Write Your software base on plugin(C/C++ ABI)

一个软件，如果把所有的功能写进C++源码，维护，扩展，编译都特别麻烦。

共享库后缀名。Linux -> .so  Windows -> .dll

关于动态符号显示问题，具体可以看系统的API,现在做了个只支持Linux.

Linux 查看一个动态库的符号 nm -D plugin.so

注意Linux如果不设置符号隐藏，那么默认的动态库所有的符号都是暴露的。可以用下面的语句设置符号是可暴露。

#define TopVertexAPI __attribute__ ((visibility("default")))

为什么不读取C++类的符号，因为每个编译器编译C++的函数符号都是不一样，所以将要暴露的符号全部定义为C符号，

C符号不会被修饰。好做跨平台。关键字：extern "C"

设计方法:GLY_Plugin.h 主要管理插件读取,插件释放内存，查找符号。

　　GLY_MessagePluginAPI.h 主要是纯虚类,提供给用户的API

　　GLY_PluginRegister.h 主要是做符号显示隐藏

 //
 // Created by gearslogy on 5/9/16.
 //

 #ifndef API_DESIGN_GLY_PLUGIN_H
 #define API_DESIGN_GLY_PLUGIN_H

 #include <string>
 class GLY_Plugin
 {
 public:
     GLY_Plugin(std::string file);
     ~GLY_Plugin();
     void *operator->();
     void *getPlugin();
 private:
     std::string _dso_file;
     void *_handle;

     //a pointer to a function can point to ->  void * plugin_creator ()
     typedef void* (*plugin_creator)();

     // now create point can point any like ->  void * plugin_creator ()
     plugin_creator creator;

     //
     typedef void (*plugin_destroy)(void *);
     plugin_destroy destroyer;

     //plugin instance ,instance is a class handle
     void *_instance;
 };

 #endif //API_DESIGN_GLY_PLUGIN_H

GLY_Plugin.h

 //
 // Created by gearslogy on 5/9/16.
 //
 #ifdef __GNUC__
     #include <dlfcn.h>
 #endif
 #include "GLY_Plugin.h"
 #include <stdio.h>

 GLY_Plugin::GLY_Plugin(std::string file)
 {

     _dso_file = file;
     _handle = NULL;
     _instance = NULL;

     // LINUX platform
 #ifdef __GNUC__
     _handle = dlopen(_dso_file.c_str(),RTLD_LAZY);
     if(!_handle)
     {
         std::string so_open_error = file + " open error ";
         throw so_open_error;
     }

     //@creator() function return a pointers to the C++ class(_instance)
     creator = (plugin_creator)dlsym(_handle,"plugin_create");  // search the signal plugin_create
     if(creator == NULL)
     {
         dlclose(_handle);
         throw "plugin creator not found ";
     }
     destroyer = (plugin_destroy)dlsym(_handle, "plugin_destroy");
     if(destroyer == NULL)
     {
         dlclose(_handle);
         throw "plugin destroyer not found";
     }

     try
     {
         _instance = creator();
     }
     catch (...)
     {
         dlclose(_handle);
         _handle= NULL;
         throw std::exception();
     }
 #endif

 }
 GLY_Plugin::~GLY_Plugin()
 {
     if(_instance)
     {
         printf("GLY_Plugin Free the instance %s \n",_dso_file.c_str());
         destroyer(_instance); //free your dynamic library
     }
     if(_handle)
     {
         printf("GLY_Plugin Free the handle  %s \n",_dso_file.c_str());
         dlclose(_handle); // close the dynamic.so
     }
 }
 void *GLY_Plugin::operator->()
 {
     return  _instance;
 }
 void *GLY_Plugin::getPlugin()
 {
     return _instance;
 }

GLY_Plugin.cpp

 //
 // Created by gearslogy on 5/9/16.
 //

 #ifndef API_DESIGN_GLY_MESSAGEPLUGINAPI_H
 #define API_DESIGN_GLY_MESSAGEPLUGINAPI_H

 class MessagePlugin_interface
 {
 public:
     MessagePlugin_interface(){};
     virtual void cookMyMessage()=;
     virtual ~MessagePlugin_interface(){};

 };

 #endif //API_DESIGN_GLY_MESSAGEPLUGINAPI_H

GLY_MessagePluginAPI.h

 //
 // Created by gearslogy on 5/10/16.
 //

 #ifndef API_DESIGN_GLY_PLUGINREGISTER_H
 #define API_DESIGN_GLY_PLUGINREGISTER_H

 #ifdef _WIN32
     #define TopVertexAPI __declspec(dllexport)
 #else
     #define TopVertexAPI __attribute__ ((visibility("default")))
 #endif

 #define TopVertexHiddenAPI __attribute__((visibility("hidden")))

 // do not use C++ function style.
 extern "C" TopVertexAPI void *plugin_create();
 extern "C" TopVertexAPI void plugin_destroy(void *);

 #endif //API_DESIGN_GLY_PLUGINREGISTER_H

GLY_PluginRegister.h

主程序:

#include <iostream>
#include "GLY_Plugin.h"
#include "GLY_MessagePluginAPI.h"

using namespace std;

int main()
{
    GLY_Plugin MessagePlugin_dyn("./libapi_plugin.so");
    MessagePlugin_interface *message_plugin_handle = (MessagePlugin_interface*) (MessagePlugin_dyn.getPlugin());

    message_plugin_handle->cookMyMessage();

    return ;
}

制作一个插件

#include <GLY_MessagePluginAPI.h>
#include <stdio.h>
#include <GLY_PluginRegister.h>
class Message_Plugin:public MessagePlugin_interface
{
public:
    Message_Plugin()
    {
    }
    void cookMyMessage()
    {
        printf("MessagePlugin cookMyMessage Houdini function \n");
    }
    static Message_Plugin * plugin_create()
    {
        return new Message_Plugin;   // create the class pointer
    }
    static void *plugin_destroy(Message_Plugin *plugin)
    {
        delete plugin;
    }
    virtual ~Message_Plugin()
    {
    }
};

void *plugin_create()
{
    printf("plugin loading\n");
    return Message_Plugin::plugin_create();  // for our plugin system
}

void plugin_destroy(void *plugin)
{
    printf("plugin unloading\n");
    Message_Plugin::plugin_destroy((Message_Plugin*) plugin );
}

主程序运行结果：

plugin loading
MessagePlugin cookMyMessage Houdini function
GLY_Plugin Free the instance ./libapi_plugin.so
plugin unloading
GLY_Plugin Free the handle  ./libapi_plugin.so

GCC对一些属性的定义:

#if defined _WIN32 || defined __CYGWIN__
  #ifdef BUILDING_DLL
    #ifdef __GNUC__
      #define DLL_PUBLIC __attribute__ ((dllexport))
    #else
      #define DLL_PUBLIC __declspec(dllexport) // Note: actually gcc seems to also supports this syntax.
    #endif
  #else
    #ifdef __GNUC__
      #define DLL_PUBLIC __attribute__ ((dllimport))
    #else
      #define DLL_PUBLIC __declspec(dllimport) // Note: actually gcc seems to also supports this syntax.
    #endif
  #endif
  #define DLL_LOCAL
#else
  #if __GNUC__ >= 4
    #define DLL_PUBLIC __attribute__ ((visibility ("default")))
    #define DLL_LOCAL  __attribute__ ((visibility ("hidden")))
  #else
    #define DLL_PUBLIC
    #define DLL_LOCAL
  #endif
#endif