Re: [boost] Has someone any plans to submit a exception_ptr library to Boost?

From: Frank Mori Hess (fmhess_at_[hidden])
Date: 2008-04-20 10:08:31

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On Tuesday 15 April 2008 13:19, Sebastian Redl wrote:
> I can use current_exception() to get the currently handled exception.
> However, calling it again will yield a null pointer. That's because I
> can't abuse handlerCount properly.
>
> The exception_ptr works so far that it lets the exception escape the
> catch.
>
> I can use rethrow_exception() to rethrow the exception. However, when I
> catch it again, it won't escape the catch. I.e.

Here's a partial proof-of-concept implementation of exception_ptr for 
Linux, tested on gcc 4.1.2.  The unwind-cxx.h file is just a copy from the 
gcc source: gcc-4.1.1/libstdc++-v3/libsupc++/unwind-cxx.h.  The test 
program demonstrates that you can rethrow more than once.  There is no 
implementation for copy_exception() yet.  If you call current_exception 
multiple times for one exception, it will probably crash.  That should be 
fixable by adding a global/singleton map which would let you find if 
another exception_ptr already owns a particular exception object.

- -- 
Frank

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Content-Type: text/x-c++src; charset="iso-8859-1";
        name="exception_ptr_test.cpp"
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        filename="exception_ptr_test.cpp"

#include <boost/shared_ptr.hpp>
#include <stdexcept>
#include <iostream>
#include "unwind-cxx.h"

using namespace __cxxabiv1;

void
__cxxabiv1::__terminate (std::terminate_handler handler)
{
  try {
    handler ();
    std::abort ();
  } catch (...) {
    std::abort ();
  }
}

typedef boost::shared_ptr<__cxa_exception> exception_ptr;
namespace detail
{
        void fake_cleanup(_Unwind_Reason_Code code, struct _Unwind_Exception *exc)
        {
                std::cerr << __PRETTY_FUNCTION__ << std::endl;
        __cxa_exception *header =3D __get_exception_header_from_ue (exc);

        // If we haven't been caught by a foreign handler, then this is
        // some sort of unwind error.  In that case just die immediately.
        // _Unwind_DeleteException in the HP-UX IA64 libunwind library
        //  returns _URC_NO_REASON and not _URC_FOREIGN_EXCEPTION_CAUGHT
        // like the GCC _Unwind_DeleteException function does.
        if (code !=3D _URC_FOREIGN_EXCEPTION_CAUGHT && code !=3D _URC_NO_REASON)
                __terminate (header->terminateHandler);
        }
=09
        void exception_ptr_deleter(__cxa_exception *header)
        {
                if (header->exceptionDestructor)
                        header->exceptionDestructor(header + 1);

                __cxa_free_exception (header + 1);
        }
}

exception_ptr current_exception()
{
        __cxa_eh_globals *globals =3D __cxa_get_globals();
        __cxa_exception *header =3D globals->caughtExceptions;
        exception_ptr current(header, &detail::exception_ptr_deleter);
        header->unwindHeader.exception_cleanup =3D &detail::fake_cleanup;
        return current;
}

void rethrow_exception(exception_ptr ep)
{
#ifdef _GLIBCXX_SJLJ_EXCEPTIONS
  _Unwind_SjLj_RaiseException (&ep->unwindHeader);
#else
  _Unwind_RaiseException (&ep->unwindHeader);
#endif

  // Some sort of unwinding error.  Note that terminate is a handler.
  __cxa_begin_catch (&ep->unwindHeader);
  std::terminate ();
}

class test_exception: public std::runtime_error
{
public:
        test_exception(const char *what_string): runtime_error(what_string)
        {}
        test_exception(const std::string &what_string): runtime_error(what_string)
        {}
        ~test_exception() throw()
        {
                std::cerr << __PRETTY_FUNCTION__ << std::endl;
        }
};

int main()
{
        {
                exception_ptr ep;
                try
                {
                        throw test_exception("this is my test exception");
                }
                catch(...)
                {
                        std::cerr << "caught error, getting exception_ptr." << std::endl;
                        ep =3D current_exception();
                }
                try
                {
                        rethrow_exception(ep);
                }
                catch(test_exception &err)
                {
                std::cerr << "caught test exception: " << err.what() << std::endl;
                }
                try
                {
                        rethrow_exception(ep);
                }
                catch(test_exception &err)
                {
                std::cerr << "caught test exception again: " << err.what() << std::endl;
                }
                std::cerr << "exception_ptr going out of scope" << std::endl;
        }
    return 0;
}

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Content-Type: text/x-c++hdr;
  charset="iso-8859-1";
  name="unwind-cxx.h"
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        filename="unwind-cxx.h"

// -*- C++ -*- Exception handling and frame unwind runtime interface routin=
es.
// Copyright (C) 2001 Free Software Foundation, Inc.
//
// This file is part of GCC.
//
// GCC is free software; you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation; either version 2, or (at your option)
// any later version.
//
// GCC is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with GCC; see the file COPYING.  If not, write to
// the Free Software Foundation, 51 Franklin Street, Fifth Floor,
// Boston, MA 02110-1301, USA.

