unhandled C++ exceptions not propagating

unhandled C++ exceptions not propagating

[David McFarland]

I was just debugging a c++ app (b2 build system from boost), and noticed
that it would appear to exit unexpectedly without an error.  This turned
out to be when an unhandled C++ exception was thrown.

On a fresh install of cygwin with gcc-g++, this program will throw an
exception from the std::string constructor:

c++ -x c++ - <<END
#include <string>
int main() { std::string str(nullptr); return 0; }
END

When it's executed from the shell it returns zero, but execution stops
at the exception.

$ ./a; echo $?
0

When executed under gdb, the exception is caught, and the process exits
non-zero when continued:

(gdb) r
Starting program: /home/corngood/a
[New Thread 4300.0x1390]
[New Thread 4300.0x1d24]
[New Thread 4300.0x1d48]
[New Thread 4300.0x80c]
gdb: unknown target exception 0x20474343 at 0x7ff8d2cfa799
Thread 1 "a" received signal ?, Unknown signal.
0x00007ff8d2cfa799 in RaiseException () from /cygdrive/c/WINDOWS/System32/KERNELBASE.dll
(gdb) c
Continuing.
[Thread 4300.0x1d24 exited with code 541541187]
[Thread 4300.0x1870 exited with code 541541187]
[Thread 4300.0x1d48 exited with code 541541187]
[Thread 4300.0x1390 exited with code 541541187]
[Inferior 1 (process 4300) exited with code 04021641503]
(gdb)

When executed under strace, it exits with an error as expected:

$ strace -o /dev/null a; echo $?
67

That's as far as I've investigated so far.

Re: unhandled C++ exceptions not propagating

[JonY]

[-- Attachment #1.1: Type: text/plain, Size: 1714 bytes --]

On 8/15/20 11:38 PM, David McFarland via Cygwin wrote:
> 
> I was just debugging a c++ app (b2 build system from boost), and noticed
> that it would appear to exit unexpectedly without an error.  This turned
> out to be when an unhandled C++ exception was thrown.
> 
> On a fresh install of cygwin with gcc-g++, this program will throw an
> exception from the std::string constructor:
> 
> c++ -x c++ - <<END
> #include <string>
> int main() { std::string str(nullptr); return 0; }
> END
> 
> When it's executed from the shell it returns zero, but execution stops
> at the exception.
> 
> $ ./a; echo $?
> 0
> 
> When executed under gdb, the exception is caught, and the process exits
> non-zero when continued:
> 
> (gdb) r
> Starting program: /home/corngood/a
> [New Thread 4300.0x1390]
> [New Thread 4300.0x1d24]
> [New Thread 4300.0x1d48]
> [New Thread 4300.0x80c]
> gdb: unknown target exception 0x20474343 at 0x7ff8d2cfa799
> Thread 1 "a" received signal ?, Unknown signal.
> 0x00007ff8d2cfa799 in RaiseException () from /cygdrive/c/WINDOWS/System32/KERNELBASE.dll
> (gdb) c
> Continuing.
> [Thread 4300.0x1d24 exited with code 541541187]
> [Thread 4300.0x1870 exited with code 541541187]
> [Thread 4300.0x1d48 exited with code 541541187]
> [Thread 4300.0x1390 exited with code 541541187]
> [Inferior 1 (process 4300) exited with code 04021641503]
> (gdb)
> 
> When executed under strace, it exits with an error as expected:
> 
> $ strace -o /dev/null a; echo $?
> 67
> 
> That's as far as I've investigated so far.

Can you try if different (older) GCC versions exhibit the same problem?
You'll need to recompile your example with said version as well.



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Re: unhandled C++ exceptions not propagating

[Takashi Yano]

On Sat, 15 Aug 2020 20:38:01 -0300
David McFarland via Cygwin <cygwin@cygwin.com> wrote:
> 
> I was just debugging a c++ app (b2 build system from boost), and noticed
> that it would appear to exit unexpectedly without an error.  This turned
> out to be when an unhandled C++ exception was thrown.
> 
> On a fresh install of cygwin with gcc-g++, this program will throw an
> exception from the std::string constructor:
> 
> c++ -x c++ - <<END
> #include <string>
> int main() { std::string str(nullptr); return 0; }
> END
> 
> When it's executed from the shell it returns zero, but execution stops
> at the exception.
> 
> $ ./a; echo $?
> 0
> 
> When executed under gdb, the exception is caught, and the process exits
> non-zero when continued:
> 
> (gdb) r
> Starting program: /home/corngood/a
> [New Thread 4300.0x1390]
> [New Thread 4300.0x1d24]
> [New Thread 4300.0x1d48]
> [New Thread 4300.0x80c]
> gdb: unknown target exception 0x20474343 at 0x7ff8d2cfa799
> Thread 1 "a" received signal ?, Unknown signal.
> 0x00007ff8d2cfa799 in RaiseException () from /cygdrive/c/WINDOWS/System32/KERNELBASE.dll
> (gdb) c
> Continuing.
> [Thread 4300.0x1d24 exited with code 541541187]
> [Thread 4300.0x1870 exited with code 541541187]
> [Thread 4300.0x1d48 exited with code 541541187]
> [Thread 4300.0x1390 exited with code 541541187]
> [Inferior 1 (process 4300) exited with code 04021641503]
> (gdb)
> 
> When executed under strace, it exits with an error as expected:
> 
> $ strace -o /dev/null a; echo $?
> 67
> 
> That's as far as I've investigated so far.

Is this the same issue with
https://cygwin.com/pipermail/cygwin/2019-October/242795.html ?

As far as I tested, this does not occur in 32-bit cygwin.

