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Date: Fri, 8 Nov 2002 11:28:26 +0100 (=?ISO-8859-1?Q?Westeurop=E4ische_Normalzeit?=)
From: Thomas Pfaff <tpfaff AT gmx DOT net>
To: Danny Smith <danny_r_smith_2001 AT yahoo DOT co DOT nz>
cc: mingw-dvlpr AT lists DOT sourceforge DOT net, <cygwin-developers AT cygwin DOT com>
Subject: Exception handling in gcc-3
Message-ID: <Pine.WNT.4.44.0211081100010.308-400000@algeria.intern.net>
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Hi Danny,

i just had the time to look at the eh changes in gcc-3 for throwing
exceptions across DLL/EXE boundaries.
While the code works in 99% for cygwin and mingw it has one disadvantage
for cygwin: The atom name is lost after a fork.
See following example:

#include <stdio.h>
#include <unistd.h>
#include <sys/wait.h>
#include <pthread.h>

#include <iostream>
using namespace std;

#define WIN32_LEAN_AND_MEAN
#include <windows.h>

#define SHAREDPTR_LETTER0   'a'
#define SHAREDPTR_LETTER1   'A'

int main (void)
{
  char s[sizeof(void*) * 8 + 1];
  ATOM atom;

  try
    {
      memset (s, SHAREDPTR_LETTER0, sizeof(void*) * 8);
      s[sizeof(void*) * 8] = '\0';

      switch (fork ())
        {
        case -1:
          return 0;
        case 0:
          atom = FindAtomA (s);
          throw "child";
        default:
          atom = FindAtomA (s);
          throw "parent";
        }
    }

  catch (const char *str)
    {
      char szAtom[16];
      sprintf( szAtom, "0x%08x", (int) atom );
      cout << "got " << str << "s exception, atom " << szAtom << endl;
    }

  catch (...)
    {
      cout << "got exception" << endl;
    }

  return 0;
}

If a dll is loaded dynamically after a fork the shared information is lost
and throwing exceptions from a dll does not work anymore.

For that reason i have created a different version that uses a shared
pointer that is contained in the cygwin1.dll, now it works in all test
cases. (And since i always link against libmingwthrd.a i added this
function to mthr.c too, but this is not applicable for all mingw users).

I have attached my code.

You may notice that i have moved 2 additional static vars in the shared
area (object_mutex and marker from unwind-dw2-fde.c) and that i have
modified the shared pointer allocation a bit.

Comments are welcome.

Thomas

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