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Date: Wed, 24 Feb 2010 05:26:31 +0000
From: Dave Korn <dave DOT korn DOT cygwin AT googlemail DOT com>
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To: Dave Korn <dave DOT korn DOT cygwin AT googlemail DOT com>
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Subject: Re: Statically initialising pthread attributes in dynamic dlls.
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On 24/02/2010 04:52, Dave Korn wrote:

>   I think the answer lies here, in this comment in dll_dllcrt0_1:
> 
>>   /* Make sure that our exception handler is installed.
>>      That should always be the case but this just makes sure.
>>
>>      At some point, we may want to just remove this code since
>>      the exception handler should be guaranteed to be installed.
>>      I'm leaving it in until potentially after the release of
>>      1.7.1 */
>>   _my_tls.init_exception_handler (_cygtls::handle_exceptions);
> 
>   Well, it may or may not already be installed, depending whether we got here
> via dlopen or whether this is a statically-linked DLL being initialised at
> process startup, and if it is already installed, it's not at the front of the
> list.  So I figure the best bet would be to replace this call with a local
> stack-frame-based exception registration record, which we'll unlink if we
> return.  IOW, just the same thing as those OS-registered SEH frames are doing
> when they unwind.  I'll report back later if it works.

  Yeh, that works nicely.  Here's what I tested, along with a couple of extra
test cases I used to check whether exception handling was still working before
and after the dlopen call.  With the current state of HEAD, the first one
works (by which I mean "prints 'sig 11' forever" and the second one fails (by
which I means reaches some sort of exit without hitting the signal handler at
all).  With the attached diff, they both work (as does the original unmodified
testcase).

  This is just a brain dump because I'm off to bed now, hence no change log,
and there's still commented-out stuff and inadequate commenting, but I figured
 I may as well let everyone know what I found out.  'night all!

    cheers,
      DaveK


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Index: winsup/cygwin/dll_init.cc
===================================================================
RCS file: /cvs/src/src/winsup/cygwin/dll_init.cc,v
retrieving revision 1.70
diff -p -u -r1.70 dll_init.cc
--- winsup/cygwin/dll_init.cc	5 Feb 2010 15:05:22 -0000	1.70
+++ winsup/cygwin/dll_init.cc	24 Feb 2010 05:05:45 -0000
@@ -329,7 +329,14 @@ dll_dllcrt0_1 (VOID *x)
      the exception handler should be guaranteed to be installed.
      I'm leaving it in until potentially after the release of
      1.7.1 */
-  _my_tls.init_exception_handler (_cygtls::handle_exceptions);
+  /* _my_tls.init_exception_handler (_cygtls::handle_exceptions); */
+  /* Correction: install a temporary registration with our handler
+     at the head of the list, and unlink it before returning.  */
+  extern exception_list *_except_list asm ("%fs:0");
+  exception_list seh;
+  seh.handler = _cygtls::handle_exceptions;
+  seh.prev = _except_list;
+  _except_list = &seh;
 
   if (p == NULL)
     p = &__cygwin_user_data;
@@ -390,6 +397,8 @@ dll_dllcrt0_1 (VOID *x)
     res = -1;
   else
     res = (DWORD) d;
+
+  _except_list = seh.prev;
 }
 
 /* OBSOLETE: This function is obsolete and will go away in the

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#include <dlfcn.h>
#include <signal.h>
#include <stdio.h>


void my_handler (int sig)
{
  printf ("sig %d\n", sig);
}

int main( int argc, char** argv )
{
  signal (SIGSEGV, my_handler);
  *(volatile unsigned int *)0;
  void* dll = dlopen( "mutex.dll", RTLD_NOW );
  dlclose( dll );
}


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#include <dlfcn.h>
#include <signal.h>
#include <stdio.h>


void my_handler (int sig)
{
  printf ("sig %d\n", sig);
}

int main( int argc, char** argv )
{
  signal (SIGSEGV, my_handler);
  void* dll = dlopen( "mutex.dll", RTLD_NOW );
  *(volatile unsigned int *)0;
  dlclose( dll );
}



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