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Re: Problems with hook-stop
> 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.