windows dll notes

Mike Toews edited this page Jun 16, 2018 · 2 revisions

Python and DLLs on Windows

DLL loading

Loading a DLL can be implicit or explicit (see DLL linking).

Implicit loading is also called static loading or load-time linking. An executable (.exe or other .dll) has been compiled against the DLL, using a .lib import library file listing the symbols exported by the DLL. The executable file has an import address table (IAT), with entries for each DLL function that the executable file needs. When the executable is run, Windows finds and loads the DLL file named in the executable, and fills in the executable IAT with the loaded locations of each required function.

Explicit loading is also called dynamic loading or run-time linking. The executable explicitly calls one of the functions LoadLibrary, LoadLibraryEx with the name of the required DLL. Calls to functions inside the loaded DLL need to be made via a function pointer.

Thus, in the implicit case, Windows does the DLL loading based on the contents of the executable. In the explicit case, the executable does the loading.

See DLLs in Visual C++ for an overview.

Which DLL?

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

This problem is sometimes called DLL hell

Let's say your executable links implicitly to myuseful.dll. How does Windows know where to look for myuseful.dll?

By default, Windows uses the dynamic link library search order.

Here is the default DLL search order for desktop applications on Windows XP SP2 and later. The words in parentheses are abbreviations I'll use later in the document.

  1. Directory containing the loading EXE application (load-exe)
  2. System directory (e.g. C:\Windows\System32) (sys32)
  3. 16 bit system directory (sys16)
  4. Windows directory (e.g. C:\Windows) (win)
  5. Current directory (pwd)
  6. Directories listed in PATH environment variable (env-paths)

For reasons that will become clearer, call this the Safe Standard Search Order (S3O).

We can use the abbreviations to describe this search order as load-exe, sys32, sys16, win, pwd, env-paths.

There are various exceptions and ways of tuning S3O, which are (quotes are from the document above):

  • Previous loading:

    If a DLL with the same module name is already loaded in memory, the system uses the loaded DLL, no matter which directory it is in. The system does not search for the DLL.

    Preference for an already-loaded DLL of the same name can only be avoided by using application manifests and side-by-side assemblies

  • Known DLLs: Windows has a list of "Known DLL" names maintained in the registry. DLL names matching any of the known names are loaded from the system and not searched for using S3O. Again, this always occurs unless you override the DLL by using side-by-side assemblies

  • Disabling "safe DLL search mode": The "safe" in "Safe Standard Search Order" refers to "safe DLL search mode". This mode is enabled via a entry in the Windows registry. It is enabled by default in all versions of Windows from XP SP2, but disabled on previous versions of XP. If "safe search" mode is disabled, the current directory gets searched after the directory containing the loading application, rather than after the Windows directory. Using the abbreviations, this gives the search order load-exe, pwd, sys32, sys16, win, env-paths. (See loadlibrary explorer for tests that suggest that enabling safe mode in the registry doesn't change the search order from the disabled state; it looks like the tests were done on XP SP2).

  • SetDllDirectory function: You can add a single path to the DLL search order by calling the SetDllDirectory(new_path) function. This inserts new_path into the search order before the System directory in the sequence above, and removes the current directory from the search path. See SetDllDirectory API doc. Note (from that page):

    The SetDllDirectory function affects all subsequent calls to the LoadLibrary and LoadLibraryEx functions. It also effectively disables safe DLL search mode while the specified directory is in the search path.

    This mode stays in place until specifically disabled by a call SetDllDirectory(NULL). With the abbreviations this gives load-exe, new_path, sys32, sys16, win, env-paths. This order applies regardless of the setting for "safe search".

    Note also the comments on SetDllDirectory in the documentation for LoadLibraryEx call:

    However, be aware that using SetDllDirectory effectively disables safe DLL search mode while the specified directory is in the search path and it is not thread safe. If possible, it is best to use AddDllDirectory to modify a default process search path.

