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fi<br>
gcc -o dlTest dlTest.c -ldl<br>
echo Current LD_LIBRARY_PATH=$LD_LIBRARY_PATH<br>
dlTest<br>
<br>
<br>
<br>
<br>
结束语<br>
创建能在运行时被动态链接到 Linux 系统上的应用程序的共享目标代码是一项非常简单的练习。应用程序通过使用对动态链接装入器的 dlopen、dlsym 和 dlclose 函数调用来获取对共享目标文件的访问。dlerror 以字符串的形式返回任何错误，这些错误信息字符串描述 dl 函数碰到的最后一个错误。在运行时，主应用程序使用绝对路径或相对于 LD_LIBRARY_PATH 的相对路径找到共享目标库，并且请求所需的 dll 入口点的地址。当需要时，也可对 dll 进行间接函数调用，最后，关闭到共享目标文件的句柄，并且从内存中取消该目标文件映射，使之不可用。<br>
<br>
使用附加选项 -fpic 或 -fPIC 编译共享目标代码，以产生位置无关的代码，使用 -shared 选项将目标代码放进共享目标库中。<br>
<br>
Linux 中的共享目标代码库和动态链接装入器向应用程序提供了额外的功能。减少了磁盘上和内存里的可执行文件的大小。可以在需要时，装入可选的应用程序功能，可以在无须重新构建整个应用程序的情况下修正缺陷，并且应用程序可以包含第三方的插件。<br>
<br>
清单（应用程序和 dll）<br>
<br>
<br>
dlTest.c:<br>
<br>
/*************************************************************/<br>
/* Test Linux Dynamic Function Loading */<br>
/* */<br>
/* void *dlopen(const char *filename, int flag) */<br>
/* Opens dynamic library and return handle */<br>
/* */<br>
/* const char *dlerror(void) */<br>
/* Returns string describing the last error. */<br>
/* */<br>
/* void *dlsym(void *handle, char *symbol) */<br>
/* Return pointer to symbol's load point. */<br>
/* If symbol is undefined, NULL is returned. */<br>
/* */<br>
/* int dlclose (void *handle) */<br>
/* Close the dynamic library handle. */<br>
/* */<br>
/* */<br>
/* */<br>
/*************************************************************/<br>
#include<stdio.h><br>
#include <stdlib.h><br>
<br>
/* */<br>
/* 1-dll include file and variables */<br>
/* */<br>
#include <dlfcn.h><br>
void *FunctionLib; /* Handle to shared lib file */<br>
int (*Function)(); /* Pointer to loaded routine */<br>
const char *dlError; /* Pointer to error string */<br>
<br>
main( argc, argv )<br>
{<br>
int rc; /* return codes */<br>
char HelloMessage[] = "HeLlO WoRlD";<br>
<br>
/* */<br>
/* 2-print the original message */<br>
/* */<br>
printf(" dlTest 2-Original message ");<br>
printf("%s", HelloMessage);<br>
<br>
/* */<br>
/* 3-Open Dynamic Loadable Libary with absolute path */<br>
/* */<br>
FunctionLib = dlopen("/home/dlTest/UPPERCASE.so",RTLD_LAZY);<br>
dlError = dlerror();<br>
printf(" dlTest 3-Open Library with absolute path return-%s- ", dlError);<br>
if( dlError ) exit(1);<br>
<br>
/* */<br>
/* 4-Find the first loaded function */<br>
/* */<br>
Function = dlsym( FunctionLib, "printUPPERCASE");<br>
dlError = dlerror();<br>
printf(" dlTest 4-Find symbol printUPPERCASE return-%s- ", dlError);<br>
if( dlError ) exit(1);<br>
<br>
/* */<br>
/* 5-Execute the first loaded function */<br>
/* */<br>
rc = (*Function)( HelloMessage );<br>
printf(" dlTest 5-printUPPERCASE return-%s- ", dlError);<br>
<br>
/* */<br>
/* 6-Close the shared library handle */<br>
/* Note: after the dlclose, "printUPPERCASE" is not loaded */<br>
/* */<br>
rc = dlclose(FunctionLib);<br>
dlError = dlerror();<br>
printf(" dlTest 6-Close handle return-%s-",dlError); <br>
if( rc ) exit(1);<br>
<br>
<br>
/* */<br>
/* 7-Open Dynamic Loadable Libary using LD_LIBRARY path */<br>
/* */<br>
FunctionLib = dlopen("lowercase.so",RTLD_LAZY);<br>
dlError = dlerror();<br>
printf(" dlTest 7-Open Library with relative path return-%s- ", dlError);<br>
if( dlError ) exit(1);<br>
<br>
/* */<br>
/* 8-Find the second loaded function */<br>
/* */<br>
Function = dlsym( FunctionLib, "printLowercase");<br>
dlError = dlerror();<br>
printf(" dlTest 8-Find symbol printLowercase return-%s- ", dlError);<br>
if( dlError ) exit(1);<br>
<br>
/* */<br>
/* 8-execute the second loaded function */<br>
/* */<br>
rc = (*Function)( HelloMessage );<br>
printf(" dlTest 9-printLowercase return-%s- ", dlError);<br>
<br>
/* */<br>
/* 10-Close the shared library handle */<br>
/* */<br>
rc = dlclose(FunctionLib);<br>
dlError = dlerror();<br>
printf(" dlTest 10-Close handle return-%s-",dlError); <br>
if( rc ) exit(1);<br>
<br>
return(0);<br>
<br>
}<br>
<br>
<br>
<br>
<br>
<br>
<br>
UPPERCASE.c:<br>
<br>
/************************************************/<br>
/* Function to print input string as UPPER case. */<br>
/* Returns 1. */<br>
/*********************************************** */<br>
<br>
int printUPPERCASE ( inLine )<br>
char inLine[];<br>
{<br>
char UPstring[256];<br>
char *inptr, *outptr;<br>
<br>
inptr = inLine;<br>
outptr = UPstring;<br>
while ( *inptr != '</FONT><br>
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