Question

I have a command-line executable from which I would like to be able load essentially any dynamic library that contains Objective-C classes. In the Project Settings, I have specified that my program supports garbage collection (without requiring it). To this end, I use dlopen to load the libraries, as it’s globally more flexible than NSBundle (first off and most importantly, it can load libraries that aren’t in a bundle).

However, when I try to load a library or framework that requires garbage collection, dlopen fails and I get this message:

Unable to open /path/to/object: dlopen(/path/to/lib, 2): no
suitable image found. Did find:
       /path/to/lib: GC capability mismatch

If my program supports garbage collection, why can’t it load libraries that require it? How can I load libraries that require it?

I can’t really flag my executable as requiring garbage collection because it will stop working with libraries that don’t support it.

Right now I have two versions of my executable, but that’s sub-par.

Additional informations: I target Mountain Lion only. My program is a universal binary and it currently handles relaunching as its 32-bits counterpart using the posix_spawn function, however there seems to be no flag controlling garbage collection there.

Answer 1

I found the solution to my problem. The Objective-C runtime has several switches triggered by environment variables, including OBJC_DISABLE_GC that can turn off the garbage collector even if the executable wants it on. This switch is taken into account when new images are loaded (see gc_enforcer and how it uses the UseGC static variable), so turning the GC off will actually affect the way libraries are loaded into the process.

My solution, then, is to implement proper retain/release logic into my executable but compile it with -fobjc-gc-only. I then try to load the library normally; if it works, everything’s good. If it doesn’t, I programmatically relaunch the process with the same arguments, plus OBJC_DISABLE_GC=YES in its environment, so the GC is disabled and the library successfully loads this time.

This snippet excludes error handling.

char disableGC[] = "OBJC_DISABLE_GC=YES";
// resets the current process with the given argv and enables the GC as per useGC
void respawn(char* const argv[], bool useGC)
{
    posix_spawnattr_t attributes;
    posix_spawnattr_init(&attributes);
    posix_spawnattr_setflags(&attributes, POSIX_SPAWN_SETEXEC)

    // build the environment variables array
    std::vector<char*> environment;
    for (char** environ_iter = environ; *environ_iter != 0; environ_iter++)
        environment.push_back(*environ_iter);

    // disable the GC if we need it off
    if (!useGC)
    {
        environment.push_back(disableGC);
    }

    environment.push_back(nullptr);

    pid_t child;
    posix_spawn(&child, argv[0], nullptr, &attributes, argv, environment.data());
}