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Date: Thu, 27 Sep 2007 12:06:00 -0000
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From: Mark Kettenis <mark.kettenis@xs4all.nl>
To: carl.shapiro@gmail.com
CC: gdb@sourceware.org
In-reply-to: <4dcb5abd0709260133h10aede86wb60a22094c8d9884@mail.gmail.com> 	(carl.shapiro@gmail.com)
Subject: Re: Problems with hook-stop
References:  <4dcb5abd0709260133h10aede86wb60a22094c8d9884@mail.gmail.com>
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> Date: Wed, 26 Sep 2007 01:33:11 -0700
> From: "Carl Shapiro" <carl.shapiro@gmail.com>
> 
> Hi,
> 
> I am trying to write a user-defined command hook that automatically
> passes a SIGTRAP to my program.  Here is the definition I am working
> with:
> 
> define hook-stop
>   if (((* (char *) ($pc - 1)) & 0xff) == 0xcc)
>     signal SIGTRAP
>   end
> end
> 
> My target architecture is x86.  This command fires the first time I
> hit an "int3" instruction.  The next time I hit an int3, hook-stop is
> not executed and I must manually pass a SIGTRAP by evaluating "signal
> SIGTRAP".  It seems that every other time gdb is stopped with a
> SIGTRAP the stop hook is not executed.  Here's a program that can be
> used to demonstrate this:
> 
> #include <signal.h>
> #include <stdio.h>
> 
> void handler(int signum) {
>   fprintf(stderr, "handler(signum=%d)\n", signum);
> }
> 
> int main(int argc, char *argv[]) {
>   struct sigaction sa;
>   int i;
>   sa.sa_handler = handler;
>   for (i = 0;; ++i) {
>     fprintf(stderr, "i=%d\n", i);
>     asm("int3");
>   }
>   return 0;
> }
> 
> If I remove the "signal SIGTRAP" statement from hook-stop, it fires
> every time around the loop.  Does anyone know where am I going wrong
> wrong?

Since SIGTRAP has a very special meaning for debugging, it's probably
unwise to play games like this.