dlclose() doesn't work with factory function & complex static in function?

Question

I'm making a simple plugin framework in which I'd like to be able to dlopen() a shared library (i.e. plugin), inspect and use whatever factory functions is provides and eventually dlclose() it, leaving no trace.

My factory system is trivial, with a single exported function that returns a pointer to a common Base class. To check the plugin has been unloaded properly, I have a static object whose destructor sets a bool from the main program.

Here's the main program:

// dltest.cpp follows. Compile with g++ -std=c++0x dltest.cpp -o dltest -ldl
#include <dlfcn.h>
#include <iostream>
using namespace std;
int main(int argc, char** argv)
{
    if (argc > 1)
    {
        void* h = dlopen(argv[1], RTLD_NOW|RTLD_LOCAL);
        if (!h)
        {
            cerr << "ERROR: " << dlerror() << endl;
            return 1;
        }
        bool isFinilized = false;
        *(bool**)dlsym(h, "g_finilized") = &isFinilized;
        cout << boolalpha << isFinilized << endl;
        if (dlclose(h))
        {
            cerr << "ERROR: " << dlerror() << endl;
            return 2;
        }
        cout << boolalpha << isFinilized << endl;
    }
    return 0;
}

And the plugin's code is:

// libempty.cpp follows. Compile with g++ -std=c++0x libempty.cpp -o libempty.so -fPIC -shared
#include <iostream>
#include <vector>
using namespace std;
bool* g_finilized = nullptr;
struct Finilizer
{
    ~Finilizer()
    {
        cout << "~Finilizer()" << endl;
        if (g_finilized) *g_finilized = true;
    }
} g_finilizer;
class Base
{
public:
    virtual void init() = 0;
};
class Foo: public Base
{
    virtual void init()
    {
        static const vector<float> ns = { 0.f, 0.75f, 0.67f, 0.87f };
    }
};
extern "C" __attribute__ ((visibility ("default"))) Base* newBase() { return new Foo; }

If executed, the output is:

false
false
~Finilizer()

This shows the call to dlclose() doesn't work as expected and the library was not unloaded until the program's exit.

However, if we move the vector to outside the function, so the last 8 lines read:

class Foo: public Base
{
    virtual void init()
    {
    }
};
static const vector<float> ns = { 0.f, 0.75f, 0.67f, 0.87f };
extern "C" __attribute__ ((visibility ("default"))) Base* newBase() { return new Foo; }

Then dlclose() works properly and the output is:

false
~Finilizer()
true

The same results are generated if the vector is left in the function but no factory is exported:

class Foo: public Base
{
    virtual void init()
    {
        static const vector<float> ns = { 0.f, 0.75f, 0.67f, 0.87f };
    }
};
//extern "C" __attribute__ ((visibility ("default"))) Base* newBase() { return new Foo; }

Positive results are found if the vector is substituted with a C array:

class Foo: public Base
{
    virtual void init()
    {
        static const float ns[] = { 0.f, 0.75f, 0.67f, 0.87f };
    }
};
extern "C" __attribute__ ((visibility ("default"))) Base* newBase() { return new Foo; }

Is this a bug in GCC/Linux? Is there any workaround so that complex objects may be static-declared in an factorified class's member function?

Answer 1

What's happening is that there is a STB_GNU_UNIQUE symbol in libempty.so:

readelf -Ws libempty.so | grep _ZGVZN3Foo4initEvE2ns
 91: 0000000000203e80     8 OBJECT  UNIQUE DEFAULT   25 _ZGVZN3Foo4initEvE2ns
 77: 0000000000203e80     8 OBJECT  UNIQUE DEFAULT   25 _ZGVZN3Foo4initEvE2ns

The problem is that STB_GNU_UNIQUE symbols work quite un-intuitively, and persist across dlopen/dlclose calls.

The use of that symbol forces glibc to mark your library as non-unloadable here.

There are other surprises with GNU_UNIQUE symbols as well. If you use sufficiently recent gold linker, you can disable the GNU_UNIQUE with --no-gnu-unique flag.