C++: dlclose doesn't unload the shared library

Question

I have a shared library loaded using dlopen (with the flags RTLD_NOW | RTLD_GLOBAL ). If this library is using functions from the main program, then it does not unload. So I end up with the same code for this shared lib, even if I unloaded (using dlclose), changed, compiled, (re)load it.

My goal is actually to reload the same library after making changes to it, so that I do not have to relaunch the whole program to test out my code.

I am using g++ 4.2.3, on Linux Ubuntu 10.04.

(edit)

solved:

"loaded library uses a symbol due to the RTLD_GLOBAL". Indeed, I had symbols of another .a embedded when linking that were probably called back and preventing my library to close... I think it's possible to verify that a lib unloaded using dlopen(...,RTLD_NOLOAD) to check out the library has unloaded correctly.

Pye

Are you releasing the handle to your dll? – Lees

You probably need to be somewhat more specific about which platform (Linux?) and which version you are using. It is likely to matter for this sort of issue. – Rosyrot

@Komyg: I am using dlclose(handle), so I am assuming that the handle is released doing so... – Pye

@Jonathan: I am using g++ 4.2.3, on linux ubuntu 10.04 – Pye

You might want to keep an eye on Unload dynamic library needs two dlclose calls, which is related but definitely not a direct duplicate as it is referring to MacOS X. Is there any chance that your executable loads the library anyway? Does ldd your_executable list the library? – Rosyrot

@Pye : I have the same issue. I tried removing RTLD_GLOBAL, but still facing the same issue. Do you have any other suggestion to try out? – Unreflecting

Answer 1

The function dlclose() decrements the reference count on the dynamic library handle. If the reference count drops to zero and no other loaded libraries use symbols in it, then the dynamic library is unloaded.

Also the RTLD_NODELETE (upon dlopen) makes dlclose not to unload the library.

Since you have not used RTLD_NODELETE, most probable is that a loaded library uses a symbol due to the RTLD_GLOBAL.

Waite

"loaded library uses a symbol due to the RTLD_GLOBAL". indeed, I had symbols of a .a embedded when linking that were probably called back and preventing the library to close... I think I can verify if a lib unloaded using dlopen(RTLD_NOLOAD) on it to check out the library has unloded correctly – Pye