On 12.01.2016 12:52, Tony Cappellini wrote: > > Stefan, > > > To: cplusplus-sig@python.org <mailto:cplusplus-sig@python.org> > Subject: Re: [C++-sig] Passing memory allocated in C++ to Python > Message-ID: <56945036.8080...@seefeld.name > <mailto:56945036.8080...@seefeld.name>> > Content-Type: text/plain; charset=windows-1252 > > > Essentially- Python needs access to the return value from a malloc() call. > > >>Really ? Why ? You say your C++ code needs to access the pointer(s). > >>Python has no notion of "pointer", but it will happily pass around > >>atever data you expose from C++. > > Sorry about that Stefan, my post should have been clearer. > > I know Python doesn't know about pointers. > I should have said "Python needs access to the memory pointed to by > the memory allocated with malloc()". > > > >>f you expose the above class together with the constructor and thei > >>'call_ioctl()' member function I think you have all you need. > > In your something class, the data type returned from allocate_memory() > needs to be something that Python understands. Since that allocation > function (member) will be allocating 100s of MB of memory, how will > this memory map to a Python data type?
Does it have to be a Python (native) data type ? Could you explain your use-case a little more ? > > Is a bytearray better to use than a list (as far as performance is > concerned)? Is a list better to use than a string (as far as > performance is concerned). Are there other data types that should be > considered? It all depends on what you intend to do with the memory. > > The user will need to access that data as bytes, words, longs, and > possibly sequences. > > I've tried using boost's extract, to convert the char * returned from > malloc() into a Python string, > and a Python list, but these result in compile errors. That's because the extract<>() logic is meant to be used for the inverse: to get access to C/C++ objects embedded into a Python object. Once you reflect your C++ type "something" to Python, boost.python will implicitly convert between the Python (wrapper) type and the C++ "something" type for all function calls. However, sometimes you may still have a Python object, knowing that it actually contains a "something". extract<> is meant to give access to that (either by-value or by-reference, depending on your usage). If you really need to expose the memory as a buffer to Python, you may want to use one of the newer C APIs such as |PyMemoryView_FromMemory (https://docs.python.org/3/c-api/memoryview.html). Support for that isn't built into boost.python yet, so you'll have to add some glue code (e.g. http://stackoverflow.com/questions/23064407/expose-c-buffer-as-python-3-bytes). This may actually be worth including into the library. We just haven't had anyone asking for such a feature - yet. | (I have used similar approaches in the past, where memory from a C++ library I wrote is bound to a NumPy array, so the data can be accessed copy-free from other Python modules via the NumPy interface.) Regards, Stefan -- ...ich hab' noch einen Koffer in Berlin... _______________________________________________ Cplusplus-sig mailing list Cplusplus-sig@python.org https://mail.python.org/mailman/listinfo/cplusplus-sig
