Mono Android Monodroid Native Library DllNotFoundException

Question

I am attempting to get a Native C/C++ library working with Monodroid in the emulator, using DllImport. I am developing mainly in Windows/Visual Studio 2010.

I have built a native C/C++ library (ZeroMQ) using the Android NDK tools, to both x86 and armeabi platforms, using a Ubuntu virtual machine. Is this correct - x86 for emulator and armeabi for the real device? (This is certainly the case on the iPhone/MonoTouch - though in that case statically linked libzmq.a file and DllImport __Internal)

I have added the x86 version of libzmq.so to my MonoDroid project under the directory structure lib\x86\libzmq.so

When I first attempted to build/deploy to the simulator, I got an error 'cannot determine abi type', so I've added x86 to the AndroidNativeLibrary Include="lib\x86\libzmq.so"> project item group. This then deployed.

I have a DllImport for the function to call [DllImport("libzmq"

I've tried libzmq.so, lib/x86/libzmq, lib/x86/libzmq.so etc, but then I call the DllImported method (running in the emulator), I always get a DllNotFoundException.

Can anyone give me some direction?

EDIT: After reading another support answer which states that the emulator uses armeabi .so libraries, I have removed the x86/libzmq and added my lib/armeabi/libzmq.so as an AndroidNativeLibrary. I also removed the project file Abi entry, and indeed the project built and deployed fine.

However, I still get a DllNotFoundException when I try to call a DllImported function. Any ideas?

Many thanks

Answer 1

Ok, I finally got it working! For whatever reason, ZeroMQ doesn't build correctly using the current/latest NDK r7. The instructions at http://www.zeromq.org/build:android use NDK r6. I also downloaded an old version of the NDK r5b (it was the easiest old version to find a download for), and rebuilt ZeroMQ using it. Result, it now works on emulator and phone!

Answer 2

I don't have any immediate ideas why it isn't working for you; [DllImport("zmq")] should work.

The SanityTests sample exercises the DllImport attribute.

• The DllImport: https://github.com/xamarin/monodroid-samples/blob/master/SanityTests/Hello.cs#L240
• The Android.mk to build libfoo.so: https://github.com/xamarin/monodroid-samples/blob/master/SanityTests/jni/Android.mk
• Building libfoo.so by calling ndk-build: https://github.com/xamarin/monodroid-samples/blob/master/SanityTests/SanityTests.csproj#L82
• Including libfoo.so into the .apk: https://github.com/xamarin/monodroid-samples/blob/master/SanityTests/SanityTests.csproj#L96

Answer 3

(This is a repeat of my reply to your email in the ZeroMQ mailing list).

This error could be caused by using an incorrect platform target in your Mono project. I'm not sure if Mono/MonoDevelop supports this, but you probably need to create an ARM platform target, as opposed to x86/x64/Any CPU.

You would experience the same errors if you tried to do P/Invoke interop between an x64-targeted assembly with an x86 native library or vice-versa.

If you're using the clrzmq bindings for bridging Mono and libzmq, you will need to create a new platform target for ARM and rebuild the bindings. The solution only defines x86 and x64 platform targets currently.