Native Library Loading in .NET 5

Ryan Lucia loader

After years of work, Mono can now be built out of the dotnet/runtime repository in a .NET 5-compatible mode! This mode means numerous changes in the available APIs, managed and embedding, as well as internal runtime behavioral changes to better align Mono with CoreCLR and the .NET ecosystem.

One area with multiple highly impactful changes to the runtime internals is library loading. For managed assemblies, Mono now follows the algorithms outlined on this page, which result from the removal of AppDomains and the new AssemblyLoadContext APIs. The only exception to this is that Mono still supports bundles registered via the embedding API, and so the runtime will check that as part of the probing logic.

The managed loading changes are fairly clear and well documented, but unmanaged library loading has changed in numerous ways, some of them far more subtle.

Summary of changes:

  • New P/Invoke resolution algorithm
  • Dropped support for DllMap
  • Unmanaged library loading defaults to RTLD_LOCAL
  • Added support for DefaultDllImportSearchPathsAttribute
  • On non-Windows platforms, Mono and CoreCLR no longer attempt to probe for A/W variants of symbols
  • Default loader log level changed from INFO to DEBUG, and new log entries added for the new algorithm

More detail where appropriate in the sections below.

Dropped support for DllMap

The new unmanaged loading algorithm makes no mention of DllMap, as Mono has removed its functionality almost entirely in .NET 5. DllMap’s XML config files have have been disabled on every platform out of security concerns. The DllMap embedding APIs are also disabled on desktop platforms, though this may change.

In place of DllMap, users are encouraged to utilize the NativeLibrary resolution APIs, which are set in managed code, and the runtime hosting properties, which are set by embedders with the monovm_initialize function.

We recognize that this does not sufficiently cover some existing mono/mono scenarios. If the NativeLibrary APIs are insufficient for your use case, please tell us about it! We’re always looking to improve our interop functionality, and in particular with .NET 6 will be evaluating NativeLibrary, so community input would be greatly appreciated.

Unmanaged library loading defaults to RTLD_LOCAL

A more subtle, yet no less impactful change is that native library loading now defaults to RTLD_LOCAL to be consistent with CoreCLR and Windows, as opposed to our historical behavior of RTLD_GLOBAL. What this means in practice is that on Unix-like platforms, libraries are no longer loaded into a single global namespace and when looking up symbols, the library must be correctly specified. This change prevents symbol collision, and will both break and enable various scenarios and libraries. For more information on the difference, see the dlopen man page.

For an example: historically in Mono on Linux, it was possible to load library foo containing symbol bar, and then invoke bar with a P/Invoke like so:

// note the incorrect library name
public static extern int bar();

This will no longer work. For that P/Invoke to function correctly, the attribute would need to use the correct library name: [DllImport("foo")]. A lot of code in the wild that was using incorrect library names will need to be updated. However, this means that when loading two libraries containing the same symbol name, there is no longer a conflict.

There have been some embedding API changes as part of this. MONO_DL_MASK is no longer a full mask, as MONO_DL_GLOBAL has been introduced to specify RTLD_GLOBAL. If both MONO_DL_LOCAL and MONO_DL_GLOBAL, are set, Mono will use local. See mono/utils/mono-dl-fallback.h for more info.

This also means that dynamically linking libmonosgen and attempting to resolve Mono symbols from dlopen(NULL, ...) will no longer work. __Internal has been preserved as a Mono-specific extension, but its meaning has been expanded. When P/Invoking into __Internal, the runtime will check both dlopen(NULL) and the runtime library in the case that they differ, so that users attempting to call Mono APIs with __Internal will not have those calls break.

Added support for DefaultDllImportSearchPathsAttribute

Mono now supports the DefaultDllImportSearchPathsAttribute attribute, which can be found in System.Runtime.InteropServices. In particular, passing DllImportSearchPath.AssemblyDirectory is now required to have the loader search the executing assembly’s directory for native libraries, and the other Windows-specific loader options should be passed down when appropriate.


And that’s it! If you have any further questions, feel free to ping us on Discord or Gitter.