Wrong ctypes assignation

Question

I made a CPP DLL and I'm trying to call function inside it from python. I've achieved this multiple times for other functions, but this one, I just can't find my mistake.

dll_name = "..\\src\\x64\\Debug\\2019-3A-IBD-MLDLL.dll"
dllabspath = os.path.dirname(os.path.abspath(__file__)) + os.path.sep + dll_name
myDll = CDLL(dllabspath)

#fit_reg_RBF_naive
myDll.fit_reg_RBF_naive.argtypes = [ct.c_void_p, ct.c_double,  ct.c_void_p, ct.c_int, ct.c_int]
myDll.fit_reg_RBF_naive.restypes = ct.c_void_p

#predict_reg_RBF_naive
myDll.predict_reg_RBF_naive.argtypes = [ct.c_void_p, ct.c_void_p, ct.c_void_p, ct.c_int, ct.c_double, ct.c_int]
myDll.predict_reg_RBF_naive.restypes = ct.c_double

def fit_reg_RBF_naive(pyXTrain, pyGamma, pyYTrain, pySampleCount, pyInputCountPerSample):
    XTrain = (ct.c_double * len(pyXTrain))(*pyXTrain)
    YTrain = (ct.c_double * len(pyYTrain))(*pyYTrain)
    inputCountPerSample = ct.c_int(pyInputCountPerSample)
    sampleCount = ct.c_int(pySampleCount)
    gamma = ct.c_double(pyGamma)
    return myDll.fit_reg_RBF_naive(XTrain, gamma, YTrain, sampleCount, inputCountPerSample)

def predict_reg_RBF_naive(pyW, pyXTrain, pyXpredict ,pyInputCountPerSample, pyGamma, pySampleCount):
    XTrain = (ct.c_double * len(pyXTrain))(*pyXTrain)
    inputCountPerSample = ct.c_int(pyInputCountPerSample)
    sampleCount = ct.c_int(pySampleCount)
    gamma = ct.c_double(pyGamma)
    Xpredict = (ct.c_double * len(pyXpredict))(*pyXpredict)
    return myDll.predict_reg_RBF_naive(W, XTrain, Xpredict, inputCountPerSample, gamma, sampleCount)

Basically I load my DLL, set Ctypes for arguments and result for both of my fonctions. Then I make a python wrapper so that the user does not have to retype every cast from python to cpp.

My types on the cpp side seems good too:

extern "C" {

    SUPEREXPORT double predict_reg_RBF_naive(double* W, double* X, double* Xpredict, int inputCountPerSample, double gamma, int N);
    SUPEREXPORT double* fit_reg_RBF_naive(double* XTrain, double gamma, double* YTrain, int sampleCount, int inputCountPerSample);
}

I have no warning from the compiler for the cpp part, I've printed the memory adresse before the return inside fit_reg_RBF_naive from cpp and the W in python and they are the same.

000002B358384980 // cpp address of W before return
0x58384980       # Python address of W after function call

For me it seems the same address. Maybe I'm wrong.

So when I try to call my second cpp function it said

myDll.predict_reg_RBF_naive(W, XTrain, Xpredict,inputCountPerSample, gamma, sampleCount) OSError: exception: access violation reading 0x000000007C7380A0

It crashed in the cpp when it tries to read W. They are no free or 'delete' in the cpp and the variable is properly allocated : double* W = new double[2];

Also, when I print W type in python I get <class 'int'>.

How comes my W seems to have the same address regarding the language, but has not the good type? Changing the result type of fit_reg_RBF_naive to POINTER(ct.c_double * 2) makes no change.

EDIT:

Here is how I call my functions:

from dll_load import predict_reg_RBF_naive, fit_reg_RBF_naive

gamma = 50
sampleCount = 2
inputCountPerSample = 3
XTrain = [1.0, 1.0, 1.0, 3.0, 3.0, 3.0]
YTrain = [-1.0, 1.0]
Xpredict = [1.0, 1.0, 1.0]

W = fit_reg_RBF_naive(XTrain, gamma, YTrain, sampleCount, inputCountPerSample)

print(predict_reg_RBF_naive(W, XTrain, Xpredict, inputCountPerSample, gamma, sampleCount))

000002B358384980 and 0x58384980 are not the same at all, the first starts with 2B and then has eight more digits and the second has only the final eight digits. This is symptomatic of 32 vs 64 bit errors, though where those errors are occurring I'm not sure off hand. — torek
– torek, Commented Jun 10, 2019 at 21:41
I agree with you on this. But I don't understand why this phenomenon exist while all my other functions seems to be coded the same way... I'll take a loot on your idea. — BeGreen
– BeGreen, Commented Jun 10, 2019 at 21:45
Also, that <class 'int'> thing seems suspicious. You may want POINTER applied, it's been a while since I fussed with ctypes. — torek
– torek, Commented Jun 10, 2019 at 21:47
I agree with you. W should be ct.c_void_p regarding myDll.fit_reg_RBF_naive.restypes = ct.c_void_p?. Edit: I'll add my call file in python — BeGreen
– BeGreen, Commented Jun 10, 2019 at 21:48
@torek from an other function that works from my DLL/python I get the same type <class 'int'> for W — BeGreen
– BeGreen, Commented Jun 10, 2019 at 21:55

CristiFati · Accepted Answer · 2019-06-11 07:02:29Z

1

[Python 3.Docs]: ctypes - A foreign function library for Python.

You misspelled restypes (it should be restype). By doing so, restype is not initialized, and defaults to int (this wouldn't be a problem on 32bit), and you ran into:

[SO]: Python ctypes cdll.LoadLibrary, instantiate an object, execute its method, private variable address truncated (@CristiFati's answer)
[SO]: python ctypes issue on different OSes (@CristiFati's answer)

Besides that, there are several problems in the code:

If the C function specifies a pointer (double* in this case), don't use ctypes.c_void_p (in argtypes or restype) to map it, as it might be too wide, use (for this case) ctypes.POINTER(ctypes.c_double) instead
For me this doesn't even compile (I wonder how were you able to run that code). I'm going to exemplify on XTrain only, but applies to YTrain and Xpredict as well. ctypes doesn't know to convert a Python list to a ctypes.POINTER(ctypes.c_double) (or ctypes.c_void_p), and the conversion must be made manually (to a ctypes.c_double array):
```
XTrain = [1.0, 1.0, 1.0, 3.0, 3.0, 3.0]
xtrain_ctypes = (ctypes.c_double * len(XTrain))(*XTrain)
```
and pass xtrain_ctypes to the functions.

edited Jun 11, 2019 at 7:02

answered Jun 11, 2019 at 6:18

CristiFati

41.5k9 gold badges68 silver badges114 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

BeGreen Over a year ago

I knew it... only a typo... Thank you sir. For the output type, I don't really need to be specific on the type. Some times I pass complex structure from my DLL tp python to DLL again. How comes python does not give me an error or warning on this type of errors? Usually it says something like 'ImportError: No module named restypes' ?

BeGreen Over a year ago

I do make the conversions for all my python variables to ctypes. I make the conversion for lists already. My code looks like a mess because i use the same names in python and cpp. I agree it is confusing.

CristiFati Over a year ago

ImportError is a totally different issue. Python lets you add members to an instance on the fly (that's why you don't get an error). I didn;t say the code is confusing. Apparently there are some missing parts :).

BeGreen Over a year ago

Ok thank you for your comment, i'm not used to python, I'm more of a C programmer. Indeed code is not full and a bit heavy to read.

Collectives™ on Stack Overflow

Wrong ctypes assignation

1 Answer 1

4 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related