From owner-freebsd-hackers  Tue Sep 15 04:15:02 1998
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Date: Tue, 15 Sep 1998 07:16:00 -0500 (EST)
From: Alfred Perlstein <bright@hotjobs.com>
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To: Konrad Heuer <kheuer@gwdu60.gwdg.de>
cc: freebsd-hackers@FreeBSD.ORG
Subject: Re: Catching SIGSEGV
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i had to write a short test program, but simply when SIGSEGV is caught it
seems that the return address is set to as to try to "retry" the operation
that failed.  take a look at the output of this program:

#include <signal.h>

int nsegv = 0;
int ptr;
int zero = 0;
int *inv = 0;

void sighdl ( int sig )
{
  ptr = &zero;

   printf("%x\n",inv);fflush(0);
   inv++; /* remove this to see 0 vs behaviour */
  nsegv++;
}

int main( )
{
  int buf;

  signal(SIGSEGV, sighdl);

  ptr = *(++inv);  /* note this only happens once, put the ++ after and
it starts at 0 rather than 4 */
  buf = ptr;
   return 1;
} 

it compiles with a warning... sue me i'm up 24hrs :P


Alfred Perlstein - Programmer, HotJobs Inc. - www.hotjobs.com
-- There are operating systems, and then there's FreeBSD.
-- http://www.freebsd.org/                        3.0-current

On Tue, 15 Sep 1998, Konrad Heuer wrote:

> 
> May I ask my question here although it's not correlated to FreeBSD
> development?
> 
> I write a program which shall run under FreeBSD and want to catch SIGSEGV
> to count these events. So the program is like this: 
> 
> int nsegv = 0;
> int *ptr;
> int zero = 0; 
> 
> void sighdl ( int sig )
> {
>   ptr = &zero;
>   nsegv++; 
> }
> 
> int dowork ( )
> {
>   int buf;
> 
>   signal(SIGSEGV, sighdl);
> 
>   ptr = InvalidAddress;
>   buf = *ptr;
> }
> 
> Ok, the signal handler gets called, ptr will be reset to a legal address,
> but nevertheless after returning from the signal handler it will be called
> again immediately resulting in an infinite loop.
> 
> Where's my mistake? What can I do to prevent that loop?
> Thanks for any reply.
> 
> Regards
> Konrad Heuer
> 
> // Gesellschaft fuer wissenschaftliche Datenverarbeitung mbH
> // Goettingen (GWDG), Am Fassberg, D-37077 Goettingen, Germany
> //
> // kheuer@gwdu60.gwdg.de
> 
> 
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