-- 
Takashi Yano <takashi.yano@nifty.ne.jp>

Re: unhandled C++ exceptions not propagating

[Ken Brown]

On 8/15/2020 11:53 PM, Takashi Yano via Cygwin wrote:
> On Sat, 15 Aug 2020 20:38:01 -0300
> David McFarland via Cygwin <cygwin@cygwin.com> wrote:
>>
>> I was just debugging a c++ app (b2 build system from boost), and noticed
>> that it would appear to exit unexpectedly without an error.  This turned
>> out to be when an unhandled C++ exception was thrown.
>>
>> On a fresh install of cygwin with gcc-g++, this program will throw an
>> exception from the std::string constructor:
>>
>> c++ -x c++ - <<END
>> #include <string>
>> int main() { std::string str(nullptr); return 0; }
>> END
>>
>> When it's executed from the shell it returns zero, but execution stops
>> at the exception.
>>
>> $ ./a; echo $?
>> 0
>>
>> When executed under gdb, the exception is caught, and the process exits
>> non-zero when continued:
>>
>> (gdb) r
>> Starting program: /home/corngood/a
>> [New Thread 4300.0x1390]
>> [New Thread 4300.0x1d24]
>> [New Thread 4300.0x1d48]
>> [New Thread 4300.0x80c]
>> gdb: unknown target exception 0x20474343 at 0x7ff8d2cfa799
>> Thread 1 "a" received signal ?, Unknown signal.
>> 0x00007ff8d2cfa799 in RaiseException () from /cygdrive/c/WINDOWS/System32/KERNELBASE.dll
>> (gdb) c
>> Continuing.
>> [Thread 4300.0x1d24 exited with code 541541187]
>> [Thread 4300.0x1870 exited with code 541541187]
>> [Thread 4300.0x1d48 exited with code 541541187]
>> [Thread 4300.0x1390 exited with code 541541187]
>> [Inferior 1 (process 4300) exited with code 04021641503]
>> (gdb)
>>
>> When executed under strace, it exits with an error as expected:
>>
>> $ strace -o /dev/null a; echo $?
>> 67
>>
>> That's as far as I've investigated so far.
> 
> Is this the same issue with
> https://cygwin.com/pipermail/cygwin/2019-October/242795.html ?
> 
> As far as I tested, this does not occur in 32-bit cygwin.

This doesn't seem to be a new issue.  I found the following report from 6 years 
ago, on Cygwin 1.7.30 with GCC 4.9.0:

 
https://stackoverflow.com/questions/24402412/program-executed-on-cygwin-does-not-report-a-thrown-exception

Ken

Re: unhandled C++ exceptions not propagating

[Takashi Yano]

On Sun, 16 Aug 2020 09:21:24 -0400
Ken Brown via Cygwin <cygwin@cygwin.com> wrote:
> On 8/15/2020 11:53 PM, Takashi Yano via Cygwin wrote:
> > On Sat, 15 Aug 2020 20:38:01 -0300
> > David McFarland via Cygwin <cygwin@cygwin.com> wrote:
> >>
> >> I was just debugging a c++ app (b2 build system from boost), and noticed
> >> that it would appear to exit unexpectedly without an error.  This turned
> >> out to be when an unhandled C++ exception was thrown.
> >>
> >> On a fresh install of cygwin with gcc-g++, this program will throw an
> >> exception from the std::string constructor:
> >>
> >> c++ -x c++ - <<END
> >> #include <string>
> >> int main() { std::string str(nullptr); return 0; }
> >> END
> >>
> >> When it's executed from the shell it returns zero, but execution stops
> >> at the exception.
> >>
> >> $ ./a; echo $?
> >> 0
> >>
> >> When executed under gdb, the exception is caught, and the process exits
> >> non-zero when continued:
> >>
> >> (gdb) r
> >> Starting program: /home/corngood/a
> >> [New Thread 4300.0x1390]
> >> [New Thread 4300.0x1d24]
> >> [New Thread 4300.0x1d48]
> >> [New Thread 4300.0x80c]
> >> gdb: unknown target exception 0x20474343 at 0x7ff8d2cfa799
> >> Thread 1 "a" received signal ?, Unknown signal.
> >> 0x00007ff8d2cfa799 in RaiseException () from /cygdrive/c/WINDOWS/System32/KERNELBASE.dll
> >> (gdb) c
> >> Continuing.
> >> [Thread 4300.0x1d24 exited with code 541541187]
> >> [Thread 4300.0x1870 exited with code 541541187]
> >> [Thread 4300.0x1d48 exited with code 541541187]
> >> [Thread 4300.0x1390 exited with code 541541187]
> >> [Inferior 1 (process 4300) exited with code 04021641503]
> >> (gdb)
> >>
> >> When executed under strace, it exits with an error as expected:
> >>
> >> $ strace -o /dev/null a; echo $?
> >> 67
> >>
> >> That's as far as I've investigated so far.
> > 
> > Is this the same issue with
> > https://cygwin.com/pipermail/cygwin/2019-October/242795.html ?
> > 
> > As far as I tested, this does not occur in 32-bit cygwin.
> 
> This doesn't seem to be a new issue.  I found the following report from 6 years 
> ago, on Cygwin 1.7.30 with GCC 4.9.0:
> 
> https://stackoverflow.com/questions/24402412/program-executed-on-cygwin-does-not-report-a-thrown-exception

Thanks for the information. But I still wonder why 32-bit and 64-bit
cygwin behave differently.


-- 
Takashi Yano <takashi.yano@nifty.ne.jp>

Re: unhandled C++ exceptions not propagating

[David McFarland]

Takashi Yano <takashi.yano@nifty.ne.jp> writes:

> On Sat, 15 Aug 2020 20:38:01 -0300
> David McFarland via Cygwin <cygwin@cygwin.com> wrote:
>> 
>> I was just debugging a c++ app (b2 build system from boost), and noticed
>> that it would appear to exit unexpectedly without an error.  This turned
>> out to be when an unhandled C++ exception was thrown.
>> 
>> On a fresh install of cygwin with gcc-g++, this program will throw an
>> exception from the std::string constructor:
>> 
>> c++ -x c++ - <<END
>> #include <string>
>> int main() { std::string str(nullptr); return 0; }
>> END
>> 
>> When it's executed from the shell it returns zero, but execution stops
>> at the exception.
>> 
>> $ ./a; echo $?
>> 0
>> 
>> When executed under gdb, the exception is caught, and the process exits
>> non-zero when continued:
>> 
>> (gdb) r
>> Starting program: /home/corngood/a
>> [New Thread 4300.0x1390]
>> [New Thread 4300.0x1d24]
>> [New Thread 4300.0x1d48]
>> [New Thread 4300.0x80c]
>> gdb: unknown target exception 0x20474343 at 0x7ff8d2cfa799
>> Thread 1 "a" received signal ?, Unknown signal.
>> 0x00007ff8d2cfa799 in RaiseException () from /cygdrive/c/WINDOWS/System32/KERNELBASE.dll
>> (gdb) c
>> Continuing.
>> [Thread 4300.0x1d24 exited with code 541541187]
>> [Thread 4300.0x1870 exited with code 541541187]
>> [Thread 4300.0x1d48 exited with code 541541187]
>> [Thread 4300.0x1390 exited with code 541541187]
>> [Inferior 1 (process 4300) exited with code 04021641503]
>> (gdb)
>> 
>> When executed under strace, it exits with an error as expected:
>> 
>> $ strace -o /dev/null a; echo $?
>> 67
>> 
>> That's as far as I've investigated so far.
>
> Is this the same issue with
> https://cygwin.com/pipermail/cygwin/2019-October/242795.html ?
>
> As far as I tested, this does not occur in 32-bit cygwin.