// As a special exception, you may use this file as part of a free software
// library without restriction.  Specifically, if other files instantiate
// templates or use macros or inline functions from this file, or you compi=
le
// this file and link it with other files to produce an executable, this
// file does not by itself cause the resulting executable to be covered by
// the GNU General Public License.  This exception does not however
// invalidate any other reasons why the executable file might be covered by
// the GNU General Public License.

// This is derived from the C++ ABI for IA-64.  Where we diverge
// for cross-architecture compatibility are noted with "@@@".

#ifndef _UNWIND_CXX_H
#define _UNWIND_CXX_H 1

// Level 2: C++ ABI

#include <typeinfo>
#include <exception>
#include <cstddef>
#include "unwind.h"

#pragma GCC visibility push(default)

namespace __cxxabiv1
{

// A C++ exception object consists of a header, which is a wrapper around
// an unwind object header with additional C++ specific information,
// followed by the exception object itself.

struct __cxa_exception
{=20
  // Manage the exception object itself.
  std::type_info *exceptionType;
  void (*exceptionDestructor)(void *);=20

  // The C++ standard has entertaining rules wrt calling set_terminate
  // and set_unexpected in the middle of the exception cleanup process.
  std::unexpected_handler unexpectedHandler;
  std::terminate_handler terminateHandler;

  // The caught exception stack threads through here.
  __cxa_exception *nextException;

  // How many nested handlers have caught this exception.  A negated
  // value is a signal that this object has been rethrown.
  int handlerCount;

#ifdef __ARM_EABI_UNWINDER__
  // Stack of exceptions in cleanups.
  __cxa_exception* nextPropagatingException;

  // The nuber of active cleanup handlers for this exception.
  int propagationCount;
#else
  // Cache parsed handler data from the personality routine Phase 1
  // for Phase 2 and __cxa_call_unexpected.
  int handlerSwitchValue;
  const unsigned char *actionRecord;
  const unsigned char *languageSpecificData;
  _Unwind_Ptr catchTemp;
  void *adjustedPtr;
#endif

  // The generic exception header.  Must be last.
  _Unwind_Exception unwindHeader;
};

// Each thread in a C++ program has access to a __cxa_eh_globals object.
struct __cxa_eh_globals
{
  __cxa_exception *caughtExceptions;
  unsigned int uncaughtExceptions;
#ifdef __ARM_EABI_UNWINDER__
  __cxa_exception* propagatingExceptions;
#endif
};

// The __cxa_eh_globals for the current thread can be obtained by using
// either of the following functions.  The "fast" version assumes at least
// one prior call of __cxa_get_globals has been made from the current
// thread, so no initialization is necessary.
extern "C" __cxa_eh_globals *__cxa_get_globals () throw();
extern "C" __cxa_eh_globals *__cxa_get_globals_fast () throw();

// Allocate memory for the exception plus the thown object.
extern "C" void *__cxa_allocate_exception(std::size_t thrown_size) throw();

// Free the space allocated for the exception.
extern "C" void __cxa_free_exception(void *thrown_exception) throw();

// Throw the exception.
extern "C" void __cxa_throw (void *thrown_exception,
                             std::type_info *tinfo,
                             void (*dest) (void *))
     __attribute__((noreturn));

// Used to implement exception handlers.
extern "C" void *__cxa_get_exception_ptr (void *) throw();
extern "C" void *__cxa_begin_catch (void *) throw();
extern "C" void __cxa_end_catch ();
extern "C" void __cxa_rethrow () __attribute__((noreturn));

// These facilitate code generation for recurring situations.
extern "C" void __cxa_bad_cast ();
extern "C" void __cxa_bad_typeid ();

// @@@ These are not directly specified by the IA-64 C++ ABI.

