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Date: Wed, 26 Sep 2007 16:06:00 -0000
From: "Carl Shapiro" <carl.shapiro@gmail.com>
To: gdb@sourceware.org
Subject: Problems with hook-stop
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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?

Thanks,

Carl