  • LOAD_LIBRARY_SEARCH_* flags: You can specify an exact search order using the LOAD_LIBRARY_SEARCH_* flags, in systems that support them. Support for this API arrived in 2011 via a security update to Vista and later, called KB2533623. A version of this update seems be present on a routinely updated copy of Windows 7 that I have. Using this API, you can either pass LOAD_LIBRARY_SEARCH_* flags to LoadLibraryEx to affect one call, or use SetDefaultDllDirectories to set the default flags for the process. See LoadLibraryEx API doc for detail. Flags are:

    • LOAD_LIBRARY_SEARCH_APPLICATION_DIR - search directory containing application.
    • LOAD_LIBRARY_SEARCH_SYSTEM32 - search %windows%\system32 directory.
    • LOAD_LIBRARY_SEARCH_USER_DIRS - search any directories added either with (calls to AddDllDirectory; or by a call to SetDllDirectory) (see below).
    • LOAD_LIBRARY_SEARCH_DEFAULT_DIRS - combination of three flags above.
    • LOAD_LIBRARY_SEARCH_DLL_LOAD_DIR - search for other DLLs in the same directory as the DLL to be loaded. This works when passing an absolute DLL path to LoadLibraryEx. It only applies to the immediate dependencies of the DLL being loaded with LoadLibraryEx.

    This means you can add multiple paths to the search order with several calls to AddDllDirectory(new_path) etc. (see AddDllDirectory API). Most unfortunately, the search order is undefined within the directories you added with AddDllDirectory - see AddDllDirectory API:

    If AddDllDirectory is used to add more than one directory to the process DLL search path, the order in which those directories are searched is unspecified.

    If you specify any of these LOAD_LIBRARY_SEARCH_* flags in a particular LoadLibraryEx call, or via SetDefaultDllDiectories, then Windows only searches the directories you specified with the flags, and does not use the S3O search order at all.

  • LOAD_WITH_ALTERED_SEARCH_PATH: If you call LoadLibraryEx("c:\path\to\my.dll") with the LOAD_WITH_ALTERED_SEARCH_PATH flag, then, for executables resolved on that call, Windows replaces the load-exe path with the directory containing the DLL - here c:\path\to. Call this the load-dll path. Using abbreviations this gives load-dll, sys32, sys16, win, pwd, env-paths in the default safe-search-enabled mode, and load-dll, pwd, sys32, sys16, win, env-paths when safe-search is disabled. (This is what the documentation says, but the author of loadlibrary explorer claims that, in fact, if c:\path\to\my.dll does not exist, the call fails and the other paths do not get searched, at least on XP SP2). Microsoft refers to this search order as the alternate DLL search order.

    Experiments suggest that, if you first set DLL search order with LOAD_LIBRARY_SEARCH_* flags to SetDefaultDllDiectories, then call LoadLibraryEx with LOAD_WITH_ALTERED_SEARCH_PATH, then the load-dll path will be prefixed to the search order given by your LOAD_LIBRARY_SEARCH_* flags.

  • Redirection file: You can force Windows to look first for DLLs in the directory containing the loading application, by making a redirection file. The redirection file is named for the application EXE, but with the extra suffix of .local. If you have an application c:\path\myapp.exe the redirection file would be called c:\path\myapp.exe.local. Windows ignores the contents of the redirection file, it can be empty. So, if c:\path\myapp.exe calls LoadLibraryEx("c:\somewhere\mylib.dll") and a file exists named c:\path\myapp.exe.local then the system will first look for c:\path\mylib.dll before looking for c:\somewhere\mylib.dll.

    The relocation "file" can also be a directory. For our example, if c:\path\myapp.exe.local is a directory, and you call LoadLibraryEx("c:\somewhere\mylib.dll") then the system will first look for c:\path\myapp.exe.local\mylib.dll before looking for c:\somewhere\mylib.dll.

    As for other S3O modifications, "Known" DLLs can't be redirected in this way. Any such redirection is ignored if the application has a side by side assemblies application manifest (see below). Also see loadlibrary explorer for dissent as to whether this redirection actually works, tested on XP SP2.

Side by side assemblies

You can specify exactly which DLLs get loaded for any executable using side by side assemblies.

These are often abbreviated to SxS.

The terminology of SxS can be very confusing. To help, here is a careful set of SxS definitions

An "executable" in the following can be a DLL or an EXE (.exe).

Let us say we have a DLL called mylib.dll. mylib.dll can get loaded by any of several applications, so doesn't have any control over libraries loaded previously. In turn, mylib.dll will load another DLL called myruntime.dll. We want to make sure that mylib.dll loads exactly this myruntime.dll regardless of any whether any other DLL called myruntime.dll has already been loaded, and regardless of whether there is another myruntime.dll somewhere else on the DLL search path.