// Handles re-checking the exception specification if unexpectedHandler
// throws, and if bad_exception needs to be thrown.  Called from the
// compiler.
extern "C" void __cxa_call_unexpected (void *) __attribute__((noreturn));
extern "C" void __cxa_call_terminate (void*) __attribute__((noreturn));

#ifdef __ARM_EABI_UNWINDER__
// Arm EABI specified routines.
typedef enum {
  ctm_failed =3D 0,
  ctm_succeeded =3D 1,
  ctm_succeeded_with_ptr_to_base =3D 2
} __cxa_type_match_result;
extern "C" bool __cxa_type_match(_Unwind_Exception*, const std::type_info*,
                                 bool, void**);
extern "C" void __cxa_begin_cleanup (_Unwind_Exception*);
extern "C" void __cxa_end_cleanup (void);
#endif

// Invokes given handler, dying appropriately if the user handler was
// so inconsiderate as to return.
extern void __terminate(std::terminate_handler) __attribute__((noreturn));
extern void __unexpected(std::unexpected_handler) __attribute__((noreturn));

// The current installed user handlers.
extern std::terminate_handler __terminate_handler;
extern std::unexpected_handler __unexpected_handler;

// These are explicitly GNU C++ specific.

// Acquire the C++ exception header from the C++ object.
static inline __cxa_exception *
__get_exception_header_from_obj (void *ptr)
{
  return reinterpret_cast<__cxa_exception *>(ptr) - 1;
}

// Acquire the C++ exception header from the generic exception header.
static inline __cxa_exception *
__get_exception_header_from_ue (_Unwind_Exception *exc)
{
  return reinterpret_cast<__cxa_exception *>(exc + 1) - 1;
}

#ifdef __ARM_EABI_UNWINDER__
static inline bool
__is_gxx_exception_class(_Unwind_Exception_Class c)
{
  // TODO: Take advantage of the fact that c will always be word aligned.
  return c[0] =3D=3D 'G'
         && c[1] =3D=3D 'N'
         && c[2] =3D=3D 'U'
         && c[3] =3D=3D 'C'
         && c[4] =3D=3D 'C'
         && c[5] =3D=3D '+'
         && c[6] =3D=3D '+'
         && c[7] =3D=3D '\0';
}

static inline void
__GXX_INIT_EXCEPTION_CLASS(_Unwind_Exception_Class c)
{
  c[0] =3D 'G';
  c[1] =3D 'N';
  c[2] =3D 'U';
  c[3] =3D 'C';
  c[4] =3D 'C';
  c[5] =3D '+';
  c[6] =3D '+';
  c[7] =3D '\0';
}

static inline void*
__gxx_caught_object(_Unwind_Exception* eo)
{
  return (void*)eo->barrier_cache.bitpattern[0];
}
#else // !__ARM_EABI_UNWINDER__
// This is the exception class we report -- "GNUCC++\0".
const _Unwind_Exception_Class __gxx_exception_class
=3D ((((((((_Unwind_Exception_Class) 'G'=20
         << 8 | (_Unwind_Exception_Class) 'N')
        << 8 | (_Unwind_Exception_Class) 'U')
       << 8 | (_Unwind_Exception_Class) 'C')
      << 8 | (_Unwind_Exception_Class) 'C')
     << 8 | (_Unwind_Exception_Class) '+')
    << 8 | (_Unwind_Exception_Class) '+')
   << 8 | (_Unwind_Exception_Class) '\0');

static inline bool
__is_gxx_exception_class(_Unwind_Exception_Class c)
{
  return c =3D=3D __gxx_exception_class;
}

#define __GXX_INIT_EXCEPTION_CLASS(c) c =3D __gxx_exception_class

// GNU C++ personality routine, Version 0.
extern "C" _Unwind_Reason_Code __gxx_personality_v0
     (int, _Unwind_Action, _Unwind_Exception_Class,
      struct _Unwind_Exception *, struct _Unwind_Context *);

// GNU C++ sjlj personality routine, Version 0.
extern "C" _Unwind_Reason_Code __gxx_personality_sj0
     (int, _Unwind_Action, _Unwind_Exception_Class,
      struct _Unwind_Exception *, struct _Unwind_Context *);

static inline void*
__gxx_caught_object(_Unwind_Exception* eo)
{
  __cxa_exception* header =3D __get_exception_header_from_ue (eo);
  return header->adjustedPtr;
}
#endif // !__ARM_EABI_UNWINDER__

} /* namespace __cxxabiv1 */

#pragma GCC visibility pop

#endif // _UNWIND_CXX_H

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