propagation of exceptions over module/language boundaries
 

Hi

I hope you'll see this cross-post (c/c++) as appropriate. I also admit
immediately that the question *may* turn out to be
compiler-/os-specific, in which case I apologize. But I wonder if
there's an underlying truth.

We are writing a (cross-platform) 'framework' application (in C++, as it
happens) that allows users to author 'plugins' (as shared library
modules). Since the lifetime of plugins is expected to be long, and the
framework may be updated using a later compiler version, we have chosen
to offer a C interface to those plugins to avoid issues with changing
ABIs (as far as I understand it). Plugins are expected to export a
single undecorated function, Event(EventData* data), and may call back
to the framework through a number of undecorated functions exported from
an API module, 'api', also authored in C++. Commonly, then, the
framework will load a plugin, call its Event() function, and that
function will call one of these callback functions on the API.

At this point I have to own up and say I don't fully understand linking
or calling conventions, so when I say we "offer a C interface" what I
mean is that api is compiled to export its functions undecorated (sort
of with the use of extern "C", though actually there's some other stuff
going on that I'd rather not get into so I'm hoping that's a
sufficiently good approximation - api is a C++ module but its exports
are undecorated). Also, plugins export their Event() function as extern "C".

Motivation: It would be convenient if the api could throw an exception
that the framework would recognise. It would pass painlessly up through
any user code without the user having to do anything, and the framework
could find out what happened and generate a useful error report. The
alternative is that we enforce that all API functions do not throw and
instead return error codes - the onus is then on the plugin author to
catch the error and return it to the framework (or, if they are
authoring in C++, they could throw an exception themselves, but i'm not
sure if that's not the same question again).

Now, 'api' is authored in C++, and so is the framework, but user plugins
may be authored in C or C++ (or, in fact, any other language that can
meet the requirements of the interface). So my initial thinking was that
we couldn't use exceptions, much as they're great imho. However, in my
current dev copy, i'm finding exceptions work fine. My worry is that in
my current dev copy i'm compiling the framework, the api, *and* the test
plugin, all with the same compiler (cl v14, win xp), which may be hiding
problems that would arise once we go more mix and match. In addition,
I'm writing this particular plugin in C++, so everything is C++ save the
actual exported function declarations.

So... the question is, should we avoid throwing exceptions across these
module boundaries, or is it ok to do so? I could actually test this to
some extent by installing some other compilers, but I'm thinking that
that's not going to be a thorough test anyway, so I'd rather reach some
understanding. For instance...

* framework loads "plugin"
* framework calls plugin::Event()
* plugin calls api::Sum()
* error condition occurs in Sum()
* Sum() throws exception of type X

or, to put that another way, the call stack looks like:

framework::main() (C++)
--> plugin::Event() (C)
--> api::Sum() (C++)
throw X;

Now, if X is, say, an int, will it pass safely back up through "plugin"
in all cases, even if plugin is compiled with an (old, perhaps) C
compiler that is strictly not aware of exceptions? Is this a
compiler-/os-specific question? what if X is a C struct defined in api.h?

From what I have learned about exceptions and how they are implemented,
I don't immediately see a problem with passing one 'through'
exception-unaware code in this way. There will be no handlers on the
stack associated with the C-code, but I'm supposing that this means that
control will just pass (effectively) directly from the "throw" statement
in api to the "catch" block in framework. But I'm worried that the way
the stack is arranged to accomodate the two cross-module-boundary calls
will somehow interfere with this mechanism. I am also worried that the
type of the object will not be recognised when it reaches framework - I
am fine with sticking with const char* as an exception type if that's
the only reliable option, but a struct with a code, a message, and some
source information would be preferable, of course.

Phew. I hope that's clear.