Yeah, it seems to be.  Thanks for tracking that down.

I tried using ProcessMonitor, and when I run the test program from a
shell (where it appears to return 0) I get:

a.exe	7096	Process Exit		SUCCESS	Exit Status: 541541187, User Time: 0.0000000 seconds, Kernel Time: 0.0000000 seconds, Private Bytes: 4,661,248, Peak Private Bytes: 4,661,248, Working Set: 4,255,744, Peak Working Set: 4,259,840

So the win32 process does appear to be exiting non-zero.

Did you find out anything else when you investigated it? My next step
was going to be digging into why it behaves differently under strace.

Re: unhandled C++ exceptions not propagating

[Ken Brown]

On 8/16/2020 9:52 AM, Takashi Yano via Cygwin wrote:
> On Sun, 16 Aug 2020 09:21:24 -0400
> Ken Brown via Cygwin <cygwin@cygwin.com> wrote:
>> On 8/15/2020 11:53 PM, Takashi Yano via Cygwin wrote:
>>> On Sat, 15 Aug 2020 20:38:01 -0300
>>> David McFarland via Cygwin <cygwin@cygwin.com> wrote:
>>>>
>>>> I was just debugging a c++ app (b2 build system from boost), and noticed
>>>> that it would appear to exit unexpectedly without an error.  This turned
>>>> out to be when an unhandled C++ exception was thrown.
>>>>
>>>> On a fresh install of cygwin with gcc-g++, this program will throw an
>>>> exception from the std::string constructor:
>>>>
>>>> c++ -x c++ - <<END
>>>> #include <string>
>>>> int main() { std::string str(nullptr); return 0; }
>>>> END
>>>>
>>>> When it's executed from the shell it returns zero, but execution stops
>>>> at the exception.
>>>>
>>>> $ ./a; echo $?
>>>> 0
>>>>
>>>> When executed under gdb, the exception is caught, and the process exits
>>>> non-zero when continued:
>>>>
>>>> (gdb) r
>>>> Starting program: /home/corngood/a
>>>> [New Thread 4300.0x1390]
>>>> [New Thread 4300.0x1d24]
>>>> [New Thread 4300.0x1d48]
>>>> [New Thread 4300.0x80c]
>>>> gdb: unknown target exception 0x20474343 at 0x7ff8d2cfa799
>>>> Thread 1 "a" received signal ?, Unknown signal.
>>>> 0x00007ff8d2cfa799 in RaiseException () from /cygdrive/c/WINDOWS/System32/KERNELBASE.dll
>>>> (gdb) c
>>>> Continuing.
>>>> [Thread 4300.0x1d24 exited with code 541541187]
>>>> [Thread 4300.0x1870 exited with code 541541187]
>>>> [Thread 4300.0x1d48 exited with code 541541187]
>>>> [Thread 4300.0x1390 exited with code 541541187]
>>>> [Inferior 1 (process 4300) exited with code 04021641503]
>>>> (gdb)
>>>>
>>>> When executed under strace, it exits with an error as expected:
>>>>
>>>> $ strace -o /dev/null a; echo $?
>>>> 67
>>>>
>>>> That's as far as I've investigated so far.
>>>
>>> Is this the same issue with
>>> https://cygwin.com/pipermail/cygwin/2019-October/242795.html ?
>>>
>>> As far as I tested, this does not occur in 32-bit cygwin.
>>
>> This doesn't seem to be a new issue.  I found the following report from 6 years
>> ago, on Cygwin 1.7.30 with GCC 4.9.0:
>>
>> https://stackoverflow.com/questions/24402412/program-executed-on-cygwin-does-not-report-a-thrown-exception
> 
> Thanks for the information. But I still wonder why 32-bit and 64-bit
> cygwin behave differently.

I'm no C++ expert, but my understanding after a brief internet search is that an 
uncaught exception is supposed to cause std::terminate to be called.  So I 
installed gcc-debuginfo and ran the OP's test case under gdb with a breakpoint 
at 'terminate'.  On 32-bit Cygwin I got the expected behavior:

Thread 1 "exception_test" hit Breakpoint 1, std::terminate ()
     at /usr/src/debug/gcc-9.3.0-2/libstdc++-v3/libsupc++/eh_terminate.cc:96
96        __atomic_load (&__terminate_handler, &func, __ATOMIC_ACQUIRE);
(gdb) bt
#0  std::terminate ()
     at /usr/src/debug/gcc-9.3.0-2/libstdc++-v3/libsupc++/eh_terminate.cc:96
#1  0x57c45909 in __cxxabiv1::__cxa_throw (obj=obj@entry=0x800396e8,
     tinfo=tinfo@entry=0x57c646a8 <typeinfo for std::logic_error>,
     dest=0x57be6c60 <std::logic_error::~logic_error()>)
     at /usr/src/debug/gcc-9.3.0-2/libstdc++-v3/libsupc++/eh_throw.cc:95
#2  0x57c49223 in std::__throw_logic_error (
     __s=__s@entry=0x57c60160 <cplus_demangle_builtin_types+10272> "basic_string::
_S_construct null not valid")
     at /usr/src/debug/gcc-9.3.0-2/libstdc++-v3/src/c++11/functexcept.cc:66
[...]

