使用教材

《“笨办法” 学C语言（Learn C The Hard Way）》
https://www.jianshu.com/p/b0631208a794

• 完整源码 : learn-c-the-hard-way-lectures/ex29/

https://github.com/zedshaw/learn-c-the-hard-way-lectures/tree/master/ex29

命令行操作

• 首先，编译 .c文件 ，生成 .so文件

$ cc -fPIC -c libex29.c -o libex29.o
$ ls
dbg.h  ex29.c  libex29.c  libex29.o

$ cc -shared -o libex29.so libex29.o
$ ls
dbg.h  ex29.c  libex29.c  libex29.o  libex29.so

$ cc -Wall -g -DNDEBUG ex29.c -ldl -o ex29
$ ls
dbg.h  ex29  ex29.c  libex29.c  libex29.o  libex29.so

• 然后，在命令行输入函数名 参数，就可以直接调用函数（函数名来自于ex29.c文件里面自己写的函数）

$ ./ex29 ./libex29.so print_a_message "hello world"
A STRING: hello world

$ ./ex29 ./libex29.so uppercase "hello there"
HELLO THERE

$ ./ex29 ./libex29.so lowercase "HELLO WorlD"
hello world

• 故意诱发各种错误（测试函数 fail_on_purpose、输入一个不存在的函数名are_you_here?、输入一个不存在的库lib.so）

$ ./ex29 ./libex29.so fail_on_purpose "i fall"
[ERROR] (ex29.c:27: errno: None) Function fail_on_purpose return 1 for data: i fall
$ ./ex29 ./libex29.so fail_on_purpose
[ERROR] (ex29.c:10: errno: None) USAGE: ex29 libex29.so function data

$ ./ex29 ./libex29.so are_you_here?
[ERROR] (ex29.c:10: errno: None) USAGE: ex29 libex29.so function data
$ ./ex29 ./libex29.so are_you_here? are_you?
[ERROR] (ex29.c:23: errno: None) Did not find are_you_here? function in the library ./libex29.so: ./libex29.so: undefined symbol: are_you_here?

$ ./ex29 ./lib.so aaaaaa ccccccc
[ERROR] (ex29.c:18: errno: None) Failed to open the library ./lib.so: ./lib.so: cannot open shared object file: No such file or directory

完整源码

libex29.c

#include <stdio.h>
#include <ctype.h>
#include "dbg.h"


int print_a_message(const char *msg)
{
    printf("A STRING: %s\n", msg);

    return 0;
}


int uppercase(const char *msg)
{
    int i = 0;

    // BUG: \0 termination problems
    for(i = 0; msg[i] != '\0'; i++) {
        printf("%c", toupper(msg[i]));
    }

    printf("\n");

    return 0;
}

int lowercase(const char *msg)
{
    int i = 0;

    // BUG: \0 termination problems
    for(i = 0; msg[i] != '\0'; i++) {
        printf("%c", tolower(msg[i]));
    }

    printf("\n");

    return 0;
}

int fail_on_purpose(const char *msg)
{
    return 1;
} 

ex29.c

#include <stdio.h>
#include "dbg.h"
#include <dlfcn.h>

typedef int (*lib_function) (const char *data);

int main(int argc, char *argv[])
{
    int rc = 0;
    check(argc == 4, "USAGE: ex29 libex29.so function data");

    char *lib_file = argv[1];
    char *func_to_run = argv[2];
    char *data = argv[3];

    void *lib = dlopen(lib_file, RTLD_NOW);
    check(lib != NULL, "Failed to open the library %s: %s", lib_file,
            dlerror());

    lib_function func = dlsym(lib, func_to_run);
    check(func != NULL,
            "Did not find %s function in the library %s: %s", func_to_run,
            lib_file, dlerror());

    rc = func(data);
    check(rc == 0, "Function %s return %d for data: %s", func_to_run,
            rc, data);

    rc = dlclose(lib);
    check(rc == 0, "Failed to close %s", lib_file);

    return 0;

error:
    return 1;
}

概念解释

• 静态库，就是一系列 .o文件；
• 动态库，典型的后缀名有.so .dll，在运行程序时，操作系统会动态地加载这些文件，并且将它们随时链接到你的程序上；

代码说明

• ( ex29.c ) typedef int (*lib_function) (const char *data); ，函数指针；
• ( ex29.c ) void *lib = dlopen(lib_file, RTLD_NOW); ，调用dlopen函数来加载lib_file所表示的库
• ( ex29.c ) lib_function func = dlsym(lib, func_to_run); ，调用 dlsym函数，通过使用函数func_to_run中的字符串名称，从lib中获取了一个函数，从而实现，通过命令行argv得到字符串，之后通过这个字符串动态地获取一个指向函数的指针

[举 例 说 明]
/****    ex29.c    ****/
  char *lib_file = argv[1];
  char *func_to_run = argv[2];
  char *data = argv[3];
/****    ex29.c    ****/

$  ./ex29     ./libex29.so   print_a_message   "hello world"
argv[1] = ./libex29.so  
argv[2] = print_a_message  
argv[3] = hello world 

参考资料

• dlopen(3) - Linux man page void *lib = dlopen(lib_file, RTLD_NOW);
  https://linux.die.net/man/3/dlopen

• 解决：/usr/bin/ld: libex29.o: relocation R_X86_64_32 against `.rodata'
  can not be used when making a shared object; recompile with -fPIC
  libex29.o: error adding symbols: Bad value
  collect2: error: ld returned 1 exit status
  https://www.jianshu.com/p/91edc58041fb

• 函数指针 typedef int (*lib_function) (const char *data);
  https://www.jianshu.com/p/a63b9575c86a