Passing Numpy arrays to a C function for input and output

While not a direct answer to your original question, here's a much more convenient way to call your function. First, make the prototype of your C function exactly as you would do it in plain C. Since you don't need rowcount and colcount separately, I'll collapse them into a single size parameter:

void cfun(const double *indatav, size_t size, double *outdatav) {    size_t i;    for (i = 0; i < size; ++i)        outdatav[i] = indatav[i] * 2.0;}

Now define the ctypes prototype in the following way:

import ctypesfrom numpy.ctypeslib import ndpointerlib = ctypes.cdll.LoadLibrary("./ctest.so")fun = lib.cfunfun.restype = Nonefun.argtypes = [ndpointer(ctypes.c_double, flags="C_CONTIGUOUS"),                ctypes.c_size_t,                ndpointer(ctypes.c_double, flags="C_CONTIGUOUS")]

Now, calls to your function will be really convenient:

indata = numpy.ones((5,6))outdata = numpy.empty((5,6))fun(indata, indata.size, outdata)

You could also define a wrapper to make this even more convenient:

def wrap_fun(indata, outdata):    assert indata.size == outdata.size    fun(indata, indata.size, outdata)

Just pass all four arguments to the C function. Change your Python code from:

fun(ctypes.c_void_p(indata.ctypes.data), ctypes.c_void_p(outdata.ctypes.data))

To:

fun(ctypes.c_void_p(indata.ctypes.data), ctypes.c_int(5), ctypes.c_int(6),    ctypes.c_void_p(outdata.ctypes.data))