✨个人主页： 熬夜学编程的小林
?系列专栏： 【C语言详解】 【数据结构详解】【C++详解】【Linux系统编程】
目录
1、动态库
1.1、怎么做动态库
1.2、怎么使用动态库
2、外部库使用
1、动态库
1.1、怎么做动态库
方式一
1、将.c文件编译成.o文件
[jkl@host lib]$ gcc -c mystdio.c[jkl@host lib]$ gcc -c mymath.c[jkl@host lib]$ lslibmyc.a mylib mymath.h mystdio.c mystdio.o main.c mymath.c mymath.o mystdio.h roommate 2、将.o文件打包成动态库文件
[jkl@host lib]$ gcc *.o -o libmyc.so/usr/lib/gcc/x86_64-redhat-linux/4.8.5/../../../../lib64/crt1.o: In function `_start':(.text+0x20): undefined reference to `main'collect2: error: ld returned 1 exit status[jkl@host lib]$ gcc -shared *.o -o libmyc.so/usr/bin/ld: mystdio.o: relocation R_X86_64_32 against `.rodata' can not be used when making a shared object; recompile with -fPIC/usr/bin/ld: final link failed: Nonrepresentable section on outputcollect2: error: ld returned 1 exit status上面的方式不能打包成动态库文件，需要在编译成.o文件时加选项。
方式二
1、删除前面编译的.o文件
[jkl@host lib]$ rm *.o[jkl@host lib]$ lslibmyc.a main.c mylib mymath.c mymath.h mystdio.c mystdio.h roommate 2、将.c文件编译成.o文件(加选项)
[jkl@host lib]$ gcc -fPIC -c mymath.c[jkl@host lib]$ gcc -fPIC -c mystdio.c[jkl@host lib]$ lslibmyc.a mylib mymath.h mystdio.c mystdio.o main.c mymath.c mymath.o mystdio.h roommate 3、将.o文件打包成动态库文件
[jkl@host lib]$ gcc -shared *.o -o libmyc.so[jkl@host lib]$ lslibmyc.a main.c mymath.c mymath.o mystdio.h roommate libmyc.so mylib mymath.h mystdio.c mystdio.o 1.2、怎么使用动态库
1、先将动态库文件拷贝到lib目录下
[jkl@host lib]$ cp libmyc.so mylib/lib[jkl@host lib]$ tree mylibmylib|-- include| |-- mymath.h| `-- mystdio.h`-- lib |-- libmyc.a `-- libmyc.so2 directories, 4 files2、 通过命令编译文件
[jkl@host roommate]$ gcc main.c -I mylib/include -L mylib/lib -lmyc[jkl@host roommate]$ lsa.out log.txt main.c mylib[jkl@host roommate]$ ./a.out./a.out: error while loading shared libraries: libmyc.so: cannot open shared object file: No such file or directory[jkl@host roommate]$ ldd a.outlinux-vdso.so.1 => (0x00007ffde21da000)libmyc.so => not foundlibc.so.6 => /lib64/libc.so.6 (0x00007f4245348000)/lib64/ld-linux-x86-64.so.2 (0x00007f4245716000)没有找到这个动态库文件，原因是：动态库在程序运行的时候，要找到动态库加载并运行。
为什么静态库没有找不到文件的问题？
编译期间，已经将库中的代码拷贝到我们的可执行程序内部了！加载和库没有关系了，这也是为什么静态库所占内存空间比动态库大很多的原因。
解决办法一
将动态库安装到系统中(不推荐)。