Thanks in advance for any comments.
Ben Mitch

Re: propagation of exceptions over module/language boundaries
 

ben mitch wrote:
> Hi
>
> I hope you'll see this cross-post (c/c++) as appropriate. I also admit
> immediately that the question *may* turn out to be
> compiler-/os-specific, in which case I apologize. But I wonder if
> there's an underlying truth.
>
> We are writing a (cross-platform) 'framework' application (in C++, as it
> happens) that allows users to author 'plugins' (as shared library
> modules). Since the lifetime of plugins is expected to be long, and the
> framework may be updated using a later compiler version, we have chosen
> to offer a C interface to those plugins to avoid issues with changing
> ABIs (as far as I understand it). Plugins are expected to export a
> single undecorated function, Event(EventData* data), and may call back
> to the framework through a number of undecorated functions exported from
> an API module, 'api', also authored in C++. Commonly, then, the
> framework will load a plugin, call its Event() function, and that
> function will call one of these callback functions on the API.
>
> At this point I have to own up and say I don't fully understand linking
> or calling conventions, so when I say we "offer a C interface" what I
> mean is that api is compiled to export its functions undecorated (sort
> of with the use of extern "C", though actually there's some other stuff
> going on that I'd rather not get into so I'm hoping that's a
> sufficiently good approximation - api is a C++ module but its exports
> are undecorated). Also, plugins export their Event() function as extern
> "C".
>
> Motivation: It would be convenient if the api could throw an exception
> that the framework would recognise. It would pass painlessly up through
> any user code without the user having to do anything, and the framework
> could find out what happened and generate a useful error report. The
> alternative is that we enforce that all API functions do not throw and
> instead return error codes - the onus is then on the plugin author to
> catch the error and return it to the framework (or, if they are
> authoring in C++, they could throw an exception themselves, but i'm not
> sure if that's not the same question again).
>
> Now, 'api' is authored in C++, and so is the framework, but user plugins
> may be authored in C or C++ (or, in fact, any other language that can
> meet the requirements of the interface). So my initial thinking was that
> we couldn't use exceptions, much as they're great imho. However, in my
> current dev copy, i'm finding exceptions work fine. My worry is that in
> my current dev copy i'm compiling the framework, the api, *and* the test
> plugin, all with the same compiler (cl v14, win xp), which may be hiding
> problems that would arise once we go more mix and match. In addition,
> I'm writing this particular plugin in C++, so everything is C++ save the
> actual exported function declarations.
>
> So... the question is, should we avoid throwing exceptions across these
> module boundaries, or is it ok to do so? I could actually test this to
> some extent by installing some other compilers, but I'm thinking that
> that's not going to be a thorough test anyway, so I'd rather reach some
> understanding. For instance...
>
> * framework loads "plugin"
> * framework calls plugin::Event()
> * plugin calls api::Sum()
> * error condition occurs in Sum()
> * Sum() throws exception of type X
>
> or, to put that another way, the call stack looks like:
>
> framework::main() (C++)
> --> plugin::Event() (C)
> --> api::Sum() (C++)
> throw X;
>
> Now, if X is, say, an int, will it pass safely back up through "plugin"
> in all cases, even if plugin is compiled with an (old, perhaps) C
> compiler that is strictly not aware of exceptions? Is this a
> compiler-/os-specific question? what if X is a C struct defined in api.h?
>
> From what I have learned about exceptions and how they are implemented,
> I don't immediately see a problem with passing one 'through'
> exception-unaware code in this way. There will be no handlers on the
> stack associated with the C-code, but I'm supposing that this means that
> control will just pass (effectively) directly from the "throw" statement
> in api to the "catch" block in framework. But I'm worried that the way
> the stack is arranged to accomodate the two cross-module-boundary calls
> will somehow interfere with this mechanism. I am also worried that the
> type of the object will not be recognised when it reaches framework - I
> am fine with sticking with const char* as an exception type if that's
> the only reliable option, but a struct with a code, a message, and some
> source information would be preferable, of course.
>
> Phew. I hope that's clear.
>
> Thanks in advance for any comments.
> Ben Mitch

I have studied this problem extensively in the context of dynamically
generated C code. I will not bore you with the details, but solving
the problem you see is not easy at all.

1) Using Microsoft compiler will work if all the "clients" use also
the same compiler AND the same version. With different versions of
the compiler things could change and you would experience mysterious
crashes.

2) Avoid gcc if possible. There is no documentation almost about the
exception framework and the documentation available is highly
misleading. In any case if you have to use gcc you will want to
ALWAYS use gcc ("clients" and API server) and specially, the SAME
version of gcc. Do remember that gcc changes its behaviour from
version to version without any announcement, since all this is
not really documented.


--
jacob navia
jacob at jacob point remcomp point fr
logiciels/informatique
http://www.cs.virginia.edu/~lcc-win32

Re: propagation of exceptions over module/language boundaries
 

ben mitch <news@benmitch.net> writes:
> I hope you'll see this cross-post (c/c++) as appropriate. I also admit
> immediately that the question *may* turn out to be
> compiler-/os-specific, in which case I apologize. But I wonder if
> there's an underlying truth.
>
[SNIP]
>
> Motivation: It would be convenient if the api could throw an exception
> that the framework would recognise. It would pass painlessly up
> through any user code without the user having to do anything, and the
> framework could find out what happened and generate a useful error
> report. The alternative is that we enforce that all API functions do
> not throw and instead return error codes - the onus is then on the
> plugin author to catch the error and return it to the framework (or,
> if they are authoring in C++, they could throw an exception
> themselves, but i'm not sure if that's not the same question again).
[SNIP]

I think this is really a question for comp.lang.c++, not for
comp.lang.c (even though it does involve both languages. The C+
standard defines features for interfacing to C, but C doesn't define
any features for interfacing to C++. Furthermore, C says nothing
about C++-style exceptions.

