bpo-25655: Improve Win DLL loading failures doc (GH-22372)
Add documentation to help diagnose CDLL dependent DLL loading errors
on windows for OSError with message:
"[WinError 126] The specified module could not be found"
This error is otherwise difficult to diagnose.
diff --git a/Doc/library/ctypes.rst b/Doc/library/ctypes.rst
index c172d53..bf32d3e 100644
--- a/Doc/library/ctypes.rst
+++ b/Doc/library/ctypes.rst
@@ -1326,6 +1326,21 @@
libraries use the standard C calling convention, and are assumed to return
:c:type:`int`.
+ On Windows creating a :class:`CDLL` instance may fail even if the DLL name
+ exists. When a dependent DLL of the loaded DLL is not found, a
+ :exc:`OSError` error is raised with the message *"[WinError 126] The
+ specified module could not be found".* This error message does not contain
+ the name of the missing DLL because the Windows API does not return this
+ information making this error hard to diagnose. To resolve this error and
+ determine which DLL is not found, you need to find the list of dependent
+ DLLs and determine which one is not found using Windows debugging and
+ tracing tools.
+
+.. seealso::
+
+ `Microsoft DUMPBIN tool <https://docs.microsoft.com/cpp/build/reference/dependents>`_
+ -- A tool to find DLL dependents.
+
.. class:: OleDLL(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False, winmode=0)