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cdll.LoadLibrary allows None as an argument #78773
Description
| BPO | 34592 |
|---|---|
| Nosy | @gpshead, @vstinner, @superbobry, @mhsmith |
Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.
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GitHub fields:
assignee = None
closed_at = None
created_at = <Date 2018-09-05.21:35:41.405>
labels = ['3.11', 'invalid', 'docs']
title = 'cdll.LoadLibrary allows None as an argument'
updated_at = <Date 2021-11-19.04:06:58.621>
user = 'https://github.com/superbobry'bugs.python.org fields:
activity = <Date 2021-11-19.04:06:58.621>
actor = 'gregory.p.smith'
assignee = 'docs@python'
closed = False
closed_date = None
closer = None
components = ['Documentation']
creation = <Date 2018-09-05.21:35:41.405>
creator = 'superbobry'
dependencies = []
files = []
hgrepos = []
issue_num = 34592
keywords = []
message_count = 5.0
messages = ['324654', '361734', '361735', '361807', '406570']
nosy_count = 5.0
nosy_names = ['gregory.p.smith', 'vstinner', 'docs@python', 'superbobry', 'Malcolm Smith']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'needs patch'
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue34592'
versions = ['Python 3.11']
Activity
superbobry commentedon Sep 5, 2018
LoadLibrary in Python 2.7 through 3.7 accepts None as an argument. I wonder if this has been allowed for a reason? If not, it should probably be changed to raise a TypeError instead.
>>> ctypes.cdll.LoadLibrary(None) <CDLL 'None', handle fffffffffffffffe at 10eedbe90>Interestingly, on Python 2.7 LoadLibrary explicitly mentions None as allowed in the error message:
>>> ctypes.cdll.LoadLibrary(42) Traceback (most recent call last): File "<stdin>", line 1, in <module> File "[...]/ctypes/__init__.py", line 444, in LoadLibrary return self._dlltype(name) File "[...]/ctypes/__init__.py", line 366, in __init__ self._handle = _dlopen(self._name, mode) TypeError: dlopen() argument 1 must be string or None, not intA side-effect of None being allowed is that chaining find_library and LoadLibrary is error-prone:
>>> ctypes.cdll.LoadLibrary(find_library("foobar")) <CDLL 'None', handle fffffffffffffffe at 10ef35fd0>mhsmith commentedon Feb 10, 2020
This isn't documented, but CDLL(None) is translated to dlopen(NULL), which "causes a search for a symbol in the main program, followed by all shared libraries loaded at program startup, and then all shared libraries loaded by dlopen() with the flag RTLD_GLOBAL" (https://linux.die.net/man/3/dlopen).
This is sometimes useful because it lets you look up a symbol in an already-loaded library without needing to give the library's name. For example:
vstinner commentedon Feb 10, 2020
I'm using LoadLibrary(None) commonly to load symbols in Python itself ;-)
vstinner commentedon Feb 11, 2020
In clear, it's a feature, not a bug.
gpshead commentedon Nov 19, 2021
We do need to document this in ctypes.rst. Quite a bit of code depends on ctypes.CDLL(None) and similar to get at symbols linked into the interpreter or already dlopened into the process these days. It works on Linux; I'm assuming it likely does on everything POSIXy.
encukou commentedon Feb 4, 2025
Indeed. POSIX says:
pythongh-78773: Document CDLL(None)
pythongh-78773: Reorganize dynamic library loading docs
gh-78773: Improve ctypes dynamic library loading docs (GH-145313)
pythongh-78773: Improve ctypes dynamic library loading docs (pythonGH…
[3.14] gh-78773: Improve ctypes dynamic library loading docs (GH-145313…
encukou commentedon Mar 9, 2026
Docs are now in.
pythongh-78773: Improve ctypes dynamic library loading docs (pythonGH…