ctypesgen: Leave c_void_p alone in the generated POINTER wrapper

lib/gis/tests/lib_gis_ctypes_void_pointer_test.py

@@ -0,0 +1,30 @@
+"""Tests of the ctypes preamble shared by the grass.lib modules
+
+POINTER(None) in the generated modules is ctypes.c_void_p itself, so the
+preamble must not replace its from_param: that would change every void pointer
+argument in the process, including those of other packages.
+"""
+
+import ctypes
+import sys
+
+import pytest
+
+import grass.lib.gis  # noqa: F401  # the import is what is being tested
+
+
+def test_void_pointer_converts_integers():
+    """After importing grass.lib, c_void_p still converts an integer address"""
+    assert not isinstance(ctypes.c_void_p.from_param(2**40), int)
+
+
+@pytest.mark.skipif(sys.platform != "win32", reason="C long is 32 bits on Windows")
+def test_void_pointer_argument_takes_a_64_bit_address():
+    """A void pointer argument declared after the import takes any address"""
+    memmove = ctypes.CDLL("msvcrt").memmove
+    memmove.argtypes = (ctypes.c_void_p, ctypes.c_void_p, ctypes.c_size_t)
+    memmove.restype = ctypes.c_void_p
+    source = ctypes.create_string_buffer(b"grass")
+    target = ctypes.create_string_buffer(len(source))
+    memmove(ctypes.addressof(target), ctypes.addressof(source), len(source))
+    assert target.value == b"grass"

python/libgrass_interface_generator/ctypesgen/printer_python/printer.py

@@ -166,6 +166,11 @@ def _copy_preamble_loader_files(self, path):
         pointer = """def POINTER(obj):
     p = ctypes.POINTER(obj)
 
+    # POINTER(None) is ctypes.c_void_p itself, which the whole process shares,
+    # and its own from_param already turns None into NULL
+    if p is ctypes.c_void_p:
+        return p
+
     # Convert None to a real NULL pointer to work around bugs
     # in how ctypes handles None on 64-bit platforms
     if not isinstance(p.from_param, classmethod):