Passing a byte array to a C++ function that accepts a void pointer

Question

I'm using boost::python to export some C++ functions that expect a void*. They interpret it as raw memory (an array of bytes) internally. Think read and write for some very special purpose device.

How do I pass a Python bytearray to such a function?

I have tried using ctypes.c_void_p.from_buffer(mybytearray) but this doesn't match the signature of the function.

Here's the minimal example:

#include <boost/python.hpp>

void fun_voidp(void*) {}

BOOST_PYTHON_MODULE(tryit) {
    using namespace boost::python;
    def("fun_voidp", fun_voidp);
}

And at the python side:

import tryit
import ctypes
b = bytearray(100)
tryit.fun_voidp(b) # fail
tryit.fun_voidp(ctypes.c_void_p.from_buffer(b)) # fail
tryit.fun_voidp(ctypes.c_void_p.from_buffer(b).value) # fail

sehe · Accepted Answer · 2017-09-12 09:11:40Z

2

I'd very much fix the function prototype to take char* and go with the last python line.

There's no reason to use void* in C++. I understand if the API does, but it shouldn't be hard to wrap it with your own function.

answered Sep 12, 2017 at 9:11

sehe

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3 Comments

n. m. could be an AI Over a year ago

It's a legacy API in use by other projects, changing it is impossible and wrapping it defeats the idea of exporting it as is. I could wrap it in a function that accepts a python object...

sehe Over a year ago

"defeats the idea" - what is the actual worth of that idea? (honest question)

n. m. could be an AI Over a year ago

There are many such functions and writing a wrapper for each one would be a lot of work. Perhaps I will go this route anyway, to make the interface more idiomatic Python (i.e. return a new byte array instead of having one as an argument and filling it). Still I'd like to know if there's an option to use a byte array as void* with boost (one can easily do that with ctypes for example).

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