-L -Wl,-rpath-link -Wl,-rpath区别精讲

目录

• 前言
• 源码准备
  • 源码内容
  • 尝试编译，保证源码没有问题
• 编译
  • 首先编译world.c
  • 编译并链接hello.c
  • 调试编译test.c
• 结论

 

前言

关于gcc这三个参数，参考了诸多文档后，仍然理解上有偏差，仿照下面博客中的方法，自己调试了一波，总算是理解了。还是建议大家动手实践一下。

参考资料如下:

• https://blog.csdn.net/q1302182594/article/details/42102961
• https://blog.csdn.net/openme_openwrt/article/details/7860580

源码准备

新建三个文件：test.c hello.c world.c ，其源码依赖关系为：test.c 依赖 hello.c；hello.c 依赖 world.c

源码内容

test.c

#include <stdio.h>
void world(void);
void hello(void)
{
    printf("hello ");
    world();
}

hello.c

#include<stdio.h>
void hello(void);
void main(void)
{
    hello();
}

world.c

#include<stdio.h>
void world(void)
{
	printf("world.\n");
}

尝试编译，保证源码没有问题

# -o 指定输出文件
[[email protected] testc]# ls
hello.c  test.c  world.c
[[email protected] testc]# gcc -o hehe *.c
[[email protected] testc]# ls
hehe  hello.c  test.c  world.c
[[email protected] testc]# ./hehe 
hello world.

编译

首先编译world.c

# -shared 编译链接库
# -fPIC 我的理解是编译链接库时给代码动态分配内存
[[email protected] testc]# gcc -shared -fPIC -o libworld.so world.c
[[email protected] testc]# ls
hello.c  libworld.so  test.c  world.c
[[email protected] testc]# ldd libworld.so 
	linux-vdso.so.1 =>  (0x00007ffd7498f000)
	libc.so.6 => /lib64/libc.so.6 (0x00007fcb49815000)
	/lib64/ld-linux-x86-64.so.2 (0x00007fcb49dea000)

#上述命令和下面的等价，建议选取单条命令即可：gcc -shared -fPIC -o libworld.so world.c
# -c 只**预处理,编译,和汇编,也就是他只把程序做成obj文件
[[email protected] testc]# gcc -c -fPIC world.c 
[[email protected] testc]# ls
hello.c  test.c  world.c  world.o
[[email protected] testc]# gcc -shared -fPIC -o libworld.so world.o
[[email protected] testc]# ls
hello.c  libworld.so  test.c  world.c  world.o
[[email protected] testc]# ldd libworld.so 
	linux-vdso.so.1 =>  (0x00007ffd0dfa9000)
	libc.so.6 => /lib64/libc.so.6 (0x00007f4357dcb000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f43583a0000)

编译并链接hello.c

# -lxxx 指定需要动态链接的库文件
# -L 指定动态连接库文件的位置(编译时)
# . 指当前路径
# 下面编译出的libhello.so文件已经显式依赖libworld.so文件，但是没有找到libworld.so的位置
[[email protected] testc]# gcc -shared -fPIC -o libhello.so hello.c -lworld -L.
[[email protected] testc]# ls
hello.c  libhello.so  libworld.so  test.c  world.c
[[email protected] testc]# ldd libhello.so 
	linux-vdso.so.1 =>  (0x00007ffe61b89000)
	libworld.so => not found
	libc.so.6 => /lib64/libc.so.6 (0x00007ff73cc1e000)
	/lib64/ld-linux-x86-64.so.2 (0x00007ff73d1f4000)

调试编译test.c

[[email protected] testc]# ls
hello.c  libhello.so  libworld.so  test.c  world.c

# 编译出错，提示找不到hello
[[email protected] testc]# gcc -o haha test.c 
/tmp/ccQCWcSW.o: In function 'main':
test.c:(.text+0x5): undefined reference to 'hello'
collect2: error: ld returned 1 exit status

# 添加libhello.so的链接索引，并指定库的搜索路径为'.'(当前路径)
# 依然编译失败，提示找不到libworld.so，该库被libhello.so依赖，并提示建议使用-rpath 或 -rpath-link解决
[[email protected] testc]# gcc -o haha test.c -lhello -L.
/usr/bin/ld: warning: libworld.so, needed by ./libhello.so, not found (try using -rpath or -rpath-link)
./libhello.so: undefined reference to 'world'
collect2: error: ld returned 1 exit status

# 手动添加libworld.so的依赖，编译通过，查看haha的链接库，已经显式指出依赖，但是没有找到其位置
[[email protected] testc]# gcc -o haha test.c -lhello -L. -lworld
[[email protected] testc]# ldd haha 
	linux-vdso.so.1 =>  (0x00007fff556ea000)
	libhello.so => not found
	libworld.so => not found
	libc.so.6 => /lib64/libc.so.6 (0x00007f1ff0c97000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f1ff106b000)

# 执行编译出的haha，执行报错，提示找不到依赖库
[[email protected] testc]# ./haha 
./haha: error while loading shared libraries: libhello.so: cannot open shared object file: No such file or directory

