dlopen on Android ndk

Question

I made a c++ main aplication that loads a so library also made by me. Both sources shares a common header (TestFlags.h). Inside TestFlags.h I have an class and a pointer declaration of it which is intended to be global to the whole application, that is define a instance in the main app and use it inside a library function.

class TestFlags {
public:
    TestFlags() : behaviour(1)
        {}
    int behaviour;
};

extern __attribute__ ((visibility("default"))) TestFlags * gpTestFlags;

then a sequence of execution steps followed to reach the named goal are:

1. main application creates a new instance of TestFlags ---> gpTestFlags = new TestFlags();
2. main application load the library ---> dlopen(library.so, RTLD_LAZY|RTLD_GLOBAL)
3. invoke a function that resides inside the library which uses previous instance declared ---> gpTestFlags->behaviour = 2;
4. Received a SIGSEGV: Segmentation fault because gpTestFlags is NULL

It seems that inside the library gpTestFlags instance is not seen for some reason. Same thing also happens with other static class I have, values which are configured on the main application not seen inside the library.

As far I can research it seems that the library manages a totally different memory space for those declarations like if it was duplicated.

Answer 1

This is the expected way dlopen() would work.

The two modules have independent global symbols called gpTestFlags. If you try to link them together, the linker would scream about duplicates.

You can declare the pointer in library as weak, or you can use dlsym() to resolve the linkage programmatically.