Question

我试图为我的C ++项目创建一个共享对象，其目的是：每次我想带来一个新类时，我只会重新编译共享对象。

下面是测试代码：

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：类TestSo从Test派生。

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

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

和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;
}

我使用以下命令行编译代码：
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

编译器没有给我任何警告或错误。它说我运行程序时undefined symbol: getObject。

最后，我发现我必须在getObject的范围extern "C"中声明test.h。

dlsym从libtest.so获取getObject的地址，程序将跳转到该地址。为什么我必须声明getObject？这违反了我的目的。或者，如果我完全走错了路，该怎么办？如果没有，我该如何解决？

编辑：

arm-none-linux-gnueabi-nm -D libfunc.so 
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0000f344 B __bss_start__
         U __cxa_begin_catch
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         U __cxa_end_cleanup
         w __cxa_finalize
         U __cxa_pure_virtual
         U __cxa_rethrow
0000f344 D _edata
0000f34c B _end
0000f34c B __end__
00006664 T _fini
         w __gmon_start__
         U __gxx_personality_v0
00003e18 T _init
         w _ITM_deregisterTMCloneTable
         w _ITM_registerTMCloneTable
         w _Jv_RegisterClasses
         w __pthread_key_create
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Answer 1

您的方法行不通，因为C ++会在编译期间更改名称，也许可以在网上或例如在this thread中找到有关此主题的更多信息。当您编译名为std::shared_ptr<Test> getObject()的函数时，它会以名为_Z9getObjectv的符号结尾，因此您需要打开此符号dlsym(handle, "_Z9getObjectv");

每次我想带一个新类时，我只会重新编译共享对象。

我不明白为什么您不希望仅链接到您的程序。 dlsym的创建是为了允许与链接程序有关的程序处理加载，您的程序似乎不需要此功能。对于您的程序，只需添加该函数的前向声明（不带任何外部C，因为名称由C ++修饰）：

std::shared_ptr<Test> getObject();

到您的test.h文件，然后使用：

-ltest

与现在一样（！）将链接到共享库。这就是共享库首先存在的原因-因此您可以更改基础代码而无需更改客户端代码。同样，在链接的情况下，如果找不到符号，链接器将发出符号解析错误，因此您具有另一层保护，您不必担心名称混乱。只需将您的main.cpp更改为：

#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;
}

它应该可以工作（具有正确的LD_LIBRARY_PATH）...

Answer 2

以下是适合您的示例。

不需要extern "C"链接，因为您可以找到并使用经过改写的C ++名称。

此外，工厂函数应返回std::unique_ptr，以向用户传达他们现在拥有该对象。除非工厂保留对该对象的引用，否则在这种情况下，std::shared_ptr是一个不错的选择。

共享库头：

// 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

共享库源：

// 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;
}

构建共享库并找到Test::create的错误名称：

$ 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

用户来源：

// 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';
}

构建并运行用户代码：

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

如何创建具有高可维护性的共享库？

2 个答案: