I am working on a project where I have to build a GUI for a development board with python with which I am new to as well. I am given the DLL which has required functions to communicate with the development board. I have LabVIEW equivalent function prototype which looks something like this:
int32_t WriteFPGARegsC(int16_t *USBdev, int32_t *DUTSelect, int32_t *Array_RegsIn, int32_t *Array_RegsOut, int32_t *Array_RegEnable);
And the same prototype also looks like this for Visual Basic:
Public Declare Function WriteFPGARegsC Lib "USB_IO_for_VB6.dll" (ByRef USBdev As Short, ByVal DUTSelect As Integer, ByRef Array_RegsIn As Integer, ByRef Array_RegsOut As Integer, ByRef Array_RegEnable As Integer) As Integer
I am trying to access this function with python instead of LabVIEW because of a lot of issues.
Last three parameters to be passed to the function needs to be an address to an array of 255 elements.
I have no clue how to pass pointers in function in python!!
I wrote the following shortcode to access this function in python:
USBdev = [0]
DUTSelect = [0]
RegsIn = [0] * 255
RegsOut = [0] * 255
RegEnable = [0] * 255
from ctypes import*
mydll = cdll.LoadLibrary("USB_IO_for_VB6.dll")
retmydll = mydll.WriteFPGARegsC(USBdev, DUTSelect, RegsIn, RegsOut, RegEnable)
After executing this code I get following error message:
Traceback (most recent call last):
File "D:\Kushal\ATLASS\Source_code\Atlass.py", line 12, in <module>
retmydll = mydll.WriteFPGARegsC(id(USBdev), id(DUTSelect), id(RegsIn), id(RegsOut), id(RegEnable))
ValueError: Procedure called with not enough arguments (20 bytes missing) or wrong calling convention
Any help will be appreciated!! Thanks a lot!
Listing [Python.Docs]: ctypes - A foreign function library for Python.
Before everything, check [SO]: C function called from Python via ctypes returns incorrect value (@CristiFati's answer) for a common pitfall when working with CTypes (calling functions).
Anyway, I prepared a dummy example (don't mind the error proneness), it's just for illustrating the mechanism.
dll00.c:
#if defined(_WIN32)
# define DLL00_EXPORT_API __declspec(dllexport)
#else
# define DLL00_EXPORT_API
#endif
#include <stdint.h>
#include <stdio.h>
#define DIM 10 // Must be the SAME (well, or lower) than the constant defined in the Python code
DLL00_EXPORT_API int32_t __stdcall WriteFPGARegsC(int16_t *USBdev, int32_t *DUTSelect, int32_t *ArrayRegsIn, int32_t *ArrayRegsOut, int32_t *ArrayRegEnable)
{
int32_t ret = 99;
printf("From C:\n\t*USBdev: %d\n\t*DUTSelect: %d\n\tArray_RegsIn[0]: %d\n\tArray_RegsOut[0]: %d\n\tArray_RegEnable[0]: %d\n\n\tReturning: %d\n",
*USBdev, *DUTSelect, ArrayRegsIn[0], ArrayRegsOut[0], ArrayRegEnable[0], ret);
for (int i = 0; i < DIM; i++) {
ArrayRegsOut[i] *= *DUTSelect;
}
return ret;
}
code00.py:
#!/usr/bin/env python
import ctypes as cts
import sys
DIM = 10 # Should be 255, but for demo purposes keep it smaller
UIntArr = cts.c_uint * DIM
UShortArr = cts.c_ushort * DIM
UIntPtr = cts.POINTER(cts.c_uint)
UShortPtr = cts.POINTER(cts.c_ushort)
def main(*argv):
dll = cts.WinDLL("./USB_IO_for_VB6.dll")
dll_func = dll.WriteFPGARegsC
dll_func.argtypes = (UShortPtr, UIntPtr, UIntPtr, UIntPtr, UIntPtr)
dll_func.restype = cts.c_uint
usb_dev = cts.c_ushort(25)
dut_select = cts.c_uint(2)
regs_in = UIntArr(*range(20, 20 + DIM))
regs_out = UIntArr(*range(30, 30 + DIM))
reg_enable = UIntArr(*range(40, 40 + DIM))
nth = 5
print("Output register array: {:s}".format(" ".join(["{:d}".format(item) for item in regs_out])))
print("\tIts {:d}th element: {:d}\n".format(nth, regs_out[nth]))
ret = dll_func(cts.byref(usb_dev), cts.byref(dut_select), regs_in, regs_out, reg_enable)
print("\nFunction returned: {:d}".format(ret))
print("Output register array: {:s}".format(" ".join(["{:d}".format(item) for item in regs_out])))
print("\tIts {:d}th element: {:d}\n".format(nth, regs_out[nth]))
if __name__ == "__main__":
print(
"Python {:s} {:03d}bit on {:s}\n".format(
" ".join(elem.strip() for elem in sys.version.split("\n")),
64 if sys.maxsize > 0x100000000 else 32,
sys.platform,
)
)
rc = main(*sys.argv[1:])
print("\nDone.\n")
sys.exit(rc)
Output:
(py35x64_test) e:\Work\Dev\StackOverflow\q051289410> "c:\Install\x86\Microsoft\Visual Studio Community\2015\vc\vcvarsall.bat" amd64 (py35x64_test) e:\Work\Dev\StackOverflow\q051289410> dir /b dll00.c code00.py (py35x64_test) e:\Work\Dev\StackOverflow\q051289410> cl /nologo dll00.c /link /DLL /OUT:USB_IO_for_VB6.dll dll00.c Creating library USB_IO_for_VB6.lib and object USB_IO_for_VB6.exp (py35x64_test) e:\Work\Dev\StackOverflow\q051289410> dir /b dll00.c dll00.obj code00.py USB_IO_for_VB6.dll USB_IO_for_VB6.exp USB_IO_for_VB6.lib (py35x64_test) e:\Work\Dev\StackOverflow\q051289410> "e:\Work\Dev\VEnvs\py35x64_test\Scripts\python.exe" ./code00.py Python 3.5.4 (v3.5.4:3f56838, Aug 8 2017, 02:17:05) [MSC v.1900 64 bit (AMD64)] 064BIT on win32 Output register array: 30 31 32 33 34 35 36 37 38 39 Its 5th element: 35 From C: *USBdev: 25 *DUTSelect: 2 Array_RegsIn[0]: 20 Array_RegsOut[0]: 30 Array_RegEnable[0]: 40 Returning: 99 Function returned: 99 Output register array: 60 62 64 66 68 70 72 74 76 78 Its 5th element: 70
Modified answer to be as close as possible to the real scenario, and also to address questions in the comments