CODEWITHSUNDEEP
androidc++android-ndk
Egor
Egor

Reputation: 809

Android app crashes after switching from .lib to .so

I am using a number of static pre-built static libraries in my native android application and everything works fine. Now I want to switch one of my static libraries to be .so. I was successfully able to build .so library by replacing BUILD_STATIC_LIBRARY with BUILD_SHARED_LIBRARY in its android.mk and adding required dependencies.

I was also able to build my application by replacing corresponding PREBUILT_STATIC_LIBRARY with PREBUILT_SHARED_LIBRARY in its android.mk. The resulting application now fails to start. I cannot even get to point where debugger attaches to the application.

Besides that what I do not understand is how the build system knows that the function should be imported from the library. My so library should export one function, but I did not declare it as dllexport/import or something. Still there are no unresolved symbols in my application (when I remove my prebuilt library from the list, the unresolved symbol appears as expected).

The other question is that I see there are two .so files generated. One big file in obj/local/$(TARGET_ARCH_ABI) folder and another small one in libs/$(TARGET_ARCH_ABI). When declaring my prebuilt library I reference the second one in libs folder.

I did try to search stackoverflow for answers and found quite a few related posts:

but I do not see how these posts related to my problem since I can successfully build and even link my application.

Upvotes: 2

Views: 2618

Answers (4)

Alex Cohn
Alex Cohn

Reputation: 57203

There may be two reasons why the app fails to start after the change of STATIC -> SHARED.

  1. The prebuilt library is not installed. With your device connected, run adb ls -l /data/your.package.name/lib/. Do you see the library there?

  2. The prebuilt library is not loaded. In your main Java class, try

    static { System.loadLibrary("prebuiltname"); System.loadLibrary("yourlib"); }

    This is a static constructor, the safest place to load JNI library.

Upvotes: 2

mstorsjo
mstorsjo

Reputation: 13317

You need to load the libraries in reverse dependency order in the java code. You previously probably have something like this:

System.loadLibrary("mylib");

Now if your prebuilt library (that was previously a static library, now a shared library) is named dependencylib, you need to change the code for loading the libraries into this:

System.loadLibrary("dependencylib");
System.loadLibrary("mylib");

As for your question how the linker can figure it out; when linking libmylib.so, it looks for all undefined symbols in all the other libraries you specified (i.e. in libdependencylib.so, and in libc.so and other system libraries). As long as all undefined symbols are found somewhere, the linker is ok. Then at runtime, when libmylib.so is loaded, it does the same routine again; all undefined symbols are looked up in the list of symbols loaded in the current process. On linux, you normally don't need to manually mark symbols as dllexport as you do on windows - all non-static symbols are exported by default.

Upvotes: 4

jww
jww

Reputation: 102376

Now I want to switch one of my static libraries to be .so. I was successfully able to build .so library by replacing BUILD_STATIC_LIBRARY with BUILD_SHARED_LIBRARY in its android.mk and adding required dependencies.

I don't think you can do it if its a C++ library. From <doc>/CPLUSPLUS-SUPPORT.html:

Please keep in mind that the static library variant of a given C++ runtime SHALL ONLY BE LINKED INTO A SINGLE BINARY for optimal conditions.

What this means is that if your project consists of a single shared library, you can link against, e.g., stlport_static, and everything will work correctly.

On the other hand, if you have two shared libraries in your project (e.g. libfoo.so and libbar.so) which both link against the same static runtime, each one of them will include a copy of the runtime's code in its final binary image. This is problematic because certain global variables used/provided internally by the runtime are duplicated.

This is likely to result in code that doesn't work correctly, for example:

* memory allocated in one library, and freed in the other would leak or even corrupt the heap.
* exceptions raised in libfoo.so cannot be caught in libbar.so (and may simply crash the program).
* the buffering of cout not working properly

This problem also happens if you want to link an executable and a shared library to the same static library.

In other words, if your project requires several shared library modules, then use the shared library variant of your C++ runtime.

From above, it means everything needs to link against the same C++ standard runtime shared object.

Upvotes: 0

fukanchik
fukanchik

Reputation: 2865

If you are on linux you will see exported symbols using nm -D. example nm -D libzip.so:

...
0000000000009dc0 T zip_unchange
0000000000009dd0 T zip_unchange_all
0000000000009e30 T zip_unchange_archive
0000000000009e60 T _zip_unchange_data

If you want to control visibility of your functions use __attribute__ ((visibility ("default"))) and command line -fvisibility=hidden. More information here.

Upvotes: 1

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