On 64-bit Cygwin, however, I got the behavior reported by the OP:

gdb: unknown target exception 0x20474343 at 0x7ff8cccca719

Thread 1 "exception_test" received signal ?, Unknown signal.
0x00007ff8cccca719 in RaiseException () from /c/WINDOWS/System32/KERNELBASE.dll
(gdb) c
Continuing.
[Thread 4508.0x3e20 exited with code 541541187]
[Thread 4508.0x5bc exited with code 541541187]
[Thread 4508.0x5ee8 exited with code 541541187]
[Thread 4508.0x1cd0 exited with code 541541187]
[Inferior 1 (process 4508) exited with code 04021641503]

Note that the breakpoint at 'terminate' is not reached.  I tried putting a 
breakpoint at __cxa_throw and stepping through from there, but I don't really 
understand what's going on.  Someone who knows the innards of exception handling 
will have to look at this.

Ken

Re: unhandled C++ exceptions not propagating

[Ken Brown]

On 8/16/2020 10:56 AM, Ken Brown via Cygwin wrote:
> On 8/16/2020 9:52 AM, Takashi Yano via Cygwin wrote:
>> On Sun, 16 Aug 2020 09:21:24 -0400
>> Ken Brown via Cygwin <cygwin@cygwin.com> wrote:
>>> On 8/15/2020 11:53 PM, Takashi Yano via Cygwin wrote:
>>>> On Sat, 15 Aug 2020 20:38:01 -0300
>>>> David McFarland via Cygwin <cygwin@cygwin.com> wrote:
>>>>>
>>>>> I was just debugging a c++ app (b2 build system from boost), and noticed
>>>>> that it would appear to exit unexpectedly without an error.  This turned
>>>>> out to be when an unhandled C++ exception was thrown.
>>>>>
>>>>> On a fresh install of cygwin with gcc-g++, this program will throw an
>>>>> exception from the std::string constructor:
>>>>>
>>>>> c++ -x c++ - <<END
>>>>> #include <string>
>>>>> int main() { std::string str(nullptr); return 0; }
>>>>> END
>>>>>
>>>>> When it's executed from the shell it returns zero, but execution stops
>>>>> at the exception.
>>>>>
>>>>> $ ./a; echo $?
>>>>> 0
>>>>>
>>>>> When executed under gdb, the exception is caught, and the process exits
>>>>> non-zero when continued:
>>>>>
>>>>> (gdb) r
>>>>> Starting program: /home/corngood/a
>>>>> [New Thread 4300.0x1390]
>>>>> [New Thread 4300.0x1d24]
>>>>> [New Thread 4300.0x1d48]
>>>>> [New Thread 4300.0x80c]
>>>>> gdb: unknown target exception 0x20474343 at 0x7ff8d2cfa799
>>>>> Thread 1 "a" received signal ?, Unknown signal.
>>>>> 0x00007ff8d2cfa799 in RaiseException () from 
>>>>> /cygdrive/c/WINDOWS/System32/KERNELBASE.dll
>>>>> (gdb) c
>>>>> Continuing.
>>>>> [Thread 4300.0x1d24 exited with code 541541187]
>>>>> [Thread 4300.0x1870 exited with code 541541187]
>>>>> [Thread 4300.0x1d48 exited with code 541541187]
>>>>> [Thread 4300.0x1390 exited with code 541541187]
>>>>> [Inferior 1 (process 4300) exited with code 04021641503]
>>>>> (gdb)
>>>>>
>>>>> When executed under strace, it exits with an error as expected:
>>>>>
>>>>> $ strace -o /dev/null a; echo $?
>>>>> 67
>>>>>
>>>>> That's as far as I've investigated so far.
>>>>
>>>> Is this the same issue with
>>>> https://cygwin.com/pipermail/cygwin/2019-October/242795.html ?
>>>>
>>>> As far as I tested, this does not occur in 32-bit cygwin.
>>>
>>> This doesn't seem to be a new issue.  I found the following report from 6 years
>>> ago, on Cygwin 1.7.30 with GCC 4.9.0:
>>>
>>> https://stackoverflow.com/questions/24402412/program-executed-on-cygwin-does-not-report-a-thrown-exception 
>>>
>>
>> Thanks for the information. But I still wonder why 32-bit and 64-bit
>> cygwin behave differently.
> 
> I'm no C++ expert, but my understanding after a brief internet search is that an 
> uncaught exception is supposed to cause std::terminate to be called.  So I 
> installed gcc-debuginfo and ran the OP's test case under gdb with a breakpoint 
> at 'terminate'.  On 32-bit Cygwin I got the expected behavior:
> 
> Thread 1 "exception_test" hit Breakpoint 1, std::terminate ()
>      at /usr/src/debug/gcc-9.3.0-2/libstdc++-v3/libsupc++/eh_terminate.cc:96
> 96        __atomic_load (&__terminate_handler, &func, __ATOMIC_ACQUIRE);
> (gdb) bt
> #0  std::terminate ()
>      at /usr/src/debug/gcc-9.3.0-2/libstdc++-v3/libsupc++/eh_terminate.cc:96
> #1  0x57c45909 in __cxxabiv1::__cxa_throw (obj=obj@entry=0x800396e8,
>      tinfo=tinfo@entry=0x57c646a8 <typeinfo for std::logic_error>,
>      dest=0x57be6c60 <std::logic_error::~logic_error()>)
>      at /usr/src/debug/gcc-9.3.0-2/libstdc++-v3/libsupc++/eh_throw.cc:95
> #2  0x57c49223 in std::__throw_logic_error (
>      __s=__s@entry=0x57c60160 <cplus_demangle_builtin_types+10272> "basic_string::
> _S_construct null not valid")
>      at /usr/src/debug/gcc-9.3.0-2/libstdc++-v3/src/c++11/functexcept.cc:66
> [...]
> 
> On 64-bit Cygwin, however, I got the behavior reported by the OP:
> 
> gdb: unknown target exception 0x20474343 at 0x7ff8cccca719
> 
> Thread 1 "exception_test" received signal ?, Unknown signal.
> 0x00007ff8cccca719 in RaiseException () from /c/WINDOWS/System32/KERNELBASE.dll
> (gdb) c
> Continuing.
> [Thread 4508.0x3e20 exited with code 541541187]
> [Thread 4508.0x5bc exited with code 541541187]
> [Thread 4508.0x5ee8 exited with code 541541187]
> [Thread 4508.0x1cd0 exited with code 541541187]
> [Inferior 1 (process 4508) exited with code 04021641503]
> 
> Note that the breakpoint at 'terminate' is not reached.  I tried putting a 
> breakpoint at __cxa_throw and stepping through from there, but I don't really 
> understand what's going on.  Someone who knows the innards of exception handling 
> will have to look at this.

