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
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