cdll.LoadLibrary allows None as an argument

[superbobry]

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']

[superbobry]

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 int

A 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]

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:

>>> import ctypes
>>> dll = ctypes.CDLL(None)
>>> dll.printf
<_FuncPtr object at 0x7f3427d547c0>
>>> dll.PyObject_Str
<_FuncPtr object at 0x7f3427d54880>

[vstinner]

I'm using LoadLibrary(None) commonly to load symbols in Python itself ;-)

[vstinner]

In clear, it's a feature, not a bug.

[gpshead]

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.