how to create a shared object with high maintainab

问题:

I try to create a shared object for my C++ project with the purpose: every time I want to bring a new class, I recompile shared object only.

below is test code:

test.h:

#if !defined(_TEST_)
#define _TEST_

#include <string>
#include <memory>

class Test {
private:

public:
    Test() {

    }
    ~Test() {

    }

    virtual void show(const std::string &) = 0;
};

#ifdef __cplusplus
extern "C" {
#endif

// std::shared_ptr<Test> getObject();

#ifdef __cplusplus
}
#endif

#endif // _TEST_  

getObject.cpp: class TestSo derive from Test.

#include <memory>
#include "testso.h"

std::shared_ptr<Test> getObject()
{
    return std::make_shared<TestSo>();
}  

and the main.cpp:

#include "test.h"

#include <dlfcn.h>
#include <string>

#include <memory>

std::shared_ptr<Test> (*getTestSo)();

int main(int argc, char const *argv[])
{
    //...
    void *handle = dlopen("/usr/lib/libfunc.so", RTLD_NOW);
    if(handle == NULL)
        return 1;

    getTestSo = (std::shared_ptr<Test> (*)())dlsym(handle, "getObject");

    const char *dlmsg = dlerror();
    if(dlmsg != NULL) {
        printf("dlsym: %s\n", dlmsg);
        dlclose(handle);
        return 1;
    }

    std::shared_ptr<Test> classSo = getTestSo();

    classSo->show("Hi");

    dlclose(handle);
    //...
    return 0;
}  

I compile code with the following command line:
arm-none-linux-gnueabi-g++ -std=c++11 -c -fPIC getObject.cpp
arm-none-linux-gnueabi-g++ -shared getObject.o -o libtest.so
arm-none-linux-gnueabi-g++ -std=c++11 -L./ main.cpp -o test -ldl -ltest

compiler doesn't give me any warning or error. it says that undefined symbol: getObject when I run program.

finally I find that I must declare getObject in scope extern "C" within test.h.

dlsym fetch address of getObject from libtest.so and the program would jump to that address. why I must declare getObject? that is against my purpose. Or if I am totally going a wrong way, how do I do? if not, how can I fix it?

Edit:

arm-none-linux-gnueabi-nm -D libfunc.so 
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回答1:

You approach does not work, because C++ mangles names during compilation, maybe more on this topic can be found over the net or for example in this thread. When you compile a function named std::shared_ptr<Test> getObject() it ends up as a symbol named _Z9getObjectv, so in you need to open this symbol dlsym(handle, "_Z9getObjectv");

every time I want to bring a new class, I recompile shared object only.

I don't understand why do you don't want to just link against your program. dlsym was created to allow linker-related programs to handle the loading, the functionality seems not needed in your program. In case of your program simply adding a forward declaration of the function (without any extern C, because the name is mangled by C++):

std::shared_ptr<Test> getObject();

to your test.h file and then compiling with:

-ltest

as you do now(!) will link against shared library. That's exactly the reason why shared libraries exists in the first place - so you can change the underlying code without changing the client code. Also in case of linking the linker will issue symbol resolution error if the symbol is not found, so you have another layer of protection and you don't have to worry about mangled names. Just change your main.cpp to:

#include "test.h"

#include <dlfcn.h>
#include <string>

#include <memory>

int main(int argc, char const *argv[])
{
    std::shared_ptr<Test> getObject();
    std::shared_ptr<Test> (*getTestSo)() = getObject;

    std::shared_ptr<Test> classSo = getTestSo();

    return 0;
}  

and it should work (with proper LD_LIBRARY_PATH)...

回答2:

Below is a working example for you.

There is no need for extern "C" linkage as you can find out and use the mangled C++ name just fine.

Also, factory functions should return std::unique_ptr to convey to the user that they now own the object. Unless the factory keeps a reference to the object, in which case std::shared_ptr would be a good choice.

Shared library header:

// shared.h
#pragma once

#include <memory>
#include <string>
#include <iosfwd>

namespace shared {

struct Test {
    virtual ~Test() = 0;
    virtual void print(std::ostream&) const = 0;

    // Factory function.
    static std::unique_ptr<Test> create(std::string const& type);
};

inline std::ostream& operator<<(std::ostream& s, Test const& t) {
    t.print(s);
    return s;
}

} // namespace shared

Shared library source:

// shared.cc
#include "shared.h"
#include <iostream>

namespace {

struct A : shared::Test {
    void print(std::ostream& s) const override {
        s << __PRETTY_FUNCTION__;
    }
};

} // namespace

shared::Test::~Test() = default;

std::unique_ptr<shared::Test> shared::Test::create(std::string const& type) {
    std::unique_ptr<Test> result;
    if(type == "A")
        result.reset(new A);
    return result;
}

Build the shared library and find the mangled name of Test::create:

$ g++ -c -fPIC -W{all,extra,error} -std=c++11 shared.cc
$ g++ -o libshared.so -shared shared.o
$ nm --demangle --defined-only --dynamic libshared.so | grep Test::create
0000000000001894 T shared::Test::create(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&)
$ nm --defined-only --dynamic libshared.so | grep 0000000000001894
0000000000001894 T _ZN6shared4Test6createERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE

The user source:

// test.cc
#include "shared.h"
#include <iostream>
#include <dlfcn.h>

struct DlClose { void operator()(void* handle) const { ::dlclose(handle); } };
using dl_ptr = std::unique_ptr<void, DlClose>;

int main() {
    dl_ptr handle(::dlopen("./libshared.so", RTLD_NOW));
    if(!handle)
        std::abort();

    using FactoryFn = std::unique_ptr<shared::Test>(std::string const&);
    FactoryFn* shared_create = reinterpret_cast<FactoryFn*>(::dlsym(handle.get(), "_ZN6shared4Test6createERKNSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEE"));
    if(!shared_create)
        std::abort();

    auto a = shared_create("A");
    std::cout << *a << '\n';
}

Build and run the user code:

$ g++ -W{all,extra,error} -std=c++11 -o test -ldl test.cc
$ ./test
virtual void {anonymous}::A::print(std::ostream&) const