Date: Fri, 27 Dec 1996 15:10:02 -0700 (MST) From: Softweyr LLC <softweyr@xmission.com> To: kline@tera.com (Gary Kline) Cc: questions@freebsd.org Subject: Re: multi-media app runs out of forking capabilities Message-ID: <199612272210.PAA21484@xmission.xmission.com> In-Reply-To: <199612271912.LAA11283@athena.tera.com> from "Gary Kline" at Dec 27, 96 11:12:30 am
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Gary Kline asked:
> Is there a way that one program can send a flag
> to another program? Assuming yes, where is some
> sample code??
According to Softweyr LLC:
% Sure. Signals are a simple, but error prone way. A UNIX-domain socket
% or a FIFO would work well, or even a MSGQ. You can find instructions
% on using most (or maybe all) of these in W. Richard Stevens' excellent
% books _UNIX Network Programming_ (0-13-949876-1) and _Advanced
% Programming in the UNIX Environment_ (0-201-56317-7), both from
% Prentice Hall.
> I've already tried a FIFO; close, but no cigar. Looks like
> spawning a new proc is the bbest way. I have an older version
> of Stevens' networking text. Probably ought to check out the
> _Advanced_ book. Do you know when it was published, off-hand?
> Anyway, thanks for the pointers.
You can find it currently at www.bookpool.com. The copyright date is
1990, but it is certainly still "current."
If yo've already tried a FIFO and can't get your information from one
process to another, none of the other options are going to help you;
they are more generalized forms of communication. You should carefull
consider what it is that the parent process really needs to tell the
child process.
% Your 'can't fork' messages may be caused by the per-process limits on
% child processes. Are you gathering the status of your child processes
% with wait(2) at any point? If not, all of your child processes will
% remain counted against your per-process limit until you wait() for them.
% Richards UNIX book will explain the UNIX process model and the relation-
% ship between parent and child processes better than I can here. ;^)
> This is most probably it. I can't do a wait because the
> program I'm forking infinite-loops until I change state.
> The wait() would wait infinitely.
Not unless you tell it to...
Use wait3 with options set to WNOHANG. If no chile processes have
exited, it will just return 0 immediately. You definitely need a good
book on UNIX programming; none of this is difficult, but you just
cannot find this information in the man pages.
> Hmm. There's got to be other ways of doing this....
Probably, there always are. You just need a few good lessons in
The UNIX Way (tm)
I just found this cool little snippet of code in _UNIX Network
Programming_:
sigchld_parent()
{
union wait status;
int pid;
again:
pid = wait3(&status, WNOHANG | WUNTRACED, (struct rusage *) 0);
if (pid == 0)
return;
goto again;
}
Early in your main program, you need to handle SIGCHLD with this routine:
...
signal(SIGCHLD, sigchld_parent);
...
This will automagically reap (and ignore) the return status of *all*
child processes. Does this help?
I'm certain we've gone beyone the scope of freebsd-questions now;
fell free to email directly if you have more questions.
--
"Where am I, and what am I doing in this handbasket?"
Wes Peters Softweyr LLC
http://www.xmission.com/~softweyr softweyr@xmission.com
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