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:

  if ( (newHandle = dlopen("./libCamera.so",RTLD_LAZY | RTLD_GLOBAL)) == NULL )
  {
     printf( "Could not open file : %s\n", dlerror() );   
     return 1;
  }

However this fails and I receive the following output: "Could not open file : libCamera.so: undefined symbol: ZTVN10_cxxabiv117__class_type_infoE"

How do I find out what symbols it is relying on?

Answer 1

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.