We do this by including myruntime.dll in a SxS assembly. The SxS assembly information says "I have myruntime.dll". We then tell mylib.dll that it depends on this SxS assembly. When mylib.dll tries to load myruntime.dll it will first look in the SxS assembly, find this copy of myruntime.dll and use that.


You use SxS assemblies like this:

  • The loading executable has an application manifest. Let's say the loading executable wants to load a DLL called myruntime.dll
  • The application manifest for the executable declares that the executable depends on a named assembly, say MyAssembly
  • The system looks for this assembly using the assembly searching sequence
  • The system is looking for an assembly manifest with a matching name (MyAssembly in our example). The assembly manifest declares the resources (including DLLs) included in the named assembly.
  • If the assembly manifest (here for MyAssembly) has an entry for (here) myruntime.dll, then the system loads the copy referenced by the assembly, ignoring all others, including copies previously loaded.

Making an SxS assembly

You make a SxS by creating an assembly manifest.

The assembly manifest is some XML that describes the assembly. The assembly is a collection of resources - typically one or more DLL files. The XML can be written to a file with a suitable filename (see below). If the assembly consists of a single DLL, the assembly manifest can be instead be embedded in the DLL file as a resource - see embedding a manifest. An embedded assembly manifest is always resource ID 1 in a DLL.

In our example, the assembly manifest XML can be written as a file mylib_assembly.manifest. The file name matches the name of the assembly that executables will use in their application manifests (see below). The same XML can also be embedded into mylib.dll as a resource at ID 1.

See the notes on "File name syntax" in assembly manifest for some complications of naming an assembly manifest file. These complications (from the assembly searching sequence) explain why you can't use mylib.manifest as a name for your manifest file in this case.

There's an example assembly manifest at the bottom of the assembly manifest page.

An assembly may either be a shared assembly or a private assembly. The two types of assemblies have the same structure, but a shared assembly may be shared between different applications and must be installed in a special "WinSxS" directory at %windows%\winsxs. These assemblies have to be installed with a Windows installer. A private assembly is an assembly included inside your application, and consists of files in your application folder.

Telling an executable to use a SxS assembly

An executable indicates that it is using a SxS assembly by using an application manifest. The application manifest is some XML that expresses the dependency of the executable file on a particular SxS assembly.

The XML can be a separate file or embedded as a resource in the executable. Because an EXE can't have an assembly manifest as well as an application manifest, the resource ID for an application manifest embedded in an EXE is 1. A DLL can have an assembly manifest as well as an application manifest, because the DLL might depend on other assemblies. Therefore the resource ID for an assembly manifest embedded in a DLL is 1, and the resource ID for an application manifest embedded in a DLL is 2 (see DLLs and resource ID 2 manifests).

When Windows loads the executable file, it looks for an application manifest. If it finds one, it creates a new activation context. It then looks for and loads DLLs within this new context. If a DLL gets loaded in the new context, it overrides any DLL of the same name that was loaded previously (SxS activation context). This is what allows us (in our example) to make sure that our own myruntime.dll gets loaded instead of using - say - a DLL with the same name that has already been loaded.

The application manifest specifies names of assemblies that the executable depends on. With these names, Windows searches for the actual assemblies, using the assembly searching sequence.

There's an example application manifest at the bottom of the application manifest page.

Expanding the search path for assemblies

You may have noticed from the assembly searching sequence that private assemblies must generally be in the directory of the executable that depends on them. In fact you can extend the search for assemblies by making an extra application configuration file and adding a probing element with a privatePath attribute. This allows you to specify relative paths with up to two levels of .. in which to search for assemblies. Unfortunately, this feature

is unavailable on systems earlier than Windows Server 2008 R2 and Windows 7

(see application configuration file). There is an example configuration file in this stackoverflow answer on SxS searching.

Some useful links

Analyzing DLL dependencies

Python DLLs

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

Imagine you have a package like this:


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

This leads to the general Windows "which DLL" problem.

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 causes Windows to look for DLLs first in the directory containing the extension (directory containing extension_path) (see above).

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:

Numpy and Windows DLLs