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From Stas Bekman <>
Subject Re: signals under worker mpm?
Date Tue, 30 Nov 2004 03:36:01 GMT
Jeff Trawick wrote:
> On Mon, 29 Nov 2004 17:16:31 -0500, Stas Bekman <> wrote:
>>Jeff Trawick wrote:
>>>How is SIGALRM used specifically?
>>         eval {
>>             POSIX::sigaction(SIGALRM,
>>                              POSIX::SigAction->new(sub { die "alarm" }))
>>                   or die "Error setting SIGALRM handler: $!\n";
>>             alarm 2;
>>             potentially_long_running_process()
>>             alarm 0;
>>         };
>>         do_something() if $@ && $@ =~ /alarm/;
> no, can't do that, unless you're the only thread in the process that
> does anything like that since
> 1) any other code in the process can wipe out your SIGALRM handler
> 2) only one "alarm" per process

Thanks Jeff for the explanations.

So it's a 100% no go with threads. But I understand that it should work 
just fine with prefork (and it does so far). so I have the following 

1) what would be the best place to document that in the Apache docs (mpm 
include/ap_mpm.h or should each mpm have it documented separately?

2) what's the policy on non-threaded mpms, i.e. which signals are declared 
usable in the user code (i.e. declare an API so one could rely on it)

3) in the particular case of threads, what's the alternative approach to 
the SIGALARM technique to trap long running/potentially run-away code 
(e.g. IO). So we can offer something to the users migrating their code 
from 1.3.x.

>>>Is there some thread-aware service which works sort of like the thread-unaware
>>>call which will send SIGALRM to a specific thread?
>>Nope. but if the signal is sent by the same thread that catches it, it's
>>still not guaranteed that it'll get it?
> the SIGALRM signal sent due to a call to alarm() is sent by the
> kernel; no guarantee which thread will get it, unless there is only
> one thread which doesn't have SIGALRM blocked

Got it. Thanks Jeff.

Stas Bekman            JAm_pH ------> Just Another mod_perl Hacker     mod_perl Guide --->

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