unload dynamic library needs two dlclose() calls?

Question

I have a dynamic library which I load using dlopen() and then unload using dlclose();

If I dont include any objective c code dlopen() needs one dlclose() call which is expected behavior. But when I include any objective c code to target, I have problem that I need to do two dlclose() calls to the loaded library in order to unload.

Is this something expected behavior? How can I fix it?

Answer 1

I realize that you are using dlopen, not CFBundle or NSBundle. Nevertheless, the Code Loading Programming Topics manual says this:

In Cocoa applications, you should not use CFBundle routines to load and unload executable code, because CFBundle does not natively support the Objective-C runtime. NSBundle correctly loads Objective-C symbols into the runtime system, but there is no way to unload Cocoa bundles once loaded due to a runtime limitation.

and this:

Because of a limitation in the Objective-C runtime system, NSBundle cannot unload executable code.

This makes me suspect that when you load your library, it registers itself with the Objective-C runtime, and the runtime calls dlopen on the library again (or somehow increases the library's reference count).

I searched the Objective-C runtime source code and found this:

// dylibs are not allowed to unload
// ...except those with image_info and nothing else (5359412)
if (result->mhdr->filetype == MH_DYLIB  &&  _hasObjcContents(result)) {
    dlopen(result->os.dl_info.dli_fname, RTLD_NOLOAD);
}

So yes, the Objective-C runtime is calling dlopen on your library specifically to prevent it from being unloaded. If you cheat and call dlclose twice, you should expect bad things to happen.