Hi,

I've written a shared library (in C++) I've built using libtool (through the automake/autoconf system).

I've also written an application (C++ executable) that dynamically loads this library using dlopen.

I would like the shared library to call a function implemented in the executable.
How do I link the shared library to the executable?

(I've tried adding the executable to the linking stage of the shared library automake system - but this didn't work)

Thanks!

It doesn't work. What you may try is to add the object file from the executable to the library. Or, if that solution does not suit you, you may add a new function to your library that will get a function pointer from the application. Then the pointer (pointing to the function in question, of course) may be used in the library. Or...maybe move the function that calls the function in question to the executable?

Linux defaults to interposing earlier definitions during loading, which probably gives you an easy solution. I don't know enough of the exact rules, so if I were doing it myself, I would need to experiment to see what works.

The basic idea would be to include dummy functions in the .so file that have the same name as the target functions in the main executable but do nothing.

At load time, I think the loader will interpose the functions from the main image overriding the dummy versions in the .so.

In porting code from Windows (which does not do this) to Linux, I had several functions in the main image duplicating names of intentionally different functions in DLLs. In Windows it all works fine because nothing is exported nor imported without effort to make it exported and imported. In Linux, such things are exported, imported and interposed all by default and it took lots of effort to get those symbols to be final at link time so they couldn't be interposed. You have the opposite issue, which I think the Linux default behavior makes easier.

Mara's suggestion is the one I recommend. It is best to be explicit about a library calling into an application and the pattern for doing that is Register a Callback Function.

In C++, the easiest way to do that is to have the library define an interface (ABC) that is implemented by a class in the application. The application implements a class which implements the interface. The app registers an object of this type with the library. And the library just calls a method on the registered object.

Thanks! very helpful advice!