I suspect that exception propagation across C code is system-specific,
but the folks in comp.lang.c++ can probably give you more information
on that point.

--
Keith Thompson (The_Other_Keith) kst-u@mib.org <http://www.ghoti.net/~kst>
Nokia
"We must do something. This is something. Therefore, we must do this."
-- Antony Jay and Jonathan Lynn, "Yes Minister"

Re: propagation of exceptions over module/language boundaries
 

On 2008-06-13 15:07, ben mitch wrote:
> Hi
>
> I hope you'll see this cross-post (c/c++) as appropriate. I also admit
> immediately that the question *may* turn out to be
> compiler-/os-specific, in which case I apologize. But I wonder if
> there's an underlying truth.
>
> We are writing a (cross-platform) 'framework' application (in C++, as it
> happens) that allows users to author 'plugins' (as shared library
> modules).

If you want it to truly be cross-platform you can not allow exceptions
to pass through to/from the plugins. The reason is simple: there is no
standardised way of doing this. The only (?) standardised calling
convention which will be available on multiple platforms is the C
calling convention and it does not support exceptions.

--
Erik Wikström

Re: propagation of exceptions over module/language boundaries
 

On 2008-06-13 17:59, Erik Wikström wrote:
> On 2008-06-13 15:07, ben mitch wrote:
>> Hi
>>
>> I hope you'll see this cross-post (c/c++) as appropriate. I also admit
>> immediately that the question *may* turn out to be
>> compiler-/os-specific, in which case I apologize. But I wonder if
>> there's an underlying truth.
>>
>> We are writing a (cross-platform) 'framework' application (in C++, as it
>> happens) that allows users to author 'plugins' (as shared library
>> modules).
>
> If you want it to truly be cross-platform you can not allow exceptions
> to pass through to/from the plugins. The reason is simple: there is no
> standardised way of doing this. The only (?) standardised calling
> convention which will be available on multiple platforms is the C
> calling convention and it does not support exceptions.

Sorry, I should mention that it is possible, but you would have to
require that all the code (framework and plugins) is compiled with the
same compiler and you will have to use a C++ API.

--
Erik Wikström

Re: propagation of exceptions over module/language boundaries
 

Erik Wikström wrote:
> On 2008-06-13 15:07, ben mitch wrote:
>> Hi
>>
>> I hope you'll see this cross-post (c/c++) as appropriate. I also admit
>> immediately that the question *may* turn out to be
>> compiler-/os-specific, in which case I apologize. But I wonder if
>> there's an underlying truth.
>>
>> We are writing a (cross-platform) 'framework' application (in C++, as it
>> happens) that allows users to author 'plugins' (as shared library
>> modules).
>
> If you want it to truly be cross-platform you can not allow exceptions
> to pass through to/from the plugins. The reason is simple: there is no
> standardised way of doing this. The only (?) standardised calling
> convention which will be available on multiple platforms is the C
> calling convention and it does not support exceptions.
>

This is not true. Most C++ compilers support C and will allow C and
C++ to coexist peacefully. Problems arise when you mix different
compilers and versions or you want to generate code dynamically.



--
jacob navia
jacob at jacob point remcomp point fr
logiciels/informatique
http://www.cs.virginia.edu/~lcc-win32

Re: propagation of exceptions over module/language boundaries
 

On 2008-06-13 18:04, jacob navia wrote:
> Erik Wikström wrote:
>> On 2008-06-13 15:07, ben mitch wrote:
>>> Hi
>>>
>>> I hope you'll see this cross-post (c/c++) as appropriate. I also admit
>>> immediately that the question *may* turn out to be
>>> compiler-/os-specific, in which case I apologize. But I wonder if
>>> there's an underlying truth.
>>>
>>> We are writing a (cross-platform) 'framework' application (in C++, as it
>>> happens) that allows users to author 'plugins' (as shared library
>>> modules).
>>
>> If you want it to truly be cross-platform you can not allow exceptions
>> to pass through to/from the plugins. The reason is simple: there is no
>> standardised way of doing this. The only (?) standardised calling
>> convention which will be available on multiple platforms is the C
>> calling convention and it does not support exceptions.
>>
>
> This is not true. Most C++ compilers support C and will allow C and
> C++ to coexist peacefully. Problems arise when you mix different
> compilers and versions or you want to generate code dynamically.

