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Subject:	Re: iostream exceptions uncaught with CYGWIN GCC/7.3.0
To:	cygwin AT cygwin DOT com
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Date:	Fri, 2 Nov 2018 10:32:41 +0000
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From: JonY <10walls AT gmail DOT com>
To: cygwin AT cygwin DOT com
Message-ID: <f5c36860-aa2f-abe3-e926-be235a153191 AT gmail DOT com>
Subject: Re: iostream exceptions uncaught with CYGWIN GCC/7.3.0
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On 11/01/2018 08:42 PM, Lavrentiev, Anton (NIH/NLM/NCBI) [C] via cygwin
wrote:
> Hi,
>=20
> Please consider the following code:
>=20
> $ cat bug.cpp=20
> #include <iostream>
>=20
> using namespace std;
>=20
> void fun()
> {
>     string dummy;
>     cin >> dummy;
> }
>=20
>=20
> int main()
> {
>     cout << "FAIL =3D 0x" << hex << ios::failbit << endl;
>     cout << "EOF  =3D 0x" << hex << ios::eofbit  << endl;
>     cout << "BAD  =3D 0x" << hex << ios::badbit  << endl;
>=20
>     cin.exceptions(ios::eofbit | ios::failbit);
>     try {
>         fun();
>     }
>     catch (ios_base::failure&) {
>         cerr << "Failure caught!" << endl;
>     }
>     catch (...) {
>         cerr << "Failure uncaught! 0x" << hex << cin.rdstate() << endl;
>     }
>     return 0;
> }
>=20
> When this program is compiled and run on Linux, the exception gets caught:
>=20
> $ uname -a
> Linux iebdev11 3.10.0-862.14.4.el7.x86_64 #1 SMP Wed Sep 26 15:12:11 UTC =
2018 x86_64 x86_64 x86_64 GNU/Linux
>=20
> $ g++ --version
> g++ (GCC) 7.3.0
> Copyright (C) 2017 Free Software Foundation, Inc.
> This is free software; see the source for copying conditions.  There is NO
> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOS=
E.
>=20
> $ g++ -Wall bug.cpp -o bug
>=20
> $ ./bug < /dev/null
> FAIL =3D 0x4
> EOF  =3D 0x2
> BAD  =3D 0x1
> Failure caught!
>=20
> However, same commands on Cygwin, and the exception goes unhandled:
>=20
> $ uname -a
> CYGWIN_NT-10.0 NCBIPC9135 2.11.1(0.329/5/3) 2018-09-05 10:24 x86_64 Cygwin
>=20
> $ gcc --version
> gcc (GCC) 7.3.0
> Copyright (C) 2017 Free Software Foundation, Inc.
> This is free software; see the source for copying conditions.  There is NO
> warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOS=
E.
>=20
> $ g++ -Wall -o bug bug.cpp
>=20
> $ ./bug < /dev/null
> FAIL =3D 0x4
> EOF  =3D 0x2
> BAD  =3D 0x1
> Failure uncaught! 0x6
>=20
> We've seen this behavior before on Linux too, when the C++ ABI was change=
d (w/GCC 5.x).  I guess CYGWIN packages a version of C++ STDLIB whose ABI i=
s incompatible with default compiler settings.
>=20
> Here's some explanation of what might be the culprit.
>=20
> https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_dual_abi.html says =
that:
>=20
>> Using the default configuration options for GCC the default value of the=
 macro is 1 which causes the new ABI to be active, so to use the old ABI yo=
u must explicitly define the macro to 0 before including any library header=
s.
>>  (Be aware that some GNU/Linux distributions configure GCC 5 differently=
 so that the default value of the macro is 0 and users must define it to 1 =
to enable the new ABI.)
>=20
>> One exception type does change when using the new ABI, namely std::ios_b=
ase::failure. This is necessary because the 2011 standard changed its base =
class from std::exception to std::system_error, which causes its layout to =
change. Exceptions due to iostream errors are thrown by a function inside l=
ibstdc++.so, so whether the thrown exception uses the old std::ios_base::fa=
ilure type or the new one depends on the ABI that was active when libstdc++=
.so was built, not the ABI active in the user code that is using iostreams.=
 This means that for a given build of GCC the type thrown is fixed. In curr=
ent releases the library throws a special type that can be caught by handle=
rs for either the old or new type, but for GCC 7.1, 7.2 and 7.3 the library=
 throws the new std::ios_base::failure type, and for GCC 5.x and 6.x the li=
brary throws the old type. Catch handlers of type std::ios_base::failure wi=
ll only catch the exceptions if using a newer release, or if the handler is=
 compiled with the same ABI as the type thrown by the library. Handlers for=
 std::exception will always catch iostreams exceptions, because the old and=
 new type both inherit from std::exception.

Cygwin GCC is configured and built with the old C++ ABI, I'm guess
that's the problem?


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