How to pass a raw pointer to Boost.Python?
I'm trying to use Boost.Python as a wrapper for a C++ function that receives a pointer, modifies the data (managed on Python side as numpy array for example) and returns. How do I get Python numpy and Boost.Python to collaborate and to give me the raw pointer inside the function?
#include <boost/python.hpp>
namespace
{
char const *greet(double *p)
{
*p = 2.;
return "hello world";
}
}
BOOST_PYTHON_MODULE(module)
{
boost::python::def("greet", &greet);
}
In Python when I try,
import numpy as np
a = np.empty([2], dtype=np.double)
raw_ptr = a.data
print cmod.greet(raw_ptr)
I get the error,
Boost.Python.ArgumentError: Python argument types in
<...>.module.greet(buffer)
did not match C++ signature:
greet(double*)
One way that seems to work, suggested by Andreas Kloeckner, comments and alternatives are welcome. The greet() is modified as follows,
char const *greet(boost::python::object obj) {
PyObject* pobj = obj.ptr();
Py_buffer pybuf;
if(PyObject_GetBuffer(pobj, &pybuf, PyBUF_SIMPLE)!=-1)
{
void *buf = pybuf.buf;
double *p = (double*)buf;
*p = 2.;
*(p+1) = 3;
PyBuffer_Release(&pybuf);
}
return "hello world";
}
in Python just use:
print cmod.greet(a)
You will probably need to use the numpy ctypes interface to get a raw pointer to the storage, and then make a ctypes pointer to double to pass into the call. ndarray.data is a buffer type, not a pointer.
I don't have any experience with boost.python , but I suspect something like
In [28]: x=np.array([1,2,3,4],dtype=np.double)
In [29]: p=x.ctypes.data_as(ctypes.POINTER(ctypes.c_double))
In [30]: type(p)
Out[30]: <class 'ctypes.LP_c_double'>
will work if passed to a wrapped C++ function expecting a pointer as an argument.
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