What is not true? To my knowledge there are two ways for C and C++ to
coexist: either you compile the C code as C++ (in which case it is no
longer C), or you use extern "C", in which case you use the C calling
convention.

Further mote it is true that there is no standardised cross-platform way
of propagating exceptions. An, as far as I know, the C calling
convention is the only wide-spread standardised calling convention.

--
Erik Wikström

Re: propagation of exceptions over module/language boundaries
 

Erik Wikström wrote:
> On 2008-06-13 18:04, jacob navia wrote:
>> Erik Wikström wrote:
>>> If you want it to truly be cross-platform you can not allow exceptions
>>> to pass through to/from the plugins. The reason is simple: there is no
>>> standardised way of doing this. The only (?) standardised calling
>>> convention which will be available on multiple platforms is the C
>>> calling convention and it does not support exceptions.
>>>
>> This is not true. Most C++ compilers support C and will allow C and
>> C++ to coexist peacefully. Problems arise when you mix different
>> compilers and versions or you want to generate code dynamically.
>
> What is not true? To my knowledge there are two ways for C and C++ to
> coexist: either you compile the C code as C++ (in which case it is no
> longer C), or you use extern "C", in which case you use the C calling
> convention.
>

Calling conventions have nothing to do here. You are confusing calling
convention (function call interface) with propagating the exceptions
(i.e. letting a throw pass through your code), what is completely
different.

> Further mote it is true that there is no standardised cross-platform way
> of propagating exceptions.

Yes. That is why I said that if you stick to the same
compiler and version it will work.

> An, as far as I know, the C calling
> convention is the only wide-spread standardised calling convention.
>

Again, calling conventions are NOT the issue here.

jacob

--
jacob navia
jacob at jacob point remcomp point fr
logiciels/informatique
http://www.cs.virginia.edu/~lcc-win32

Re: propagation of exceptions over module/language boundaries
 

jacob navia wrote:
> Erik Wikström wrote:
>> On 2008-06-13 15:07, ben mitch wrote:
>>> Hi
>>>
>>> I hope you'll see this cross-post (c/c++) as appropriate. I also
>>> admit immediately that the question *may* turn out to be
>>> compiler-/os-specific, in which case I apologize. But I wonder if
>>> there's an underlying truth.
>>>
>>> We are writing a (cross-platform) 'framework' application (in C++, as
>>> it happens) that allows users to author 'plugins' (as shared library
>>> modules).
>>
>> If you want it to truly be cross-platform you can not allow exceptions
>> to pass through to/from the plugins. The reason is simple: there is no
>> standardised way of doing this. The only (?) standardised calling
>> convention which will be available on multiple platforms is the C
>> calling convention and it does not support exceptions.
>>
>
> This is not true. Most C++ compilers support C and will allow C and
> C++ to coexist peacefully.

They do, provided you use C linkage functions (extern "C" in the C++
world). Exceptions are alien to C, so you simply can't propagate
exceptions through a C function.

> Problems arise when you mix different
> compilers and versions or you want to generate code dynamically.
>
A problem in C++, but in C? The C ABI is determined by the platform,
not the compiler (at least in the Linux/Unix world). If a compiler used
a different ABI, how could it link to system libraries?

--
Ian Collins.

Re: propagation of exceptions over module/language boundaries
 

Ian Collins wrote:
> jacob navia wrote:
[snip]
>> This is not true. Most C++ compilers support C and will allow C and
>> C++ to coexist peacefully.
>
> They do, provided you use C linkage functions (extern "C" in the C++
> world). Exceptions are alien to C, so you simply can't propagate
> exceptions through a C function.
>

Yes you can. I have done this under gcc and msvc, and I think
all C++ compilers will generate compatible code.

>> Problems arise when you mix different
>> compilers and versions or you want to generate code dynamically.
>>
> A problem in C++, but in C? The C ABI is determined by the platform,
> not the compiler (at least in the Linux/Unix world). If a compiler used
> a different ABI, how could it link to system libraries?
>

If you use different compilers, the throw machinery ill be different
and passing throws will just not work.


--
jacob navia
jacob at jacob point remcomp point fr
logiciels/informatique
http://www.cs.virginia.edu/~lcc-win32