[issue29399] python3.dll export forwarders not resolved in all situations due to missing python3?.dll dependency and DLL search path behavior
Elli Pirelli added the comment:
I should perhaps clarify my sentence "As described, as long as forwarder.dll
does not contain a import dependency on real.dll, loading forwarder.dll and
doing GetProcAddress("Func") will fail, if real.dll is not in the standard DLL
search path."
What i tried to say was:
As described, as long as forwarder.dll does not contain a import dependency on
real.dll, loading forwarder.dll succeeds, but doing GetProcAddress("Func") will
fail if real.dll is not in the standard DLL search path. If this forwarder.dll
is put in one of the standard DLL search paths, GetProcAddress("Func")
successfully resolves the address of the dummy function Func in real.dll.
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[issue29399] python3.dll export forwarders not resolved in all situations due to missing python3?.dll dependency and DLL search path behavior
Elli Pirelli added the comment:
It does not work on Windows 7 Pro x64.
And i am not the only one. The maintainer of the "python-embedded-launcher"
also encountered the issue (see discussion here:
https://github.com/zsquareplusc/python-embedded-launcher/issues/3)
Are you sure it really works on Windows 10? Perhaps you have the python3?.dll
in the standard DLL search path already, making you mistakenly believe that it
works properly? I would suggest to use a tool like SysInternal's Process
Monitor to see and verify where the python3?.dll is really being loaded from.
The issue is not with incorrectly using LoadLibraryEx, SetDllDirectory and
AddDllDirectory. The problem can be reduced to the python3.dll not having a
dependcy on python3?.dll.
I did verify this with a little project, emulating how python3.dll uses export
forwarders. I made a DLL (real.dll) with a dummy function Func and another DLL
(forwarder.dll) containing just an export forwarder Func=real.Func.
As described, as long as forwarder.dll does not contain a import dependency on
real.dll, loading forwarder.dll and doing GetProcAddress("Func") will fail, if
real.dll is not in the standard DLL search path.
As soon as forwarder.dll includes a dependency to real.dll, the export
forwarder can be resolved by GetProcAddress("Func") properly.
If i were using LoadLibraryEx/SetDllDirectory/AddDllDirectory incorrectly,
attempting to load the forwarder.dll with the dependency on real.dll would just
fail -- which it doesn't.
Unfortunately, the whole business with export forwarders is very poorly
documented. It could perhaps be that MS did change something in Win8/2K12 or
Win10 to fix this issue. Since i currently don't have access to either of these
OS, i myself cannot confirm whether the problem would also occur or not on Win
10...
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[issue29399] python3.dll export forwarders not resolved in all situations due to missing python3?.dll dependency and DLL search path behavior
Changes by Elli Pirelli <gonzelm...@gmail.com>: -- components: +Windows nosy: +paul.moore, steve.dower, tim.golden, zach.ware ___ Python tracker <rep...@bugs.python.org> <http://bugs.python.org/issue29399> ___ ___ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
[issue29399] python3.dll export forwarders not resolved in all situations due to missing python3?.dll dependency and DLL search path behavior
New submission from Elli Pirelli:
Let me first apologize if this issue has already been reported and discussed
before. I would be surprised if there are not a few developers that had to deal
with this issue before, but i was unable to find an existing bug report or
discussion about this. So, here it goes...
Importing/using Py_* symbols from python3.dll instead directly from
python3?.dll can be a problem when python3.dll and python3?.dll reside in a
directory that is not part of the standard DLL search path. An example would be
an application that tries loading Python from a sub directory of the
application.
The functions exported in python3.dll are export forwarders referencing the
respective functions in python3?.dll.
python3.dll itself has no import dependency on python3?.dll.
Without an import dependency on python3?.dll, this DLL will be loaded when one
of the export forwarders in python3.dll will be resolved, for example when
calling GetProcAddress("Py_Main").
However, in such a case an altered DLL search path will be completely ignored,
and python3?.dll will only be looked for in the standard DLL search paths.
Since the sub directory with the python DLLs is not part of the standard DLL
search paths, the resolution of the export fowarder(s) fails.
Trying to alter the DLL search path has no effect. Loading python3.dll with
LoadLibraryEx and the flag LOAD_WITH_ALTERED_SEARCH_PATH does not help. Neither
does calling SetDllDirectory or AddDllDirectory before loading python3.dll
and/or before invoking GetProcAddress.
Note that loading python3.dll itself works fine. However, resolving one of the
exports with GetProcAddress will fail if python3?.dll cannot be found in the
standard DLL search path.
I don't know, if there is a way to make it work properly...
As of now the only two options out of this dilemma are either to put the python
DLLs (and assorted files) in the same directory as the application exe (ugly
mess) or to load python3?.dll instead of python3.dll (which sort of defeats the
purpose of having python3.dll, i guess).
My suggestion for future Python 3 versions is to let python3.dll have a DLL
dependency on python3?.dll. Linking python3.dll to python3?.dll (i.e., loading
python3.dll will cause python3?.dll being loaded) allows using altered DLL
search paths.
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messages: 286544
nosy: Elli Pirelli
priority: normal
severity: normal
status: open
title: python3.dll export forwarders not resolved in all situations due to
missing python3?.dll dependency and DLL search path behavior
type: behavior
versions: Python 3.5, Python 3.6, Python 3.7
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