Cpp + Python: static data dynamic initialization in *nix shared lib?
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From: Alf P. Steinbach /Usenet on 9 Jul 2010 11:52 [Cross-posted comp.lang.python and comp.lang.c++] I lack experience with shared libraries in *nix and so I need to ask... This is about "cppy", some support for writing Python extensions in C++ that I just started on (some days ago almost known as "pynis" (not funny after all)). For an extension module it seems that Python requires each routine to be defined as 'extern "C"'. And although e.g. MSVC is happy to mix 'extern "C"' and C++ linkage, using a routine declared as 'static' in a class as a C callback, formally they're two different kinds, and I seem to recall that /some/ C++ compiler balks at that kind of mixing unless specially instructed to allow it. Perhaps it was the Sun compiler? Anyway, to be formally correct I cannot generate the required C routines via templating, and I ended up using macros that the user must explicitly invoke, like, here the Py doc's first extension module example recoded using cppy, ------------------------------------------------------------------ <code file="spam.cpp"> #include <progrock/cppx/devsupport/better_experience.h> #include <progrock/cppy/Module.h> using namespace progrock; class Spam: public cppy::Module { public: Spam(): cppy::Module( "spam" ) { setDocString( L"bl�b�rsyltet�y er bl�tt" ); } PyObject* system( PyObject* args ) { const char *command; int sts; if( !PyArg_ParseTuple( args, "s", &command ) ) { return NULL; } sts = ::system( command ); return Py_BuildValue( "i", sts ); } }; CPPY_MODULE_CROUTINE( Spam, system, "Execute a shell command" ) PyMODINIT_FUNC PyInit_spam() { return cppy::init< Spam >(); } </code> ------------------------------------------------------------------ It works in Windows. But here CPPY_MODULE_CROUTINE does three things: A Defining the 'extern "C"' routine. I cannot think of any problem here. B Defining installation data for that routine. Possible problem: initializing a static with address of routine? C -> Adding that install data record into a linked list! Possible problem: are dynamic initialization actions guaranteed to be performed in *nix shared library? Problem (C) is outside the realm of the C++ standard, since the C++ standard doesn't support shared libraries, and I've never actually used *nix shared libraries so I don't /know/... Is such dynamic initialization guaranteed? For completeness, the macro definition (the 0 in there is a list next-pointer): <code> #define CPPY_MODULE_CROUTINE_DEF( cppClassName, name ) \ extern "C" \ static PyObject* cppClassName##_##name( PyObject*, PyObject* args ) \ { \ return ::progrock::cppy::module<cppClassName>().name( args ); \ } #define CPPY_MODULE_CROUTINE_INSTALLDATA( cppClassName, name, docString ) \ static ::progrock::cppy::detail::ModuleRoutineDescriptor \ cppClassName##_##name##_descriptor = { \ 0, \ #name, \ docString, \ &cppClassName##_##name \ }; \ \ static bool cppClassName##_##name##_descriptor_installed = \ ::progrock::cppy::detail::addToList< cppClassName >( \ cppClassName##_##name##_descriptor \ ); #define CPPY_MODULE_CROUTINE( cppClassName, name, docString ) \ CPPY_MODULE_CROUTINE_DEF( cppClassName, name ) \ CPPY_MODULE_CROUTINE_INSTALLDATA( cppClassName, name, docString ) </code> TIA., - Alf -- blog at <url: http://alfps.wordpress.com>
From: geremy condra on 9 Jul 2010 17:43 On Fri, Jul 9, 2010 at 5:22 PM, Ian Collins <ian-news(a)hotmail.com> wrote: > On 07/10/10 03:52 AM, Alf P. Steinbach /Usenet wrote: >> >> [Cross-posted comp.lang.python and comp.lang.c++] >> >> I lack experience with shared libraries in *nix and so I need to ask... >> >> This is about "cppy", some support for writing Python extensions in C++ >> that I just started on (some days ago almost known as "pynis" (not funny >> after all)). >> >> For an extension module it seems that Python requires each routine to be >> defined as 'extern "C"'. And although e.g. MSVC is happy to mix 'extern >> "C"' and C++ linkage, using a routine declared as 'static' in a class as >> a C callback, formally they're two different kinds, and I seem to recall >> that /some/ C++ compiler balks at that kind of mixing unless specially >> instructed to allow it. Perhaps it was the Sun compiler? > > Yes, it will (correctly) issue a warning. > > As the is a bit OT, contact me directly and we can work through it. I have > had similar fun and games adding PHP modules! I'd appreciate it if you'd either leave this on-list or cc me in on this, as I'm working through a similar issue. Geremy Condra
