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gh-52008: Document that ctypes.CDLL can only observe errno from its own C runtime #159239

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gh-52008: Document that ctypes.CDLL can only observe errno from its own C runtime #159239

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5 changes: 4 additions & 1 deletion Doc/library/ctypes.rst
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Expand Up @@ -1579,7 +1579,10 @@ way is to instantiate :py:class:`CDLL` or one of its subclasses:
:mod:`!ctypes` maintains a thread-local copy of the system's :data:`errno`
variable; if you call foreign functions created with ``use_errno=True`` then the
:data:`errno` value before the function call is swapped with the ctypes private
copy, the same happens immediately after the function call.
copy, the same happens immediately after the function call. This mechanism only
works for libraries that use the same C runtime library as the interpreter itself.
A library linked against a different C runtime (for example, a different CRT on
Windows) has its own :data:`errno`, which :mod:`!ctypes` cannot observe.

The function :func:`ctypes.get_errno` returns the value of the ctypes private
copy, and the function :func:`ctypes.set_errno` changes the ctypes private copy
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