help: SIGALRM on OS X

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Chris - 10 Jan 2006 16:32 GMT

Hello,
I'm pretty much a newbie when it comes to Mac OS X so go easy on me. I have
a Perl applicaton that communicates with the Asterisk VoIP phone system
through AGI (Asterisk Gateway Interface) very similar to CGI. Asterisk
starts my perl application and the two communicate over STDIN and STDOUT
file handles. When the user on the phone presses the # key my AGI perl
application is suppose to start recording what the users says until they
press # again. The way this works, is the perl applications writes the
RECORD command to STDOUT and Asterisk starts to record and then the perl
program waits on STDIN for the response which would be the # key when the
user presses it. I call the "alarm(3)" perl command (having set $SIG{ALRM}
= \&handleAlarm; earlier) before issuing the RECORD command to have perl
call my handleAlarm() routine to perform some tasks three seconds after the
recording starts (like check that the file actually has some sound and to
notify others that a messages is currently being recorded).

We've been using this application on a Linux machine for over a year and
recently I tried to port the whole thing to a Macmini with OS X 10.3.
Almost everything works great except, when the alarm goes off the wait on
STDIN stops prematurly before Asterisk has actually stopped recording. If I
comment out the alarm command the wait on STDIN works fine.

I've posted this already in the Asterisk and AstMasters mailing list but I
think it may be more perl related then Asterisk related and having something
to do wtih the whole alarm subsystem on Mac OS X vs Linux. Asterisk is
writen in straight C so perhaps they are both using the same alarm timer.
Any thoughts or suggestions would be greatly appreciated because I'm
stumped.

Thanks,
Chris

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Danny Hembree - 10 Jan 2006 19:04 GMT

There is no difference between signals in Linux and signals in OSX, they
behave the same. However, the way the processes setup may differ. There
is an OSX asterisk version that should have this sorted out for you.
There is a lot of help here:

http://www.voip-info.org/tiki-index.php?page=Asterisk%20MacOSX%20Support

> Hello,
> I'm pretty much a newbie when it comes to Mac OS X so go easy on me. I have
[quoted text clipped - 11 lines]
> recording starts (like check that the file actually has some sound and to
> notify others that a messages is currently being recorded).

There are several ways to fork a child process and the way Linux and BSD
do this has minor variations. The way that it is forked determines
whether the parent intercepts a signal or passes it to the child. The
wait mask likewise varies between the OS's. Reading the manual pages
over and over and experimentation is the only way I've ever gotten these
things to work. In principal, the wait returns an integer, by masking
the proper bits one can determine what broke the wait, then decide to
continue waiting or exit.

> We've been using this application on a Linux machine for over a year and
> recently I tried to port the whole thing to a Macmini with OS X 10.3.
[quoted text clipped - 11 lines]
> Thanks,
> Chris

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Chris - 10 Jan 2006 23:07 GMT

Hello Danny,
Wow, thank you for the speedy reply and the suggestions. I've been searching
the Asterisk Mac community (including the voip-info wiki and astmasters.org)
for some time and have been reading the man pages on "alarm" and how
STDIN/OUT are handled both on linux and mac and couldn't come up with any
reason that the code should behave differently.

I was thinking of trying a fork instead of the alarm but figured at a lower
level the alarm was essentially doing the same thing.

Although very simplified, the perl code essentially looks like this:

alarm(3);
$fhO = \*STDOUT if (!$fhO);
select ((select ($fhO), $| = 1)[0]);
print $fhO "$command\n";
$fhI = \*STDIN if (!$fhI);
$response = <$fhI> || '200 result=-1 (noresponse)';
chomp($response);
$timeleft = alarm(0);

On linux the handleAlarm routine runs and finishes without a problem and
when the user finishes recording and presses the # key, then the line
$response = <$fhI> continues. On the Mac, as soon as the alarm goes off
$response gets '200 result=-1 (noresponse)'. If I comment out the alarm(3),
then it works. It's so simple on the Perl side that I now think the problem
must be in the Asterisk C code.

Thanks again,
Chris

> There is no difference between signals in Linux and signals in OSX, they
> behave the same. However, the way the processes setup may differ. There
[quoted text clipped - 49 lines]
>> Thanks,
>> Chris

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Mac Forum / Programming / Perl / January 2006

help: SIGALRM on OS X