One further comment, and then I'll shut up and leave it to people who know what 
they're talking about: In the 64-bit case, the function _Unwind_RaiseException 
in libgcc/unwind-seh.c gets called, and it calls the Win32 function 
RaiseException as follows:

   RaiseException (STATUS_GCC_THROW, 0, 1, (ULONG_PTR *)&exc);

Then the following comment appears:

   /* The exception handler installed in crt0 will continue any GCC
      exception that reaches there (and isn't marked non-continuable).
      Returning allows the C++ runtime to call std::terminate.  */

Apparently this doesn't actually happen.

Ken

[Attn: gcc maintainers] Re: unhandled C++ exceptions not propagating

[Ken Brown]

On 8/16/2020 6:38 PM, Ken Brown via Cygwin wrote:
> On 8/16/2020 10:56 AM, Ken Brown via Cygwin wrote:
>> On 8/16/2020 9:52 AM, Takashi Yano via Cygwin wrote:
>>> On Sun, 16 Aug 2020 09:21:24 -0400
>>> Ken Brown via Cygwin <cygwin@cygwin.com> wrote:
>>>> On 8/15/2020 11:53 PM, Takashi Yano via Cygwin wrote:
>>>>> On Sat, 15 Aug 2020 20:38:01 -0300
>>>>> David McFarland via Cygwin <cygwin@cygwin.com> wrote:
>>>>>>
>>>>>> I was just debugging a c++ app (b2 build system from boost), and noticed
>>>>>> that it would appear to exit unexpectedly without an error.  This turned
>>>>>> out to be when an unhandled C++ exception was thrown.
>>>>>>
>>>>>> On a fresh install of cygwin with gcc-g++, this program will throw an
>>>>>> exception from the std::string constructor:
>>>>>>
>>>>>> c++ -x c++ - <<END
>>>>>> #include <string>
>>>>>> int main() { std::string str(nullptr); return 0; }
>>>>>> END
>>>>>>
>>>>>> When it's executed from the shell it returns zero, but execution stops
>>>>>> at the exception.
>>>>>>
>>>>>> $ ./a; echo $?
>>>>>> 0
>>>>>>
>>>>>> When executed under gdb, the exception is caught, and the process exits
>>>>>> non-zero when continued:
>>>>>>
>>>>>> (gdb) r
>>>>>> Starting program: /home/corngood/a
>>>>>> [New Thread 4300.0x1390]
>>>>>> [New Thread 4300.0x1d24]
>>>>>> [New Thread 4300.0x1d48]
>>>>>> [New Thread 4300.0x80c]
>>>>>> gdb: unknown target exception 0x20474343 at 0x7ff8d2cfa799
>>>>>> Thread 1 "a" received signal ?, Unknown signal.
>>>>>> 0x00007ff8d2cfa799 in RaiseException () from 
>>>>>> /cygdrive/c/WINDOWS/System32/KERNELBASE.dll
>>>>>> (gdb) c
>>>>>> Continuing.
>>>>>> [Thread 4300.0x1d24 exited with code 541541187]
>>>>>> [Thread 4300.0x1870 exited with code 541541187]
>>>>>> [Thread 4300.0x1d48 exited with code 541541187]
>>>>>> [Thread 4300.0x1390 exited with code 541541187]
>>>>>> [Inferior 1 (process 4300) exited with code 04021641503]
>>>>>> (gdb)
>>>>>>
>>>>>> When executed under strace, it exits with an error as expected:
>>>>>>
>>>>>> $ strace -o /dev/null a; echo $?
>>>>>> 67
>>>>>>
>>>>>> That's as far as I've investigated so far.
>>>>>
>>>>> Is this the same issue with
>>>>> https://cygwin.com/pipermail/cygwin/2019-October/242795.html ?
>>>>>
>>>>> As far as I tested, this does not occur in 32-bit cygwin.
>>>>
>>>> This doesn't seem to be a new issue.  I found the following report from 6 years
>>>> ago, on Cygwin 1.7.30 with GCC 4.9.0:
>>>>
>>>> https://stackoverflow.com/questions/24402412/program-executed-on-cygwin-does-not-report-a-thrown-exception 
>>>>
>>>
>>> Thanks for the information. But I still wonder why 32-bit and 64-bit
>>> cygwin behave differently.
>>
>> I'm no C++ expert, but my understanding after a brief internet search is that 
>> an uncaught exception is supposed to cause std::terminate to be called.  So I 
>> installed gcc-debuginfo and ran the OP's test case under gdb with a breakpoint 
>> at 'terminate'.  On 32-bit Cygwin I got the expected behavior:
>>
>> Thread 1 "exception_test" hit Breakpoint 1, std::terminate ()
>>      at /usr/src/debug/gcc-9.3.0-2/libstdc++-v3/libsupc++/eh_terminate.cc:96
>> 96        __atomic_load (&__terminate_handler, &func, __ATOMIC_ACQUIRE);
>> (gdb) bt
>> #0  std::terminate ()
>>      at /usr/src/debug/gcc-9.3.0-2/libstdc++-v3/libsupc++/eh_terminate.cc:96
>> #1  0x57c45909 in __cxxabiv1::__cxa_throw (obj=obj@entry=0x800396e8,
>>      tinfo=tinfo@entry=0x57c646a8 <typeinfo for std::logic_error>,
>>      dest=0x57be6c60 <std::logic_error::~logic_error()>)
>>      at /usr/src/debug/gcc-9.3.0-2/libstdc++-v3/libsupc++/eh_throw.cc:95
>> #2  0x57c49223 in std::__throw_logic_error (
>>      __s=__s@entry=0x57c60160 <cplus_demangle_builtin_types+10272> 
>> "basic_string::
>> _S_construct null not valid")
>>      at /usr/src/debug/gcc-9.3.0-2/libstdc++-v3/src/c++11/functexcept.cc:66
>> [...]
>>
>> On 64-bit Cygwin, however, I got the behavior reported by the OP:
>>
>> gdb: unknown target exception 0x20474343 at 0x7ff8cccca719
>>
>> Thread 1 "exception_test" received signal ?, Unknown signal.
>> 0x00007ff8cccca719 in RaiseException () from /c/WINDOWS/System32/KERNELBASE.dll
>> (gdb) c
>> Continuing.
>> [Thread 4508.0x3e20 exited with code 541541187]
>> [Thread 4508.0x5bc exited with code 541541187]
>> [Thread 4508.0x5ee8 exited with code 541541187]
>> [Thread 4508.0x1cd0 exited with code 541541187]
>> [Inferior 1 (process 4508) exited with code 04021641503]
>>
>> Note that the breakpoint at 'terminate' is not reached.  I tried putting a 
>> breakpoint at __cxa_throw and stepping through from there, but I don't really 
>> understand what's going on.  Someone who knows the innards of exception 
>> handling will have to look at this.
> 
> One further comment, and then I'll shut up

