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I'm using python and of course you can't loop through every pixel of a large image very quickly, so I defer to a C DLL.

I want to do something like this:

img = QImage("myimage.png").constBits()
imgPtr = c_void_p(img)
found = ctypesDLL.myImageSearchMethod(imgPtr, width, height)
But this line
    imgPtr = c_void_p(img)
yelds 
  
builtins.TypeError: cannot be converted to pointer
I don't need to modify the bits.  Please teach me your Jedi ways in this area.
So you should be able to build a c_void_p passing the return value of sip.voidptr.__int__() method to its constructor:
imgPtr = c_void_p(img.__int__())
I tested this solution this way:
from PyQt5 import QtGui
from ctypes import *
lib = CDLL("/usr/lib/libtestlib.so")
image = QtGui.QImage("so.png")
bits = image.constBits()
bytes = image.bytesPerLine()
lib.f(c_void_p(bits.__int__()), c_int(image.width()), c_int(image.height()), c_int(bytes))
Which works fine with a function like:
#include <cstdio>
#include <QImage>
extern "C" {
    void f(unsigned char * c, int width, int height, int bpl)
        printf("W:%d H:%d BPL:%d\n", width, height, bpl);
        QImage image(c, width, height, bpl, QImage::Format_RGB32);
        image.save("test.bmp");
        
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