[Python-Dev] Why does python use relative instead of absolute path when calling LoadLibrary*

[Brett Cannon]

On Wed, Mar 11, 2015 at 4:37 PM David Cournapeau <cournape at gmail.com> wrote:

> Hi,
>
> While looking at the import code of python for C extensions, I was
> wondering why we pass a relative path instead of an absolute path to
> LoadLibraryEx (see bottom for some context).
>
> In python 2.7, the full path existence was even checked before calling
> into LoadLibraryEx (
> https://github.com/python/cpython/blob/2.7/Python/dynload_win.c#L189),
> but it looks like this check was removed in python 3.x branch.
>

I personally don't know of any specific reason.

-Brett


>
> Is there any defined behaviour that depends on this path to be relative ?
>
> Context
> -----------
>
> The reason why I am interested in this is the potential use of
> SetDllDirectory to share dlls between multiple python extensions.
> Currently, the only solutions I am aware of are:
>
> 1. putting the dlls in the PATH
> 2. bundling the dlls side by side the .pyd
> 3. patching packages to use preloading (using e.g. ctypes)
>
> I am investigating a solution 4, where the dlls would be put in a separate
> "private" directory only known of python itself, without the need to modify
> PATH.
>
> Patching python to use SetDllDirectory("some private paths specific to a
> python interpreter") works perfectly, except that it slightly changes the
> semantics of LoadLibraryEx not to look for dlls in the current directory.
> This breaks importing extensions built in place, unless I modify the call
> in ;https://github.com/python/cpython/blob/2.7/Python/dynload_win.c#L195
> from:
>
>     hDLL = LoadLibraryEx(pathname, NULL LOAD_WITH_ALTERED_SEARCH_PATH)
>
> to
>
>     hDLL = LoadLibraryEx(pathbuf, NULL LOAD_WITH_ALTERED_SEARCH_PATH)
>
> That seems to work, but I am quite worried about changing any import
> semantics by accident.
>
> David
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