Accessing .so libraries using dlopen() throws undefined symbol error

Question

I\'m trying to dynamically load a camera library .so file into a Linux executable to gain access to simple camera functions.

I\'m attempting to do this by:

Answer 1

In your source code for libCamera.so, you have unresolved external symbol. It means that type_infoE have no definition in your source code and should be resolved.

Answer 2

Most likely, libCamera.so uses a symbol defined in a shared library without depending on that library.

1. Find a culprit. Take a real executable which links against libCamera.so (and it works). List its dependencies with ldd /path/to/executable. Among them should be a library which has a definition for ZTVN10_cxxabiv117__class_type_infoE (use grep to select likely candidates, nm -D on a library to be sure). That library won't be in the list shown by ldd ./libCamera.so.

2. Solve a problem. Load the library found in step 1 by dlopen first (use RTLD_GLOBAL there as well).

3. If there is a problem with another symbol, goto step 1.

4. If newly-added libraries have the same problem too, goto step 1.

5. Tell library authors to please fix their linking.

It could also happen that one of the prerequisites in ldd ./libCamera.so got upgraded and lost a symbol definition (maybe it was recompiled with a compiler which does name mangling differently). Then you won't find the culprit in step 1, and there is no solution but downgrading something again.

Answer 3

I had a similar problem. It was to do with a .a library, which should have been linked to my .so and statically linked into the archive being left out.

I determined this with (OP object name used here):

nm mylibrary.so | grep ZTVN10_cxxabiv117__class_type_infoE
0000ABC0 U ZTVN10_cxxabiv117__class_type_infoE

The U here means that the symbol is "undefined". You can find the demangled name of the missing object with --demangle:

 $ nm --demangle mylibrary.so | grep 0000ABC0 
0000ABC0 U abi::class_type_info(params...)

(or something like that) this should help you figure out which library is missing.

In my case, even after including the library on the compiler line I still had the issue. Eventually, after some tinkering I discovered that the library-file (.a) has to come after its dependent object (.o) file like:

g++ -Wl,-E -g -m32 ... -fPIC myobjects1.o myobjects2.o missing_library.a -shared -o mylibrary.so

Now I get (no more U):

 $ nm --demangle mylibrary.so | grep 0000ABC0 
0000ABC0 T abi::class_type_info(params...)

and most importantly I don't get the error any more!

Answer 4

The ldd command can be used to display shared library dependencies.

ldd libCamera.so

Once you know the dependencies, you can use nm to show the symbols in each library.

nm -DC libCamera.so