How can I unload a DLL using ctypes in Python?

Question

I\'m using ctypes to load a DLL in Python. This works great.

Now we\'d like to be able to reload that DLL at runtime.

The straightforward approach would s

Answer 1

windows and linux compatible minimal reproducible example from 2020

overview of similar discussion

Here an overview of similar discussions (where I constructed this answer from).

• How can I unload a DLL using ctypes in Python?
• ctypes unload dll
• Unload shared library inside ctypes loaded shared library
• forcing ctypes.cdll.LoadLibrary() to reload library from file

minimal reproducible example

This is for windows and linux, hence there are 2 scripts given for compilation. Tested under:

• Win 8.1, Python 3.8.3 (anaconda), ctypes 1.1.0, mingw-w64 x86_64-8.1.0-posix-seh-rt_v6-rev0
• Linux Fedora 32, Python 3.7.6 (anaconda), ctypes 1.1.0, g++ 10.2.1

cpp_code.cpp

extern "C" int my_fct(int n)
{
    int factor = 10;
    return factor * n;
}

compile-linux.sh

#!/bin/bash
g++ cpp_code.cpp -shared -o myso.so

compile-windows.cmd

set gpp="C:\Program Files\mingw-w64\x86_64-8.1.0-posix-seh-rt_v6-rev0\mingw64\bin\g++.exe"
%gpp% cpp_code.cpp -shared -o mydll.dll
PAUSE

Python code

from sys import platform
import ctypes


if platform == "linux" or platform == "linux2":
    # https://stackoverflow.com/a/50986803/7128154
    # https://stackoverflow.com/a/52223168/7128154

    dlclose_func = ctypes.cdll.LoadLibrary('').dlclose
    dlclose_func.argtypes = [ctypes.c_void_p]

    fn_lib = './myso.so'
    ctypes_lib = ctypes.cdll.LoadLibrary(fn_lib)
    handle = ctypes_lib._handle

    valIn = 42
    valOut = ctypes_lib.my_fct(valIn)
    print(valIn, valOut)

    del ctypes_lib
    dlclose_func(handle)

elif platform == "win32": # Windows
    # https://stackoverflow.com/a/13129176/7128154
    # https://stackoverflow.com/questions/359498/how-can-i-unload-a-dll-using-ctypes-in-python

    lib = ctypes.WinDLL('./mydll.dll')
    libHandle = lib._handle

    # do stuff with lib in the usual way
    valIn = 42
    valOut = lib.my_fct(valIn)
    print(valIn, valOut)

    del lib
    ctypes.windll.kernel32.FreeLibrary(libHandle)

A more general solution (object oriented for shared libraries with dependencies)

If a shared library has dependencies, this does not necessarily work anymore (but it can - depends on the dependency ^^). I did not investigate the very details, but it looks like the mechanism is the following: library and dependency are loaded. As the dependency is not unloaded, the library can not get unloaded.

I found, that if I include OpenCv (Version 4.2) into my shared library, this messes up the unloading procedure. The following example was only tested on the linux system:

code.cpp

#include 
#include  


extern "C" int my_fct(int n)
{
    cv::Mat1b mat = cv::Mat1b(10,8,(unsigned char) 1 );  // change 1 to test unloading
    
    return mat(0,1) * n;
}

Compile with g++ code.cpp -shared -fPIC -Wall -std=c++17 -I/usr/include/opencv4 -lopencv_core -o so_opencv.so

Python code

from sys import platform
import ctypes


class CtypesLib:

    def __init__(self, fp_lib, dependencies=[]):
        self._dependencies = [CtypesLib(fp_dep) for fp_dep in dependencies]

        if platform == "linux" or platform == "linux2":  # Linux
            self._dlclose_func = ctypes.cdll.LoadLibrary('').dlclose
            self._dlclose_func.argtypes = [ctypes.c_void_p]
            self._ctypes_lib = ctypes.cdll.LoadLibrary(fp_lib)
        elif platform == "win32":  # Windows
            self._ctypes_lib = ctypes.WinDLL(fp_lib)

        self._handle = self._ctypes_lib._handle

    def __getattr__(self, attr):
        return self._ctypes_lib.__getattr__(attr)

    def __del__(self):
        for dep in self._dependencies:
            del dep

        del self._ctypes_lib

        if platform == "linux" or platform == "linux2":  # Linux
            self._dlclose_func(self._handle)
        elif platform == "win32":  # Windows
            ctypes.windll.kernel32.FreeLibrary(self._handle)


fp_lib = './so_opencv.so'

ctypes_lib = CtypesLib(fp_lib, ['/usr/lib64/libopencv_core.so'])

valIn = 1
ctypes_lib.my_fct.argtypes = [ctypes.c_int]
ctypes_lib.my_fct.restype = ctypes.c_int
valOut = ctypes_lib.my_fct(valIn)
print(valIn, valOut)

del ctypes_lib

Let me know, when there are any issues with the code examples or the explanation given so far. Also if you know a better way! It would be great, if we could settle the issue once and for all.