Say I have an application foo that is linked against bar.so. I have another library, buzz.so that the application does not directly link, but that I use when I run the application with LD_PRELOAD like so: LD_PRELOAD=buzz.so ./foo. Now I want buzz.so to be able to use dlsym to find a symbol inside bar.so. My question is: what is the earliest point in time the dlsym call can succeed and how do run a function at that time?

GCC/Clang support annotating functions with the __constructor__((priority)) attribute to make them run before main, but AFAIK there isn’t any priority number that corresponds to being sure that all shared libraries are loaded.