I lied.

> and leave it to people who know what 
> they're talking about: In the 64-bit case, the function _Unwind_RaiseException 
> in libgcc/unwind-seh.c gets called, and it calls the Win32 function 
> RaiseException as follows:
> 
>    RaiseException (STATUS_GCC_THROW, 0, 1, (ULONG_PTR *)&exc);
> 
> Then the following comment appears:
> 
>    /* The exception handler installed in crt0 will continue any GCC
>       exception that reaches there (and isn't marked non-continuable).
>       Returning allows the C++ runtime to call std::terminate.  */
> 
> Apparently this doesn't actually happen.

I'm beginning to think this is a bug in the Cygwin build of gcc.  The file 
unwind-seh.c is compiled on 64-bit Cygwin because __SEH__ is defined.  But it 
makes assumptions, such as those in the comment quoted above, that are not valid 
on Cygwin.

In fact, if you look at the main exception handler on Cygwin 
(exceptions.cc:626), it is clear that the exception code STATUS_GCC_THROW does 
not lead to continuation.  It falls under the default case:

     default:
       /* If we don't recognize the exception, we have to assume that
	 we are doing structured exception handling, and we let
	 something else handle it.  */
       return ExceptionContinueSearch;

[STATUS_GCC_THROW is 0x20474343, which shows up in the gdb output above as an 
unknown target exception.]

So it seems to me that either Cygwin's exception handler has to learn to handle 
the exception codes defined in unwind-seh.c, or else gcc should be built with 
__SEH__ undefined.

JonY, Achim, Corinna, am I missing something?

Ken

Re: [Attn: gcc maintainers] Re: unhandled C++ exceptions not propagating

[Corinna Vinschen]

On Aug 17 13:36, Ken Brown via Cygwin wrote:
> On 8/16/2020 6:38 PM, Ken Brown via Cygwin wrote:
> > One further comment, and then I'll shut up
> 
> I lied.

...which is ok.

> > and leave it to people who know what they're talking about: In the
> > 64-bit case, the function _Unwind_RaiseException in libgcc/unwind-seh.c
> > gets called, and it calls the Win32 function RaiseException as follows:
> > 
> >    RaiseException (STATUS_GCC_THROW, 0, 1, (ULONG_PTR *)&exc);
> > 
> > Then the following comment appears:
> > 
> >    /* The exception handler installed in crt0 will continue any GCC
> >       exception that reaches there (and isn't marked non-continuable).
> >       Returning allows the C++ runtime to call std::terminate.  */
> > 
> > Apparently this doesn't actually happen.
> 
> I'm beginning to think this is a bug in the Cygwin build of gcc.  The file
> unwind-seh.c is compiled on 64-bit Cygwin because __SEH__ is defined.  But
> it makes assumptions, such as those in the comment quoted above, that are
> not valid on Cygwin.
> 
> In fact, if you look at the main exception handler on Cygwin
> (exceptions.cc:626), it is clear that the exception code STATUS_GCC_THROW
> does not lead to continuation.  It falls under the default case:
> 
>     default:
>       /* If we don't recognize the exception, we have to assume that
> 	 we are doing structured exception handling, and we let
> 	 something else handle it.  */
>       return ExceptionContinueSearch;
> 
> [STATUS_GCC_THROW is 0x20474343, which shows up in the gdb output above as
> an unknown target exception.]
> 
> So it seems to me that either Cygwin's exception handler has to learn to
> handle the exception codes defined in unwind-seh.c, or else gcc should be
> built with __SEH__ undefined.
> 
> JonY, Achim, Corinna, am I missing something?

This stuff isn't really my strong point.  IIUC the above comment correctly,
we might get away with adding another case to the exception handler switch:

  #define STATUS_GCC_THROW 0x20474343
  case STATUS_GCC_THROW:
    return ExceptionContinueExecution;

Did you try that, by any chance?


Thanks,
Corinna

-- 
Corinna Vinschen
Cygwin Maintainer

Re: [Attn: gcc maintainers] Re: unhandled C++ exceptions not propagating

[Achim Gratz]

Corinna Vinschen writes:
> This stuff isn't really my strong point.  IIUC the above comment correctly,
> we might get away with adding another case to the exception handler switch:
>
>   #define STATUS_GCC_THROW 0x20474343
>   case STATUS_GCC_THROW:
>     return ExceptionContinueExecution;
>
> Did you try that, by any chance?

There is a clear expectation that SEH is available on 64bit as expressed
in cygming.h from gcc (but not on 32bit, which explains why there's no
problem there):

#define TARGET_SEH  (TARGET_64BIT_MS_ABI && flag_unwind_tables)

and MS_ABI is the default for 64bit, flag_unwind_tables is a global
variable that is set to 1 by default and very few platforms set it to 0.


