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windows dll notes

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Matthew Brett edited this page May 7, 2014 · 22 revisions

Notes for Numpy on Windows

About DLLs

On Windows, Python extension modules have file extensions .pyd as in myextension.pyd.

DLLs are a problem for general windows builds because it can be tricky making sure your extension gets the right DLL.

Imagine you have a package like this:

mypackage/
    myextension.pyd
    runtime.dll
yourpackage/
    yourextension.pyd
    runtime.dll

myextension.pyd depends on my version of runtime.dll, and yourextension.pyd depends on your version of runtime.dll.

But, when I do: >>> import mypackage.myextension, then myextension.pyd gets loaded, as does my copy of runtime.dll. When I then do >>> import yourpackage.yourextension, Windows sees that we already have a copy of runtime.dll, so doesn't load your copy of runtime.dll. If these two DLLs are not the same, this can cause nasty crashes, and crashes that depend on the order in which the two pyd extensions get loaded.

We also need to make sure that the DLLs we need can be found when the Python extension gets loaded.

When Python loads an extension, it does it using the Windows LoadLibraryEx call, like this:

hDLL = LoadLibraryEx(extension_path, NULL,
                     LOAD_WITH_ALTERED_SEARCH_PATH);

See: http://hg.python.org/cpython/file/3a1db0d2747e/Python/dynload_win.c#l195

LOAD_WITH_ALTERED_SEARCH_PATH causes Windows to look for DLLs first in the directory containing the extension (directory containing extension_path) (see DLL search path)

Specifically, if you do:

>>> import mypackage.myextension

and myextension.pyd is in c:\Python27\Lib\site-packages\mypackage, and myextension.dll loads runtime.dll, then Windows will look for runtime.dll first in c:\Python27\Lib\site-packages\mypackage.

A common situation is that you want to put all needed DLLs in one directory, but there are extensions loading these DLLs are all over the file tree.

For example, default builds of scipy using Mingw-w64 will depend on gcc and gfortran run-time DLLs. There will be extensions needing these DLLs in several places in the scipy package tree. In that case we may want to have a single directory called dlls in the Scipy tree containing these DLLs. We can put this directory on the DLL search path with DLL path tricks used in this ctypes code fragment (thanks to Steve Dower for the fragment, Carl Kleffner for finding it).

Some unsorted links that also seemed useful:

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