c++ - Multiple instances of singleton across shared libraries on Linux

Question

My question, as the title mentioned, is obvious, and I describe the scenario in details. There is a class named singleton implemented by singleton pattern as following, in file singleton.h:

/*
 * singleton.h
 *
 *  Created on: 2011-12-24
 *      Author: bourneli
 */

#ifndef SINGLETON_H_
#define SINGLETON_H_

class singleton
{
private:
    singleton() {num = -1;}
    static singleton* pInstance;
public:
    static singleton& instance()
    {
        if (NULL == pInstance)
        {
            pInstance = new singleton();
        }
        return *pInstance;
    }
public:
    int num;
};

singleton* singleton::pInstance = NULL;

#endif /* SINGLETON_H_ */

then, there is a plugin called hello.cpp as following:

#include <iostream>
#include "singleton.h"

extern "C" void hello() {
    std::cout << "singleton.num in hello.so : " << singleton::instance().num << std::endl;
    ++singleton::instance().num;
    std::cout << "singleton.num in hello.so after ++ : " << singleton::instance().num << std::endl;
}

you can see that the plugin call the singleton and change the attribute num in the singleton.

last, there is a main function use the singleton and the plugin as following:

#include <iostream>
#include <dlfcn.h>
#include "singleton.h"

int main() {
    using std::cout;
    using std::cerr;
    using std::endl;

    singleton::instance().num = 100; // call singleton
    cout << "singleton.num in main : " << singleton::instance().num << endl;// call singleton

    // open the library
    void* handle = dlopen("./hello.so", RTLD_LAZY);

    if (!handle) {
        cerr << "Cannot open library: " << dlerror() << '
';
        return 1;
    }

    // load the symbol
    typedef void (*hello_t)();

    // reset errors
    dlerror();
    hello_t hello = (hello_t) dlsym(handle, "hello");
    const char *dlsym_error = dlerror();
    if (dlsym_error) {
        cerr << "Cannot load symbol 'hello': " << dlerror() << '
';
        dlclose(handle);
        return 1;
    }

    hello(); // call plugin function hello

    cout << "singleton.num in main : " << singleton::instance().num << endl;// call singleton
    dlclose(handle);
}

and the makefile is following:

example1: main.cpp hello.so
    $(CXX) $(CXXFLAGS)  -o example1 main.cpp -ldl

hello.so: hello.cpp
    $(CXX) $(CXXFLAGS)  -shared -o hello.so hello.cpp

clean:
    rm -f example1 hello.so

.PHONY: clean

so, what is the output? I thought there is following:

singleton.num in main : 100
singleton.num in hello.so : 100
singleton.num in hello.so after ++ : 101
singleton.num in main : 101

however, the actual output is following:

singleton.num in main : 100
singleton.num in hello.so : -1
singleton.num in hello.so after ++ : 0
singleton.num in main : 100

It proves that there are two instances of the singleton class.

Why?

Answer 1

First, you should generally use -fPIC flag when building shared libraries.

Not using it "works" on 32-bit Linux, but would fail on 64-bit one with an error similar to:

/usr/bin/ld: /tmp/ccUUrz9c.o: relocation R_X86_64_32 against `.rodata' can not be used when making a shared object; recompile with -fPIC

Second, your program will work as you expect after you add -rdynamic to the link line for the main executable:

singleton.num in main : 100
singleton.num in hello.so : 100
singleton.num in hello.so after ++ : 101
singleton.num in main : 101

In order to understand why -rdynamic is required, you need to know about the way dynamic linker resolves symbols, and about the dynamic symbol table.

First, let's look at the dynamic symbol table for hello.so:

$ nm -C -D hello.so | grep singleton
0000000000000b8c W singleton::instance()
0000000000201068 B singleton::pInstance
0000000000000b78 W singleton::singleton()

This tells us that there are two weak function definitions, and one global variable singleton::pInstance that are visible to the dynamic linker.

Now let's look at the static and dynamic symbol table for the original example1 (linked without -rdynamic):

$ nm -C  example1 | grep singleton
0000000000400d0f t global constructors keyed to singleton::pInstance
0000000000400d38 W singleton::instance()
00000000006022e0 B singleton::pInstance
0000000000400d24 W singleton::singleton()

$ nm -C -D example1 | grep singleton
$ 

That's right: even though the singleton::pInstance is present in the executable as a global variable, that symbol is not present in the dynamic symbol table, and therefore "invisible" to the dynamic linker.

Because the dynamic linker "doesn't know" that example1 already contains a definition of singleton::pInstance, it doesn't bind that variable inside hello.so to the existing definition (which is what you really want).

When we add -rdynamic to the link line:

$ nm -C  example1-rdynamic | grep singleton
0000000000400fdf t global constructors keyed to singleton::pInstance
0000000000401008 W singleton::instance()
00000000006022e0 B singleton::pInstance
0000000000400ff4 W singleton::singleton()

$ nm -C -D  example1-rdynamic | grep singleton
0000000000401008 W singleton::instance()
00000000006022e0 B singleton::pInstance
0000000000400ff4 W singleton::singleton()

Now the definition of singleton::pInstance inside the main executable is visible to the dynamic linker, and so it will "reuse" that definition when loading hello.so:

LD_DEBUG=bindings ./example1-rdynamic |& grep pInstance
     31972: binding file ./hello.so [0] to ./example1-rdynamic [0]: normal symbol `_ZN9singleton9pInstanceE'