How do I call a windows dll function from cygwin python?

Question

I have a Windows python3.7 function which successfully calls the kernel32.dll GetSystemPowerStatus function using ctypes to interrogate the power status to see if my laptop is on AC or battery power. This is a pure python solution.

I want to port this function to cygwin python3.7. Out of the box, python3 for cygwin's ctypes does not seem to allow calling a windows dll. I would prefer a pure python solution, but I can use C/C++ if necessary. Does anyone have an example of how to do this?

Edited to add the code (lines 63-67) and error messages:

elif _os.name == 'posix' and _sys.platform == 'cygwin':
    # c:\Windows\System32\kernel32.dll
    kernel32_name = '/proc/cygdrive/c/Windows/System32/kernel32.dll'
    kernel32 = CDLL(kernel32_name)
    _GetSystemPowerStatus = kernel32.GetSystemPowerStatus


$ python3.7 GetSystemPowerStatus.py
Traceback (most recent call last):
  File "GetSystemPowerStatus.py", line 82, in <module>
    result, systemPowerStatus = GetSystemPowerStatus()
  File "GetSystemPowerStatus.py", line 66, in GetSystemPowerStatus
    kernel32 = CDLL(kernel32_name)
  File "/usr/lib/python3.7/ctypes/__init__.py", line 356, in __init__
    self._handle = _dlopen(self._name, mode)
OSError: Invalid argument

python2.7 gives the same error, but at line 366.

Solved. See my own answer below.

Please give the error message that you get when you run your working windows code inside cygwin — Eric, Mar 03 '19 at 06:36
'$ python3.7 GetSystemPowerStatus.py⏎ Traceback (most recent call last):⏎ File "GetSystemPowerStatus.py", line 69, in ⏎ result, systemPowerStatus = GetSystemPowerStatus()⏎ File "GetSystemPowerStatus.py", line 57, in GetSystemPowerStatus⏎ _GetSystemPowerStatus = cdll.kernel32.GetSystemPowerStatus⏎ File "/usr/lib/python3.7/ctypes/__init__.py", line 426, in __getattr__⏎ dll = self._dlltype(name)⏎ File "/usr/lib/python3.7/ctypes/__init__.py", line 356, in __init__⏎ self._handle = _dlopen(self._name, mode)⏎ OSError: No such file or directory⏎' — Doug Henderson, Mar 03 '19 at 07:37
Edit the question with additional, formatted information. Comments obviously are a mess to read. — Mark Tolonen, Mar 03 '19 at 18:17
Quite puzzling, actually this works for me: `ctypes.CDLL("/cygdrive/c/windows/system32/user32.dll")` but not with `kernel32.dll`. I get exactly the same error. So it's possible to load a windows library from cygwin, but **not** kernel32 :( — Neitsa, Mar 04 '19 at 15:26
Thanks. Very interesting. Must dig in source of ctypes, I guess. — Doug Henderson, Mar 04 '19 at 15:29

score 0 · Answer 1 · answered Mar 04 '19 at 16:06

As you I wasn't able to get kernel32.dll (although it works with other DLLs like user32, msvcrt, kernelbase, etc.)

I found a pretty contrived way of doing it... This uses kernelbase.dll (which exports GetModuleHandle) to get an handle on kernel32.dll, then call CDLL with the handle optional keyword:

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
import sys
import ctypes

def main():
    # the idea is to load kernelbase.dll which will allow us to call GetModuleHandleW() on kernel32.dll
    try:
        kernel_base = ctypes.CDLL("/cygdrive/c/windows/system32/kernelbase.dll")
    except OSError:
        print("Can't load kernelbase.dll...")
        return -1

    gmhw = kernel_base.GetModuleHandleW
    gmhw.argtypes = (ctypes.c_wchar_p, )
    gmhw.restype = ctypes.c_void_p

    # call GetModuleHandleW on kernel32.dll (which is loaded by default in the Python process)
    kernel32_base_addr = gmhw("kernel32.dll")
    print(f"Got kernel32 base address: {kernel32_base_addr:#x}")

    # now call CDLL with optional arguments
    kernel32 = ctypes.CDLL("/cygdrive/c/windows/system32/kernel32.dll", handle=kernel32_base_addr, use_last_error=True)

    # test with GetSystemPowerStatus
    print(f"GetSystemPowerStatus: {kernel32.GetSystemPowerStatus}")

    return 0

if __name__ == "__main__":
    sys.exit(main())

Thanks. It's a bit convoluted. I found a better way. – Doug Henderson Mar 04 '19 at 19:51 — Doug Henderson, Mar 04 '19 at 19:51

score -1 · Answer 2 · answered Mar 04 '19 at 20:06

After finding that I could load user32.dll easily, I did some more investigation with the source and pdb and ldd. What follows is my solution.

elif _os.name == 'posix' and _sys.platform == 'cygwin':
    RTLD_LOCAL = 0  # from /usr/include/dlfcn.h
    RTLD_LAZY = 1
    RTLD_NOW = 2
    RTLD_GLOBAL = 4
    RTLD_NODELETE = 8
    RTLD_NOLOAD = 16
    RTLD_DEEPBIND = 32
    kernel32_name = '/proc/cygdrive/c/Windows/System32/kernel32.dll'
    kernel32 = CDLL(kernel32_name, mode=RTLD_LAZY | RTLD_NOLOAD)
    _GetSystemPowerStatus = kernel32.GetSystemPowerStatus

The common code to call the function is:

_GetSystemPowerStatus.restype = c_bool
_GetSystemPowerStatus.argtypes = [c_void_p,]

_systemPowerStatus = _SystemPowerStatus()  # not shown, see ctypes module doc

result = _GetSystemPowerStatus(byref(_systemPowerStatus))

return result, SystemPowerStatus(_systemPowerStatus)

This works fine with Python 2.7, 3.6 and 3.6 on

$ uname -a
CYGWIN_NT-10.0 host 3.0.1(0.338/5/3) 2019-02-20 10:19 x86_64 Cygwin

Thanks to all for their help.

Also check: https://stackoverflow.com/questions/53781370/is-there-an-alternative-way-to-import-ctypes-wintypes-in-cygwin. — CristiFati, Mar 09 '19 at 03:08

How do I call a windows dll function from cygwin python?

2 Answers2