python ctypes issue on different OSes

Question

I'm trying to convert C function for python 3.6 use.

code as below:

lib = ctypes.WinDLL('ftrScanAPI.dll') # provided by fingerprint scanner
class FTRSCAN_IMAGE_SIZE(ctypes.Structure):
    _fields_ = [
    ("nWidth", ctypes.c_int),
    ("nHeight", ctypes.c_int),
    ("nImageSize", ctypes.c_int)
]

print('Open device and get device handle...')
hDevice = lib.ftrScanOpenDevice()
print('handle is', hDevice)
print('Get image size...')
Image_size = FTRSCAN_IMAGE_SIZE(0, 0, 0)
if lib.ftrScanGetImageSize(hDevice, ctypes.byref(Image_size)):
    print('Get image size succeed...')
    print('  W', Image_size.nWidth)
    print('  H', Image_size.nHeight)
    print('  Size', Image_size.nImageSize)
else:
    print('Get image size failed...')

function definition:

typedef struct FTR_PACKED __FTRSCAN_IMAGE_SIZE {
    int nWidth;
    int nHeight;
    int nImageSize;
} FTRSCAN_IMAGE_SIZE, *PFTRSCAN_IMAGE_SIZE;
FTRHANDLE ftrScanOpenDevice();  # typedef void *  FTRHANDLE;
BOOL ftrScanGetImageSize(FTRHANDLE ftrHandle, 
    PFTR_SCAN_IMAGE_SIZE pImageSize);

But different OSes with the same code seems to have different result:

On Windows 7 64 bit

On Windows 10 64 bit
I don't print "handle is here"

What I've tried:
According to some answers on stack overflow, this may be caused by not assigning function argtypes and restype explicitly, so I tried and failed.

Answer 1

In 99% of the cases, inconsistencies between arguments (and / or return) type inconsistencies are the cause (check [SO]: C function called from Python via ctypes returns incorrect value (@CristiFati's answer) for more details).

Always have [Python.Docs]: ctypes - A foreign function library for Python open when working with CTypes.

I found [GitHub]: erikssm/futronics-fingerprint-reader - (master) futronics-fingerprint-reader/ftrScanAPI.h (I don't know how different it's from what you currently have, but things you posted so far seem to match), and I did some changes to your code:

• Define argtypes and restype for functions

• Define missing types (for clarity only)

• Some other insignificant changes (renames)

• One other thing that I noticed in the above file, is a #pragma pack(push, 1) macro (check [MS.Docs]: pack for more details). For this structure it makes no difference (thanks @AnttiHaapala for the hint), as the 3 int (4 byte) members alignment doesn't change, but for other structures (with "smaller" member types (e.g. char, short)) you might want to add: _pack_ = 1

Your modified code (needless to say, I didn't run it as I don't have the .dll):

#!/usr/bin/env python

import ctypes as ct
from ctypes import wintypes as wt


# ...


class FTRSCAN_IMAGE_SIZE(ct.Structure):
    # _pack_ = 1
    _fields_ = (
        ("nWidth", ct.c_int),
        ("nHeight", ct.c_int),
        ("nImageSize", ct.c_int),
    )

PFTRSCAN_IMAGE_SIZE = ct.POINTER(FTRSCAN_IMAGE_SIZE)
FTRHANDLE = ct.c_void_p

lib = ct.WinDLL("ftrScanAPI.dll")  # provided by fingerprint scanner

ftrScanOpenDevice = lib.ftrScanOpenDevice
ftrScanOpenDevice.argtypes = ()
ftrScanOpenDevice.restype = FTRHANDLE

ftrScanGetImageSize = lib.ftrScanGetImageSize
ftrScanGetImageSize.argtypes = (FTRHANDLE, PFTRSCAN_IMAGE_SIZE)
ftrScanGetImageSize.restype = wt.BOOL

print("Open device and get device handle...")
h_device = ftrScanOpenDevice()
print("Handle is", h_device)
print("Get image size...")
image_size = FTRSCAN_IMAGE_SIZE(0, 0, 0)

if ftrScanGetImageSize(h_device, ct.byref(image_size)):
    print("Get image size succeed...")
    print("  W", image_size.nWidth)
    print("  H", image_size.nHeight)
    print("  Size", image_size.nImageSize)
else:
    print("Get image size failed...")