gh-61103: use PEP 3118 codes in the ctypes

This change only buffer protocol support for complex types,
`_type_` properties aren't affected.

Author: skirpichev

Lib/test/test_ctypes/test_pep3118.py

@@ -1,3 +1,4 @@
+import ctypes
 import re
 import sys
 import unittest
@@ -231,6 +232,14 @@ class Complete(Structure):
 
     ]
 
+if hasattr(ctypes, 'c_float_complex'):
+    c_float_complex = ctypes.c_float_complex
+    c_double_complex = ctypes.c_double_complex
+    c_longdouble_complex = ctypes.c_longdouble_complex
+    native_types.extend([(c_float_complex * 4,      "<Zf", (4,), c_float_complex),
+                         (c_double_complex * 4,     "<Zd", (4,), c_double_complex),
+                         (c_longdouble_complex * 4, "<Zg", (4,), c_longdouble_complex)])
+
 
 class BEPoint(BigEndianStructure):
     _fields_ = [("x", c_long), ("y", c_long)]

Misc/NEWS.d/next/Library/2026-05-04-05-08-17.gh-issue-61103.mONh4V.rst

@@ -0,0 +1,2 @@
+Use :pep:`3118` type codes for the buffer protocol support of complex types
+in the :mod:`ctypes`.  Patch by Sergey B Kirpichev.

Modules/_ctypes/_ctypes.c

@@ -311,7 +311,7 @@ _ctypes_alloc_format_string_for_type(char code, int big_endian)
         break;
     }
 
-    result = PyMem_Malloc(3);
+    result = PyMem_Malloc(4);
     if (result == NULL) {
         PyErr_NoMemory();
         return NULL;
@@ -320,6 +320,7 @@ _ctypes_alloc_format_string_for_type(char code, int big_endian)
     result[0] = big_endian ? '>' : '<';
     result[1] = pep_code;
     result[2] = '\0';
+    result[3] = '\0';
     return result;
 }
 
@@ -3120,7 +3121,27 @@ PyCData_NewGetBuffer(PyObject *myself, Py_buffer *view, int flags)
     view->len = self->b_size;
     view->readonly = 0;
     /* use default format character if not set */
-    view->format = info->format ? info->format : "B";
+    if (!info->format) {
+        view->format = "B";
+    }
+    else {
+        view->format = info->format;
+        if (view->format[1] == 'F') {
+            view->format[1] = 'Z';
+            view->format[2] = 'f';
+            view->format[3] = '\0';
+        }
+        if (view->format[1] == 'D') {
+            view->format[1] = 'Z';
+            view->format[2] = 'd';
+            view->format[3] = '\0';
+        }
+        if (view->format[1] == 'G') {
+            view->format[1] = 'Z';
+            view->format[2] = 'g';
+            view->format[3] = '\0';
+        }
+    }
     view->ndim = info->ndim;
     view->shape = info->shape;
     view->itemsize = item_info->size;