RE: signal trouble

[Dave Korn]

Original Message
>From: znort
>Sent: 23 August 2005 10:40

> Could you tell/help me why the little code :


> Under cygwin gcc version 3.4.4 (cygming special) and Linux with gcc 3.3.5
> same "strange" results... (no segmentation fault with cygwin anyway)

  Hm.  Worked fine for me on RH8 with gcc 3.2.  There in fact _is_ a
segfault under cygwin, but it doesn't get printed out because either 1)
everything blows up before stderr can be flushed or 2) everything blows up
so badly it's no longer possible to print to stdout.

> (notes : Under Solaris x86 (gcc3.4.4) it works perfectly !?)
> 
> I think I've forgotten something but where ?
> 
> help me please


  Not sure yet.  It seems that the second time the SEGV happens, the signal
handling code inside cygwin goes wrong somehow and ends up with a SEGV of
its own and the app gets terminated without your handler being called.
Unless there's some POSIX issue I don't know about to do with the way in
which you re-enable signals or interactions between longjmp and signal
handlers, I'd say it's a cygwin bug.


cheers,
  DaveK
-- 
Can't think of a witty .sigline today


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Re: signal trouble

[Pavel Tsekov]

Hello,

On Tue, 23 Aug 2005, znort wrote:

> Could you tell/help me why the little code :

[...]

> Under cygwin gcc version 3.4.4 (cygming special) and Linux with gcc 3.3.5
> same "strange" results... (no segmentation fault with cygwin anyway)
>
> (notes : Under Solaris x86 (gcc3.4.4) it works perfectly !?)
>
> I think I've forgotten something but where ?

See the attached file - it is modified version of your program. When the
second SIGSEGV occurs SIGSEGV signals are blocked. This is because you
exited the signal handler via a longjmp() and the signal mask is not
cleared. To do what you want you must use sigsetjmp/siglongjmp pair of
calls or remove SIGSEGV from the list of blocked signals.#include 
#include 
#include 

/* jmp_buf context; */
sigjmp_buf context;

int yog(int n)
{
   char *p=NULL;

   printf("pass %d\n", n);

   if (n<=2)
 return 1;

   strcpy(p, "dfldflflmflkklfklflkmfdlfldfldlfdlkfd");// must crash of 
course !!!
   return 1;

}

void resume(int sig)
{
/*   longjmp(context, 6); */
   siglongjmp(context, 6);
   //signal(SIGSEGV, resume);

}

int main(void)
{
   int sig, i;
   char *x=NULL;
   sigset_t nset, oset;

   for (i=0; i<3; i++)
   {
 /* Remove SIGSEGV from the list of blocked signals if you use 
  * setjmp/longjmp instead of sigsetjmp/siglongjmp. */
 sigemptyset (&nset);
 sigaddset(&nset, SIGSEGV);
 sigprocmask (SIG_UNBLOCK, &nset, &oset);
 signal(SIGSEGV, resume);
 
/* sig = setjmp(context);*/
 sig = sigsetjmp(context, 1);

 if (sig == 0)
 {
   yog(1);
   yog(2);
   yog(3);
   //return 555;
 }
 else
 {
   fprintf(stderr, "ouch -- signal %d\n", sig);
 }
   }

   puts("end");
   return 0;

}
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