From 141204e632c9df3cb538b1f8f68802cc8f32a25b Mon Sep 17 00:00:00 2001
From: alteralto <182909819+alteralto@users.noreply.github.com>
Date: Mon, 26 Oct 2026 03:25:29 +0300
Subject: [PATCH] ctypesgen: Leave c_void_p alone in the generated POINTER
 wrapper

ctypes.POINTER(None) returns ctypes.c_void_p itself, so the wrapper in the
generated preamble replaced from_param on the c_void_p type the whole
process shares. The replacement returns integers unchanged, and ctypes then
passes them as a C long. On Windows that is 32 bits, so any void pointer
argument declared after grass.lib is imported fails for a 64-bit address,
for example LoadStringW in dateutil, which breaks matplotlib date axes in
g.gui.timeline and g.gui.tplot.

c_void_p's own from_param already turns None into NULL, so the wrapper now
returns it unchanged.

Fixes #2109
---
 .../tests/lib_gis_ctypes_void_pointer_test.py | 30 +++++++++++++++++++
 .../ctypesgen/printer_python/printer.py       |  5 ++++
 2 files changed, 35 insertions(+)
 create mode 100644 lib/gis/tests/lib_gis_ctypes_void_pointer_test.py

diff --git a/lib/gis/tests/lib_gis_ctypes_void_pointer_test.py b/lib/gis/tests/lib_gis_ctypes_void_pointer_test.py
new file mode 100644
index 00000000000..ed9afd9dde4
--- /dev/null
+++ b/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"
diff --git a/python/libgrass_interface_generator/ctypesgen/printer_python/printer.py b/python/libgrass_interface_generator/ctypesgen/printer_python/printer.py
index 8fd4327e650..601ef66d3b0 100755
--- a/python/libgrass_interface_generator/ctypesgen/printer_python/printer.py
+++ b/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):