Regards,
Achim.
-- 
+<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+

Factory and User Sound Singles for Waldorf rackAttack:
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Re: [Attn: gcc maintainers] Re: unhandled C++ exceptions not propagating

[Ken Brown]

On 8/17/2020 2:13 PM, Corinna Vinschen wrote:
> On Aug 17 13:36, Ken Brown via Cygwin wrote:
>> On 8/16/2020 6:38 PM, Ken Brown via Cygwin wrote:
>>> One further comment, and then I'll shut up
>>
>> I lied.
> 
> ...which is ok.
> 
>>> and leave it to people who know what they're talking about: In the
>>> 64-bit case, the function _Unwind_RaiseException in libgcc/unwind-seh.c
>>> gets called, and it calls the Win32 function RaiseException as follows:
>>>
>>>     RaiseException (STATUS_GCC_THROW, 0, 1, (ULONG_PTR *)&exc);
>>>
>>> Then the following comment appears:
>>>
>>>     /* The exception handler installed in crt0 will continue any GCC
>>>        exception that reaches there (and isn't marked non-continuable).
>>>        Returning allows the C++ runtime to call std::terminate.  */
>>>
>>> Apparently this doesn't actually happen.
>>
>> I'm beginning to think this is a bug in the Cygwin build of gcc.  The file
>> unwind-seh.c is compiled on 64-bit Cygwin because __SEH__ is defined.  But
>> it makes assumptions, such as those in the comment quoted above, that are
>> not valid on Cygwin.
>>
>> In fact, if you look at the main exception handler on Cygwin
>> (exceptions.cc:626), it is clear that the exception code STATUS_GCC_THROW
>> does not lead to continuation.  It falls under the default case:
>>
>>      default:
>>        /* If we don't recognize the exception, we have to assume that
>> 	 we are doing structured exception handling, and we let
>> 	 something else handle it.  */
>>        return ExceptionContinueSearch;
>>
>> [STATUS_GCC_THROW is 0x20474343, which shows up in the gdb output above as
>> an unknown target exception.]
>>
>> So it seems to me that either Cygwin's exception handler has to learn to
>> handle the exception codes defined in unwind-seh.c, or else gcc should be
>> built with __SEH__ undefined.
>>
>> JonY, Achim, Corinna, am I missing something?
> 
> This stuff isn't really my strong point.  IIUC the above comment correctly,
> we might get away with adding another case to the exception handler switch:
> 
>    #define STATUS_GCC_THROW 0x20474343
>    case STATUS_GCC_THROW:
>      return ExceptionContinueExecution;
> 
> Did you try that, by any chance?

No, I didn't.  It might well solve the immediate problem of this bug report. 
But then there are two other exception codes that occur in unwind-seh.c that we 
would also have to deal with: STATUS_GCC_UNWIND	and STATUS_GCC_FORCED.  Someone 
would have to look at their uses and figure out what, if anything, Cygwin should 
do with them.

I'd be willing to take a crack at this if no one more knowledgeable steps 
forward.  But keep in mind that I've never looked at the gcc code or Cygwin's 
exception handler until today, so I'm not necessarily the best person to do this.

Ken

Re: [Attn: gcc maintainers] Re: unhandled C++ exceptions not propagating

[Ken Brown]

On 8/17/2020 3:06 PM, Ken Brown via Cygwin wrote:
> On 8/17/2020 2:13 PM, Corinna Vinschen wrote:
>> On Aug 17 13:36, Ken Brown via Cygwin wrote:
>>> On 8/16/2020 6:38 PM, Ken Brown via Cygwin wrote:
>>>> One further comment, and then I'll shut up
>>>
>>> I lied.
>>
>> ...which is ok.
>>
>>>> and leave it to people who know what they're talking about: In the
>>>> 64-bit case, the function _Unwind_RaiseException in libgcc/unwind-seh.c
>>>> gets called, and it calls the Win32 function RaiseException as follows:
>>>>
>>>>     RaiseException (STATUS_GCC_THROW, 0, 1, (ULONG_PTR *)&exc);
>>>>
>>>> Then the following comment appears:
>>>>
>>>>     /* The exception handler installed in crt0 will continue any GCC
>>>>        exception that reaches there (and isn't marked non-continuable).
>>>>        Returning allows the C++ runtime to call std::terminate.  */
>>>>
>>>> Apparently this doesn't actually happen.
>>>
>>> I'm beginning to think this is a bug in the Cygwin build of gcc.  The file
>>> unwind-seh.c is compiled on 64-bit Cygwin because __SEH__ is defined.  But
>>> it makes assumptions, such as those in the comment quoted above, that are
>>> not valid on Cygwin.
>>>
>>> In fact, if you look at the main exception handler on Cygwin
>>> (exceptions.cc:626), it is clear that the exception code STATUS_GCC_THROW
>>> does not lead to continuation.  It falls under the default case:
>>>
>>>      default:
>>>        /* If we don't recognize the exception, we have to assume that
>>>      we are doing structured exception handling, and we let
>>>      something else handle it.  */
>>>        return ExceptionContinueSearch;
>>>
>>> [STATUS_GCC_THROW is 0x20474343, which shows up in the gdb output above as
>>> an unknown target exception.]
>>>
>>> So it seems to me that either Cygwin's exception handler has to learn to
>>> handle the exception codes defined in unwind-seh.c, or else gcc should be
>>> built with __SEH__ undefined.
>>>
>>> JonY, Achim, Corinna, am I missing something?
>>
>> This stuff isn't really my strong point.  IIUC the above comment correctly,
>> we might get away with adding another case to the exception handler switch:
>>
>>    #define STATUS_GCC_THROW 0x20474343
>>    case STATUS_GCC_THROW:
>>      return ExceptionContinueExecution;
>>
>> Did you try that, by any chance?
> 
> No, I didn't.  It might well solve the immediate problem of this bug report. But 
> then there are two other exception codes that occur in unwind-seh.c that we 
> would also have to deal with: STATUS_GCC_UNWIND    and STATUS_GCC_FORCED.  
> Someone would have to look at their uses and figure out what, if anything, 
> Cygwin should do with them.

This isn't an issue after all.  The comment I quoted above refers to all GCC 
exceptions, not just STATUS_GCC_THROW.  I'll submit a patch after testing.

