添加到动态库中的函数指针映射

Adding to function pointer map in dynamic library

本文关键字：函数 指针 映射 动态 添加      更新时间：2023-10-16

下面的两个代码部分显示了重要信息。真正的工作在第三个代码部分main.cpp.中完成

// For all files:
typedef void (*func_ptr)(Event&); // Event is a class with certain methods

foo.cpp

extern "C" void init(std::map<std::string, func_ptr> *func_ptrs)
{
    std::cout<<std::endl<<"Initializing..."<<std::endl; // does appear in output, so function does execute
    (*func_ptrs)["test"] = stuff; // the stuff function below
}
// this is the function we are trying to add to the func_ptrs map
// It looks like it's added in main.cpp, but calling it in main.cpp seg faults
void stuff(Event &e)
{
    std::cout<<"In stuff..."<<std::endl; // seg faults before this line
    // do things with e
}

main.cpp

// Excluding fluff for brevity, use comments to understand location of problem
// In main....
map<string, func_ptr> func_ptrs; // map of strings to func_ptrs
void (*init)(map<string, func_ptr>*);
// some other stuff
handle = dlopen("libfoo.so", RLTD_NOW);
// library opens successfully
*(void **) (&init) = dlsym(handle, "init"); // get the init function from the library
// No, I don't completely understand the above sequence of pointers and reference, but it does open successfully
(*init)(&func_ptrs);
// Executes successfully
// Iterating over func_ptrs shows that the new "test" mapping is in func_ptrs!!
// Next, this should call the stuff function in foo.cpp that was assigned to func_ptrs["test"]
(func_ptrs["test"])(event); // seg fault

综上所述，

  1. libfoo.so库正在正确加载
  2. 正在正确检索libfoo.so中的init符号
  3. 运行libfoo的init((后，映射键"test"在main.cpp中可见

错误：试图执行libfoo的东西，位于main.cpp的映射func_ptrs中，seg错误

注意：另一种选择是向main.cpp提供一个"添加到映射"函数，libfoo.so可以调用该函数将映射添加到函数列表中。这看起来更容易/更好吗？

我找到了很多类似的问题/答案，但没有一个与我所处的情况有足够密切的联系。非常感谢你们的帮助，我还没有失望。希望我没有把缩短代码搞砸。

此代码适用于gcc版本4.7.2 20121109:

d_a.cc:

#include <iostream>
#include <map>
#include <string>
#include <dlfcn.h>
class Event { 
  public: 
    void method1() { std::cout << "method1: " << name << std::endl; }
    Event(std::string& n):name(n) {}
  private: std::string name;
};
typedef void (*func_ptr)(Event&); 
int main(int argc, const char **argv) {
  std::string name = "Some";
  Event ev(name);
  std::map<std::string, func_ptr> func_ptrs; 
  void (*init)(std::map<std::string, func_ptr>*);
  void *handle = dlopen("./d_l.so", RTLD_NOW);
  *(void **) (&init) = dlsym(handle, "init");
  (*init)(&func_ptrs);
  (func_ptrs["test"])(ev);
  return 0;
}

d_l.cc:

#include <map>
#include <string>
#include <iostream>
void stuff(Event& e)
{ 
  e.method1();
  std::cout<<"In stuff..."<<std::endl; 
}
extern "C" void init(std::map<std::string, func_ptr> *func_ptrs)
{ 
  std::cout<<std::endl<<"Initializing..."<<std::endl; 
  (*func_ptrs)["test"] = stuff; 
}

编译：

g++ d_a.cc -ldl
g++ d_l.cc -fPIC -shared -o d_l.so

执行：

./a.out

输出：

Initializing...
method1: Some
In stuff...
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