ArgumentError:expected LP_c_float instance instead of pointer to LP_c_float_Array_50000

Question

I have a function in dll file that takes float pointer as one of argument(argument 9: float *result).

void generate_codebook(int *nodestatus, int *nofnode, int *noftree, int *terminal,  int *nofterminal, int *nofobservations, int *total, int *nofseries,  float *result)

Here is the python code where I am facing issue:

nofseries=c_int(len(nofobservations))
noftree=c_int(terminal.shape[1])
nofnode=c_int(nodestatus.shape[0])
total=c_int(np.sum(nofobservations,dtype=np.int64))
nofentry=ctypes.POINTER(ctypes.c_float *(len(nofobservations)*nofterminal*terminal.shape[1]))()
mydll.generate_codebook.argtypes = [POINTER(c_int), POINTER(c_int), POINTER(c_int), POINTER(c_int),
                                  POINTER(c_int), POINTER(c_int), POINTER(c_int), POINTER(c_int), POINTER(c_float)]
result=mydll.generate_codebook(nodestatus.ctypes.data_as(ctypes.POINTER(ctypes.c_int)),
                               nofnode,noftree,terminal.ctypes.data_as(ctypes.POINTER(ctypes.c_int)),
                               c_int(nofterminal),
                               nofobservations.ctypes.data_as(ctypes.POINTER(ctypes.c_int)),total,
                               nofseries,
                              ctypes.byref(nofentry))

While calling generate_codebook function I am facing argument error in last argument where LP_c_float instance is expected. Below is the error:

<ipython-input-28-f73a7383211e> in generatecodebook(nodestatus, terminal, nofterminal, nofobservations)
 16                                    nofobservations.ctypes.data_as(ctypes.POINTER(ctypes.c_int)),total,
 17                                    nofseries,
---> 18                                   ctypes.byref(nofentry))

ArgumentError: argument 9: <class 'TypeError'>: expected LP_c_float instance instead of pointer to LP_c_float_Array_50000

I went through this question's solution but not able to resolve error. Thank you in advance!

CristiFati · Accepted Answer · 2023-01-05T14:39:00.807

Your nofentry value is a pointer to an array of floats, while generate_codebook expects a pointer to float.
CTypes can't do such a conversion automatically, so it has to be performed manually (using [Python.Docs]: ctypes.cast(obj, type)).

Example:

>>> import ctypes
>>>
>>> dim = 100
>>>
>>> FloatArr100 = ctypes.c_float * dim
>>> FloatArr100Ptr = ctypes.POINTER(FloatArr100)
>>>
>>> float_arr = FloatArr100(*range(dim))
>>> float_arr[4], float_arr[38], float_arr[99]
(4.0, 38.0, 99.0)
>>>
>>> float_arr_ptr = ctypes.pointer(float_arr)  # This is the equivalent of your `nofentry`
>>> float_arr_ptr
<__main__.LP_c_float_Array_100 object at 0x000001921ED85A48>
>>> type(float_arr_ptr) is FloatArr100Ptr
True
>>>
>>> float_ptr = ctypes.cast(float_arr, ctypes.POINTER(ctypes.c_float))  # This is what you should do
>>>
>>> float_ptr
<__main__.LP_c_float object at 0x000001921ED859C8>
>>> float_ptr[4], float_ptr[38], float_ptr[99]
(4.0, 38.0, 99.0)

Translated to your code:

Change nofentry definition to:

nofentry = (ctypes.c_float * (len(nofobservations) * nofterminal * terminal.shape[1]))()  # Notice dropping `ctypes.POINTER`

When invoking mydll.generate_codebook, replace

ctypes.byref(nofentry)

with

ctypes.cast(nofentry, ctypes.POINTER(ctypes.c_float))

so at the end it will look like:

result = mydll.generate_codebook(
    nodestatus.ctypes.data_as(ctypes.POINTER(ctypes.c_int)),
    nofnode, noftree, terminal.ctypes.data_as(ctypes.POINTER(ctypes.c_int)),
    c_int(nofterminal),
    nofobservations.ctypes.data_as(ctypes.POINTER(ctypes.c_int)),
    total, nofseries,
    ctypes.cast(nofentry, ctypes.POINTER(ctypes.c_float)))

Also mentioning [SO]: C function called from Python via ctypes returns incorrect value (@CristiFati's answer).

I tried above commands and got ArgumentError: argument 9: : expected LP_c_float instance instead of pointer to LP_c_float — Hena Ghonia, Mar 19 '19 at 11:30
Hmm, that means that either there's smth wrong in the question, or you didn't do exactly what I've pointed out: `ctypes.cast(nofentry, ctypes.POINTER(ctypes.c_float))` returns a *LP\_c\_float instance*. — CristiFati, Mar 19 '19 at 11:42
Sorry, I made mistake while casting but now I get OSError: exception: access violation reading 0x0000020C7B9F2248 — Hena Ghonia, Mar 19 '19 at 11:59
Unfortunately that info will be only be available in *generate\_codebook* implementation (or docs). The function expects some pointers (that are also arrays) and most likely is trying to access memory outside of he bounds. — CristiFati, Mar 19 '19 at 14:03

ArgumentError:expected LP_c_float instance instead of pointer to LP_c_float_Array_50000

1 Answers1