Ken

Re: [Attn: gcc maintainers] Re: unhandled C++ exceptions not propagating

[Ken Brown]

On 8/17/2020 4:40 PM, Ken Brown via Cygwin wrote:
> On 8/17/2020 3:06 PM, Ken Brown via Cygwin wrote:
>> On 8/17/2020 2:13 PM, Corinna Vinschen wrote:
>>> On Aug 17 13:36, Ken Brown via Cygwin wrote:
>>>> On 8/16/2020 6:38 PM, Ken Brown via Cygwin wrote:
>>>>> One further comment, and then I'll shut up
>>>>
>>>> I lied.
>>>
>>> ...which is ok.
>>>
>>>>> and leave it to people who know what they're talking about: In the
>>>>> 64-bit case, the function _Unwind_RaiseException in libgcc/unwind-seh.c
>>>>> gets called, and it calls the Win32 function RaiseException as follows:
>>>>>
>>>>>     RaiseException (STATUS_GCC_THROW, 0, 1, (ULONG_PTR *)&exc);
>>>>>
>>>>> Then the following comment appears:
>>>>>
>>>>>     /* The exception handler installed in crt0 will continue any GCC
>>>>>        exception that reaches there (and isn't marked non-continuable).
>>>>>        Returning allows the C++ runtime to call std::terminate.  */
>>>>>
>>>>> Apparently this doesn't actually happen.
>>>>
>>>> I'm beginning to think this is a bug in the Cygwin build of gcc.  The file
>>>> unwind-seh.c is compiled on 64-bit Cygwin because __SEH__ is defined.  But
>>>> it makes assumptions, such as those in the comment quoted above, that are
>>>> not valid on Cygwin.
>>>>
>>>> In fact, if you look at the main exception handler on Cygwin
>>>> (exceptions.cc:626), it is clear that the exception code STATUS_GCC_THROW
>>>> does not lead to continuation.  It falls under the default case:
>>>>
>>>>      default:
>>>>        /* If we don't recognize the exception, we have to assume that
>>>>      we are doing structured exception handling, and we let
>>>>      something else handle it.  */
>>>>        return ExceptionContinueSearch;
>>>>
>>>> [STATUS_GCC_THROW is 0x20474343, which shows up in the gdb output above as
>>>> an unknown target exception.]
>>>>
>>>> So it seems to me that either Cygwin's exception handler has to learn to
>>>> handle the exception codes defined in unwind-seh.c, or else gcc should be
>>>> built with __SEH__ undefined.
>>>>
>>>> JonY, Achim, Corinna, am I missing something?
>>>
>>> This stuff isn't really my strong point.  IIUC the above comment correctly,
>>> we might get away with adding another case to the exception handler switch:
>>>
>>>    #define STATUS_GCC_THROW 0x20474343
>>>    case STATUS_GCC_THROW:
>>>      return ExceptionContinueExecution;
>>>
>>> Did you try that, by any chance?
>>
>> No, I didn't.  It might well solve the immediate problem of this bug report. 
>> But then there are two other exception codes that occur in unwind-seh.c that 
>> we would also have to deal with: STATUS_GCC_UNWIND    and STATUS_GCC_FORCED. 
>> Someone would have to look at their uses and figure out what, if anything, 
>> Cygwin should do with them.
> 
> This isn't an issue after all.  The comment I quoted above refers to all GCC 
> exceptions, not just STATUS_GCC_THROW.  I'll submit a patch after testing.

I've sent the patch.  The output of the OP's test case is now the same on 64-bit 
as on 32-bit:

$ ./a
terminate called after throwing an instance of 'std::logic_error'
   what():  basic_string::_S_construct null not valid
Aborted (core dumped)

Ken

Re: [Attn: gcc maintainers] Re: unhandled C++ exceptions not propagating

[JonY]

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On 8/17/20 8:50 PM, Ken Brown via Cygwin wrote:
>>
>> This isn't an issue after all.  The comment I quoted above refers to
>> all GCC exceptions, not just STATUS_GCC_THROW.  I'll submit a patch
>> after testing.
> 
> I've sent the patch.  The output of the OP's test case is now the same
> on 64-bit as on 32-bit:
> 
> $ ./a
> terminate called after throwing an instance of 'std::logic_error'
>   what():  basic_string::_S_construct null not valid
> Aborted (core dumped)
> 

Is the patch on gcc-patches? I'm not seeing it somehow.


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Re: [Attn: gcc maintainers] Re: unhandled C++ exceptions not propagating

[Ken Brown]

On 8/17/2020 8:09 PM, JonY via Cygwin wrote:
> On 8/17/20 8:50 PM, Ken Brown via Cygwin wrote:
>>>
>>> This isn't an issue after all.  The comment I quoted above refers to
>>> all GCC exceptions, not just STATUS_GCC_THROW.  I'll submit a patch
>>> after testing.
>>
>> I've sent the patch.  The output of the OP's test case is now the same
>> on 64-bit as on 32-bit:
>>
>> $ ./a
>> terminate called after throwing an instance of 'std::logic_error'
>>    what():  basic_string::_S_construct null not valid
>> Aborted (core dumped)
>>
> 
> Is the patch on gcc-patches? I'm not seeing it somehow.

No, it's on cygwin-patches.

Ken

Re: [Attn: gcc maintainers] Re: unhandled C++ exceptions not propagating

[JonY]

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On 8/18/20 12:20 AM, Ken Brown via Cygwin wrote:
> On 8/17/2020 8:09 PM, JonY via Cygwin wrote:
>> On 8/17/20 8:50 PM, Ken Brown via Cygwin wrote:
>>>>
>>>> This isn't an issue after all.  The comment I quoted above refers to
>>>> all GCC exceptions, not just STATUS_GCC_THROW.  I'll submit a patch
>>>> after testing.
>>>
>>> I've sent the patch.  The output of the OP's test case is now the same
>>> on 64-bit as on 32-bit:
>>>
>>> $ ./a
>>> terminate called after throwing an instance of 'std::logic_error'
>>>    what():  basic_string::_S_construct null not valid
>>> Aborted (core dumped)
>>>
>>
>> Is the patch on gcc-patches? I'm not seeing it somehow.
> 
> No, it's on cygwin-patches.
> 

OK thanks for working on this.



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