与dlopen / dlsym一起使用时dynamic_cast失败 [英] dynamic_cast fails when used with dlopen/dlsym

问题描述

让我为这个长期的问题道歉。

Let me apologise upfront for the long question. It is as short as I could make it, which is, unfortunately, not very short.

我定义了两个接口A和B：

I have defined two interfaces, A and B:

class A // An interface
{
public:
  virtual ~A() {}

  virtual void whatever_A()=0;
};

class B // Another interface
{
public:
  virtual ~B() {}

  virtual void whatever_B()=0;
};

然后，我有一个共享的库 testc，构造了类C的对象，同时实现了A和B ，然后传递指向其A接口的指针：

Then, I have a shared library "testc" constructing objects of class C, implementing both A and B, and then passing out pointers to their A-interface:

class C: public A, public B
{
public:
  C();
  ~C();

  virtual void whatever_A();
  virtual void whatever_B();
};

A* create()
{
  return new C();
}

最后，我有第二个共享库 testd，它需要一个 A * 作为输入，并尝试使用 dynamic_cast 将其转换为 B * code>

Finally, I have a second shared library "testd", which takes a A* as input, and tries to cast it to a B*, using dynamic_cast

void process(A* a)
{
  B* b = dynamic_cast<B*>(a);
  if(b)
    b->whatever_B();
  else
    printf("Failed!\n");
}

最后，我有主应用程序，通过了 A * 在库之间：

Finally, I have main application, passing A*'s between the libraries:

A* a = create();
process(a);

问题

如果我构建主应用程序，链接到 testc和 testd库，一切按预期进行。但是，如果我修改主应用程序以不链接到'testc'和'testd'，而是在运行时使用 dlopen / dlsym加载它们，则 dynamic_cast 失败。

我不明白为什么。有任何线索吗？

I do not understand why. Any clues?

• 已通过gcc 4.4.1，libc6测试2.10.1（Ubuntu 9.10）


• 示例代码可用

• Tested with gcc 4.4.1, libc6 2.10.1 (Ubuntu 9.10)
• Example code available

推荐答案

我找到了问题的答案此处。据我了解，我需要将 testc中的typeinfo提供给 tested库。为此，在使用 dlopen（）时，还需要做两件事：

I found the answer to my question here. As I understand it, I need to make the typeinfo available in 'testc' available to the library 'testd'. To do this when using dlopen(), two extra things need to be done:

• 链接库时，将链接器的 -E 选项传递给链接器，以确保将所有符号导出到可执行文件，而不只是导出未在其中解析的符号（因为


• 使用 dlopen（）加载库时，添加 RTLD_GLOBAL 选项，以确保 testc 导出的符号也可用于 tested

• When linking the library, pass the linker the -E option, to make sure it exports all symbols to the executable, not just the ones that are unresolved in it (because there are none)
• When loading the library with dlopen(), add the RTLD_GLOBAL option, to make sure symbols exported by testc are also available to testd

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