#修改系统环境变量'LD_LIBRARY_PATH',增加索引库的位置，查看依赖OK，执行haha, 结果OK, 清空'LD_LIBRARY_PATH'
[[email protected] testc]# export LD_LIBRARY_PATH=/home/testc/  && echo $LD_LIBRARY_PATH
/home/testc/
[[email protected] testc]# ldd haha 
	linux-vdso.so.1 =>  (0x00007ffd647d2000)
	libhello.so => /home/testc/libhello.so (0x00007fb7aa063000)
	libworld.so => /home/testc/libworld.so (0x00007fb7a9e60000)
	libc.so.6 => /lib64/libc.so.6 (0x00007fb7a9a8e000)
	/lib64/ld-linux-x86-64.so.2 (0x00007fb7aa266000)
[[email protected] testc]# ./haha 
hello world.
[[email protected] testc]# export LD_LIBRARY_PATH=  && echo $LD_LIBRARY_PATH

# 将-lworld 替换为 -Wl,-rpath-link=. ，编译OK，依然找不到索引库，添加LD_LIBRARY_PATH后，执行OK 
[[email protected] testc]# gcc -o haha test.c -lhello -L. -Wl,-rpath-link=.
[[email protected] testc]# ldd haha 
	linux-vdso.so.1 =>  (0x00007ffdf67c0000)
	libhello.so => not found
	libc.so.6 => /lib64/libc.so.6 (0x00007fbdbb94b000)
	/lib64/ld-linux-x86-64.so.2 (0x00007fbdbbd1f000)
[[email protected] testc]# ./haha 
./haha: error while loading shared libraries: libhello.so: cannot open shared object file: No such file or directory
[[email protected] testc]# export LD_LIBRARY_PATH=/home/testc  && echo $LD_LIBRARY_PATH
/home/testc
[[email protected] testc]# ldd haha 
	linux-vdso.so.1 =>  (0x00007fff89504000)
	libhello.so => /home/testc/libhello.so (0x00007fc9e75c6000)
	libc.so.6 => /lib64/libc.so.6 (0x00007fc9e71f3000)
	libworld.so => /home/testc/libworld.so (0x00007fc9e6ff1000)
	/lib64/ld-linux-x86-64.so.2 (0x00007fc9e77c9000)
[[email protected] testc]# ./haha 
hello world.
[[email protected] testc]# export LD_LIBRARY_PATH=  && echo $LD_LIBRARY_PATH

# 将-Wl,-rpath-link=. 换成 -Wl,-rpath=. 编译OK， 查看链接库OK，执行OK 
# 修改LD_LIBRARY_PATH后，链接库的位置没有变化
[[email protected] testc]# gcc -o haha test.c -lhello -L. -Wl,-rpath=.
[[email protected] testc]# ls
haha  hello.c  libhello.so  libworld.so  test.c  world.c
[[email protected] testc]# ldd haha 
	linux-vdso.so.1 =>  (0x00007fff86195000)
	libhello.so => ./libhello.so (0x00007f4c11254000)
	libc.so.6 => /lib64/libc.so.6 (0x00007f4c10e81000)
	libworld.so => ./libworld.so (0x00007f4c10c7f000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f4c11457000)
[[email protected] testc]# ./haha 
hello world.
[[email protected] testc]# export LD_LIBRARY_PATH=/home/testc/  && echo $LD_LIBRARY_PATH
/home/testc/
[[email protected] testc]# ldd haha 
	linux-vdso.so.1 =>  (0x00007ffc9f36c000)
	libhello.so => ./libhello.so (0x00007f35cf07c000)
	libc.so.6 => /lib64/libc.so.6 (0x00007f35ceca9000)
	libworld.so => ./libworld.so (0x00007f35ceaa7000)
	/lib64/ld-linux-x86-64.so.2 (0x00007f35cf27f000)
[[email protected] testc]# export LD_LIBRARY_PATH=  && echo $LD_LIBRARY_PATH

结论

• 编译时链接库需要分为两类： 直接引用 间接引用
• 直接引用 被源码中直接调用的库
• 间接引用 被调用库的依赖库
• -lxxx 指定具体的库名称，编译时需要显式指定直接引用的库名称
• -L 指定链接库的位置，编译时需要显式指定直接引用的库位置
• -Wl,-rpath-link ，用于编译时指定间接引用的库位置
  如果知道所有间接引用的库文件名称，并且不嫌麻烦，也可以用-lxxx显式指定每一个库（不推荐-lxxx）
• -Wl,-rpath ，有两个作用：
  1. 用于编译时指定间接引用的库位置，作用同-Wl,-rpath-link
  2. 用于运行时指定所有引用库的位置，作用同修改环境变量(LD_LIBRARY_PATH),并且库路径引用优先级高于LD_LIBRARY_PATH
• 使用建议
  1. 编译命令中使用-Wl,-rpath-link 指定间接引用库位置(编译时)，使用-Wl,-rpath 指定引用库位置（运行时）
  2. -Wl,-rpath-link 在 -Wl,-rpath 前