C语言入门实践之Linux动态链接

在上一篇博文中已经可以开发简单的C程序，并成功引用已有静态库。

对于中小规模的程序很好，程序达到一定规模就有问题了，磁盘空间和运行时占用空间都会成问题，因为静态链接是在编译阶段把所有以来都打包到程序中，运行时也会一次性被加载到内存中。

Linux中动态链接库（.so）有两种使用方式，动态链接和动态加载：

动态链接

动态链接需要动态库在链接时候对程序可见，动态库制作方法如下：

gcc -Wall -shared -fPIC -o libjoin.so join.c

 
或者

gcc -Wall -fPIC -c join.c
gcc -Wall -shared -fPIC join.o -o libjoin.so

 
其中用到的几个选项：

• -Wall: include warnings. See man page for warnings specified.
• -fPIC: Compiler directive to output position independent code, a characteristic required by shared libraries. Also see fpic.
• -shared: Produce a shared object which can then be linked with other objects to form an executable.

在gcc的man手册中-shared让编译器生成共享库，否则没有main方法的代码无法独立编译；-fPIC是-shared子选项，告诉编译器生成位置无关的代码。
 
经过以上步骤动态库已经编译好，但是如果要在程序中引用，还需要想办法告诉系统.so库的位置。
 
编译器通过/etc/ld.so.conf查找动态库。ldconfig用于配置运行时动态库查找路径，实际是更新/etc/ld.so.cache。
 
通过ld --verbose实际查看，默认加载路径如下：
 

 
到这里其实有三种方法可以让编译器能够寻到我的.so库：
1、修改ld.so.conf配置，把.so所在目录加入进去
2、建立软连接或者将.so直接拷贝过去
3、ldconfig `pwd`，直接让编译器到当前目录来加载
 
这里直接把.so拷贝到/usr/lib下，测试通过；
 
作为测试程序这样是做法没问题的，但是如果是正式发布的程序，需要考虑到部署、卸载过程，软连接方式更为稳妥。
 
运行时动态加载

上面的案例演示了动态链接库的一种使用方式即由Linux系统自动链接和加载动态库。

Linux提供了另外一组API，使用户程序在运行期间动态加载特定动态库而无需重新编译，为Apache、Nginx等模块化可插拔软件提供了可能性。

 

The dlopen function also automatically resolves dependencies in shared libraries. In this way, if you open an object that is dependent upon other shared libraries, it automatically loads them. The function returns a handle that is used in subsequent calls to the API. The prototype for dlopen is:

 

#include <dlfcn.h>
/**
 * Makes an object file accessible to a program
 * @param file libarary file name(full path) 
 * @param mode  RTLD_LAZY/RTLD_NOW/RTLD_GLOBAL
 */ 
void *dlopen( const char *file, int mode );

 
With a handle to the ELF object, you can identify addresses to symbols within this object using the dlsym call. This function takes a symbol name, such as the name of a function contained within the object. The return value is a resolved address to the symbol within the object:

 

/**
 * Obtains the address of a symbol within a dlopened object file
 * @param handle library handle obtained by dlopen 
 * @param name method name
 */
void *dlsym( void *handle, const char *name );

 

If an error occurs during a call with this API, you can use the dlerror function to return a human-readable string representing the error. This function has no arguments and returns a string if a prior error occurred or returns NULL if no error occurred:

/**
 * Returns a string error of the last error that occurred
 */
char *dlerror();

Finally, when no additional calls to the shared object are necessary, the application can call dlclose to inform the operating system that the handle and object references are no longer necessary. This is properly reference-counted, so that multiple users of a shared object do not conflict with one another (it remains in memory as long as there is a user for it). Any symbols resolved through dlsym for the closed object will no longer be available.

/**
 * Closes(unload) an object file
 * @param handle of library object
 */
char *dlclose( void *handle );

  

示例代码

hello_l.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <dlfcn.h>

void hello(char* name, char* familyName){
    void* lib_handle;
    char* (*fn)(char* f, char* l);
    char* error;
    lib_handle = dlopen("/home/daiyma/work/dynamic/libjoin.so", RTLD_LAZY);
    if (!lib_handle){
        fprintf(stderr, "%s\n", dlerror());
        exit(1);
    }

    fn = dlsym(lib_handle, "join");
    if ((error = dlerror()) != NULL){
        fprintf(stderr, "%s\n", error);
        exit(1);
    }

    char* who = (*fn)(name, familyName);
    printf("Hi, i am :%s\n", who);

    free(who);
    dlclose(lib_handle);
}

 这里仍然沿用之前的示例，只是动态库使用方式有变化（hello.c变为hello_l.c）。

 

编译代码：

 gcc -o main -rdynamic -ldl -L. -ljoin main.c hello_l.c

 运行结果：

 

有一点需要注意，通过ldd main查看，已经看不到所依赖的动态库了：

 

因为编译的时候增加了一下两个选项：

  -rdynamic tell the linker to add all symbols to the dynamic symbol table (to permit backtraces with the use of dlopen).
  -ldl indicates that the dllib should be linked to this program
参考连接：
[1]https://www.ibm.com/developerworks/library/l-dynamic-libraries/index.html
[2]http://www.yolinux.com/TUTORIALS/LibraryArchives-StaticAndDynamic.html
[3]https://typecodes.com/cseries/gcclderrlibrarypath.html