From: Alf P. Steinbach /Usenet on 13 Jul 2010 03:34 * geremy condra, on 09.07.2010 23:43: > On Fri, Jul 9, 2010 at 5:22 PM, Ian Collins<ian-news(a)hotmail.com> wrote: >> On 07/10/10 03:52 AM, Alf P. Steinbach /Usenet wrote: >>> >>> [Cross-posted comp.lang.python and comp.lang.c++] >>> >>> I lack experience with shared libraries in *nix and so I need to ask... >>> >>> This is about "cppy", some support for writing Python extensions in C++ >>> that I just started on (some days ago almost known as "pynis" (not funny >>> after all)). >>> >>> For an extension module it seems that Python requires each routine to be >>> defined as 'extern "C"'. And although e.g. MSVC is happy to mix 'extern >>> "C"' and C++ linkage, using a routine declared as 'static' in a class as >>> a C callback, formally they're two different kinds, and I seem to recall >>> that /some/ C++ compiler balks at that kind of mixing unless specially >>> instructed to allow it. Perhaps it was the Sun compiler? >> >> Yes, it will (correctly) issue a warning. >> >> As the is a bit OT, contact me directly and we can work through it. I have >> had similar fun and games adding PHP modules! > > I'd appreciate it if you'd either leave this on-list or cc me in on this, as > I'm working through a similar issue. Well, we got no further, but I know of three solutions: A) Punting: just say that the compiler has to support C++/C function type mingling. -> Perhaps the practical solution, but formally unsafe. B) On the script side of things, delegate all calls to single Mother Of All C func downcaller that supplies as extra arg an id of the C++ function. -> Micro-level inefficient but easy to use and formally correct. C) Let the user define the C linkage function wrappers via macros. -> Efficient and formally correct but exposes ugly macro names. I chose (C). I believe Boost's Python binding uses (A), or perhaps (B). Cheers, - Alf PS: You (the reader) may be wondering, why why why Yet Another Python/C++ binding? Well, because I had this great name for it, "pyni", unfortunately already in use. But cppy is very different from Boost: Boost is large, cppy is tiny; Boost has as main goal to expose arbitrary C++ code to Python, automating argument conversion etc., while with cppy your Python design is exposed to C++ with no enforced arg conversions and such; Boost relies on canned magic, difficult to subvert when it doesn't do what you want, while with cppy you are (or, so far, I am) in control; and I suspect that the Boost Python binding, relying on dynamic registries and stuff, is not all that efficient, while cppy is as efficient as using the Python C API to create an extension. And besides, cppy supports national characters in doc strings etc. And I'm Norwegian. So. :-) -- blog at <url: http://alfps.wordpress.com>
From: Jonathan Lee on 13 Jul 2010 10:41 > Problem (C) is outside the realm of the C++ standard, since the C++ standard > doesn't support shared libraries, and I've never actually used *nix shared > libraries so I don't /know/... > > Is such dynamic initialization guaranteed? > Not guaranteed, though I think there's a combination of dlopen options and gcc command line parameters that invoke this behavior. See the second page of http://www.linuxjournal.com/article/3687 about auto-registration. Personally, though, it never worked for me :/ --Jonathan
From: Alf P. Steinbach /Usenet on 13 Jul 2010 15:53
* Jonathan Lee, on 13.07.2010 16:41: >> Problem (C) is outside the realm of the C++ standard, since the C++ standard >> doesn't support shared libraries, and I've never actually used *nix shared >> libraries so I don't /know/... >> >> Is such dynamic initialization guaranteed? >> > > Not guaranteed, though I think there's a combination of dlopen options > and gcc command line parameters that invoke this behavior. See the > second page of > > http://www.linuxjournal.com/article/3687 > > about auto-registration. > > Personally, though, it never worked for me :/ Ah, well. :-( Thanks for the info! OK, I'll just have to replace the auto-registration with some C++ magic. For which I think I'll simply /require/ that the compiler supports mixing of C and C++ linkage, that is, that ... <code language="Not quite standard C++!"> #include <iostream> extern "C" { typedef int (*Callback)( int ); } void foo( Callback f ) { std::cout << "foo!" << f( 42 ) << std::endl; } int a( int ) { return 1; } extern "C" int b( int ) { return 2; } int main() { foo( a ); // Unholy Mix of C++ and C linkage, formally not OK. foo( b ); // Should be OK with any compiler. } </code> .... compiles, and works. Cheers, & thanks, - Alf -- blog at <url: http://alfps.wordpress.com> |