C语言插件机制(下) 转

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前言
上一篇文章简单介绍了*NIX下的动态库的使用,我们在这篇文章中实现一个计算器,计算器程序calc本身不做运算,只是将操作数传递给具体的插件(adder, suber, muler, diver)来完成实际运算。首先,计算器根据插件配置文件plugin.xml来确定插件的位置,名称,入口符号的定义,然后依次调用各个插件完成计算。



插件列表
文中涉及到的插件定义在plugin.xml中,文档结构如下:




Xml代码
<plugins>  
    <plugin name="adder">  
        <library path="/home/juntao/.libs/adder.so">  
        </library>  
        <entry name="add">  
        </entry>  
    </plugin>  
    <plugin name="suber">  
        <library path="/home/juntao/.libs/suber.so">  
        </library>  
        <entry name="sub">  
        </entry>  
   </plugin>  
   <plugin name="muler">  
        <library path="/home/juntao/.libs/muler.so">  
        </library>  
        <entry name="mul">  
        </entry>  
   </plugin>  
   <plugin name="diver">  
        <library path="/home/juntao/.libs/diver.so">  
        </library>  
        <entry name="div">  
        </entry>  
   </plugin>  
</plugins>  

<plugins>
    <plugin name="adder">
        <library path="/home/juntao/.libs/adder.so">
        </library>
        <entry name="add">
        </entry>
    </plugin>
    <plugin name="suber">
        <library path="/home/juntao/.libs/suber.so">
        </library>
        <entry name="sub">
        </entry>
   </plugin>
   <plugin name="muler">
        <library path="/home/juntao/.libs/muler.so">
        </library>
        <entry name="mul">
        </entry>
   </plugin>
   <plugin name="diver">
        <library path="/home/juntao/.libs/diver.so">
        </library>
        <entry name="div">
        </entry>
   </plugin>
</plugins> 

每个插件为一个plugin标签,plugin标签中包含library, entry两个字标签,分别定义动态库文件的路径及名称和插件函数的入口。为了简便,我们不重复设计list及xml解析,这里使用libxml2作为xml的分析器,GLIB中的GSList(单链表)来作为插件列表的链表对象。



每一个插件在C语言中的定义如下,非常简单(plugin.h)




C代码
#ifndef _PLUGIN_H_   
#define _PLUGIN_H_   
  
typedef struct{   
    char name[64];   
    char path[256];   
    char entry[128];   
    int version;   
}Plugin;   
  
#endif  

#ifndef _PLUGIN_H_
#define _PLUGIN_H_

typedef struct{
    char name[64];
    char path[256];
    char entry[128];
    int version;
}Plugin;

#endif


这里为了行文方便,Plugin结构中的字符串为静态尺寸。



计算器
计算器调用parseconf模块中的load_plugins将plugin.xml中定义的Plugin加载进一个GSList,以备后用:




C代码
#include "plugin.h"   
  
extern int load_plugins(char *config, GSList **list);  

#include "plugin.h"

extern int load_plugins(char *config, GSList **list); 

插件中的函数原型应该符合接口定义:




C代码
//pointer to function, which return a double, and get 2 double as input   
double (*pfunc)(double a, double b);  

//pointer to function, which return a double, and get 2 double as input
double (*pfunc)(double a, double b); 


计算器的主要代码如下:


C代码
int calc_test(double a, double b){   
    GSList *list = NULL, *it = NULL;   
    Plugin *pl = NULL;   
       
    //insert a null node into list at first   
    list = g_slist_append(list, NULL);   
       
    int code = 0;   
    double result;   
  
    //load plugin defined in plugin.xml into list   
    load_plugins("plugin.xml", &list);   
       
    for(it = list; it != NULL; it = it->next){   
        pl = (Plugin *)it->data;   
        if(pl == NULL){   
            continue;   
        }else{   
            //open the library   
            flib = dlopen(pl->path, RTLD_LAZY);   
            dlError = dlerror();   
  
            if(dlError){   
                fprintf(stderr, "open %s failed\n", pl->name);   
                g_slist_free(list);   
                return -1;   
            }   
               
            //get the entry   
            *(void **)(&pfunc) = dlsym(flib, pl->entry);   
            dlError = dlerror();   
            if(dlError){   
                fprintf(stderr, "find symbol %s failed\n", pl->entry);   
                g_slist_free(list);   
                return -1;   
            }   
  
            //call the function   
            result = (*pfunc)(a, b);   
            printf("%s(%f, %f) = %f\n", pl->entry, a, b, result);   
               
            //then close it   
            code = dlclose(flib);   
            dlError = dlerror();   
  
            if(code){   
                fprintf(stderr, "close lib error\n");   
                g_slist_free(list);   
                return -1;   
            }   
        }   
    }   
       
    g_slist_free(list);   
    return 0;   
}  

int calc_test(double a, double b){
    GSList *list = NULL, *it = NULL;
    Plugin *pl = NULL;
    
    //insert a null node into list at first
    list = g_slist_append(list, NULL);
    
    int code = 0;
    double result;

    //load plugin defined in plugin.xml into list
    load_plugins("plugin.xml", &list);
    
    for(it = list; it != NULL; it = it->next){
        pl = (Plugin *)it->data;
        if(pl == NULL){
            continue;
        }else{
            //open the library
            flib = dlopen(pl->path, RTLD_LAZY);
            dlError = dlerror();

            if(dlError){
                fprintf(stderr, "open %s failed\n", pl->name);
                g_slist_free(list);
                return -1;
            }
            
            //get the entry
            *(void **)(&pfunc) = dlsym(flib, pl->entry);
            dlError = dlerror();
            if(dlError){
                fprintf(stderr, "find symbol %s failed\n", pl->entry);
                g_slist_free(list);
                return -1;
            }

            //call the function
            result = (*pfunc)(a, b);
            printf("%s(%f, %f) = %f\n", pl->entry, a, b, result);
            
            //then close it
            code = dlclose(flib);
            dlError = dlerror();

            if(code){
                fprintf(stderr, "close lib error\n");
                g_slist_free(list);
                return -1;
            }
        }
    }
    
    g_slist_free(list);
    return 0;
} 

首先,定义一个GSList,然后将其传递给load_plugins,load_plugins解析plugin.xml,然后填充list返回,calc_test遍历插件列表,并调用每一个插件定义的entry.



除法器
我们来看一个具体的插件:做除法的模块




C代码
#include <stdio.h>   
  
double div(double a, double b){   
    if(b == 0){   
        fprintf(stderr, "div zero error\n");   
        return -1;   
    }else{   
        return a / b;   
    }   
}  

#include <stdio.h>

double div(double a, double b){
    if(b == 0){
        fprintf(stderr, "div zero error\n");
        return -1;
    }else{
        return a / b;
    }
}
diver.c在编译之后,生成diver.so,将其置于plugin.xml定义的位置处即可。



运行结果如下图所示:

C语言插件机制(下) 转

其他代码如xml的解析,GSList的使用等与插件机制关系不大,感兴趣的朋友可以在附件中查看。


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