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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.
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Philippe Ombredanne 2020-10-22 17:39:18 +02:00 committed by GitHub
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Doc/library/ctypes.rst
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@ -1326,6 +1326,21 @@ way is to instantiate one of the following classes:
libraries use the standard C calling convention, and are assumed to return libraries use the standard C calling convention, and are assumed to return
:c:type:`int`. :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) .. class:: OleDLL(name, mode=DEFAULT_MODE, handle=None, use_errno=False, use_last_error=False, winmode=0)