalus.com> wrote:
>
> % Process creates two threads.  In the 'main' thread module, I install
> % signal handler for SIGTERM and SIGHUP.
>
> I don't understand why you're creating two threads here. Why not simply
> fork() off the child? After all, your drone thread seems to be waiting
> for the worker to crash, but if the worker crashes, it will take the
> drone with it. This is better done by forking off a child and having
> the parent process block in waitpid() (or not block in waitpid() if
> it has something useful to do.
>
> If there is some reason to use threads, you should ensure you block
 > SIGCLD in the drone thread, and you might as well use g_shutdown to
> tell the child to shut down, rather than screwing around with signals.
>
> % PS I am also confused whether I am using LinuxThreads or NPTL when
> % linking with  glibc and making calls to pthread_* functions.
>
> You'll be better off using NPTL. It has more portable behaviour, it is
> the future, and it's here now.
> 
> -- 
> 
> Patrick TJ McPhee
> North York  Canada
> ptjm@interlog.com

.