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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:
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。