[jkl@host roommate]$ sudo cp mylib/lib/libmyc.so /lib64 # 将动态库文件拷贝到系统中，普通用户需要使用sudo提权[sudo] password for jkl: [jkl@host roommate]$ gcc main.c -I mylib/include -L mylib/lib -lmyc # 默认使用动态库编译[jkl@host roommate]$ lsa.out log.txt main.c mylib[jkl@host roommate]$ ./a.out # 执行代码10 + 20 = 30[jkl@host roommate]$ ldd a.outlinux-vdso.so.1 => (0x00007fff79128000)libmyc.so => /lib64/libmyc.so (0x00007fdef1f1b000) # 我们拷贝的动态库libc.so.6 => /lib64/libc.so.6 (0x00007fdef1b4d000)/lib64/ld-linux-x86-64.so.2 (0x00007fdef211d000)[jkl@host roommate]$ file a.out a.out: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.32, BuildID[sha1]=37f079df869cdf34f19759f0317a0da027a1973a, not stripped删除拷贝的动态库文件
[jkl@host roommate]$ sudo rm /lib64/libmyc.so # 删除动态库文件[jkl@host roommate]$ ll /lib/libmyc.so # 查看前面拷贝的动态库文件，确认删除ls: cannot access /lib/libmyc.so: No such file or directory[jkl@host roommate]$ ./a.out # 删除之后就运行不了，不需要重新编译./a.out: error while loading shared libraries: libmyc.so: cannot open shared object file: No such file or directory解决办法二
建立软链接。
[jkl@host roommate]$ sudo ln -s /home/jkl/linux-learning/lib/roommate/mylib/lib/libmyc.so /lib64/libmyc.so # 将自己的动态库文件软链接到系统的同名文件[jkl@host roommate]$ ll /lib64/libmyc.so # 查看软链接文件lrwxrwxrwx 1 root root 57 Sep 10 14:06 /lib64/libmyc.so -> /home/jkl/linux-learning/lib/roommate/mylib/lib/libmyc.so[jkl@host roommate]$ ./a.out # 运行程序10 + 20 = 30[jkl@host roommate]$ ldd a.outlinux-vdso.so.1 => (0x00007ffd94fc9000)libmyc.so => /lib64/libmyc.so (0x00007f31209de000)libc.so.6 => /lib64/libc.so.6 (0x00007f3120610000)/lib64/ld-linux-x86-64.so.2 (0x00007f3120be0000)取消软链接
[jkl@host roommate]$ sudo unlink /lib64/libmyc.so # 取消软链接[jkl@host roommate]$ ll /lib64/libmyc.so # 查看软链接文件，没有该文件ls: cannot access /lib64/libmyc.so: No such file or directory[jkl@host roommate]$ ./a.out # 程序执行不了./a.out: error while loading shared libraries: libmyc.so: cannot open shared object file: No such file or directory解决办法三
命令行导入环境变量(内存级别的，退出就没有该环境变量了)
[jkl@host roommate]$ LD_LIBRARY_PATH=$LD_LIBARY_PATH:/home/jkl/linux-learning/lib/roommate/mylib/lib[jkl@host roommate]$ echo $LD_LIBRARY_PATH:/home/jkl/.VimForCpp/vim/bundle/YCM.so/el7.x86_64:/home/jkl/linux-learning/lib/roommate/mylib/lib[jkl@host roommate]$ ./a.out10 + 20 = 30[jkl@host roommate]$ ldd[jkl@host roommate]$ ldd a.outlinux-vdso.so.1 => (0x00007ffe06fcf000)libmyc.so => /home/jkl/linux-learning/lib/roommate/mylib/lib/libmyc.so (0x00007f7dcb53e000)libc.so.6 => /lib64/libc.so.6 (0x00007f7dcb170000)/lib64/ld-linux-x86-64.so.2 (0x00007f7dcb740000)退出xshell
[jkl@host roommate]$ echo $LD_LIBRARY_PATH:/home/jkl/.VimForCpp/vim/bundle/YCM.so/el7.x86_64[jkl@host roommate]$ ./a.out./a.out: error while loading shared libraries: libmyc.so: cannot open shared object file: No such file or directory解决办法四
修改.bashrc配置文件，让环境变量永久生效。
1、编辑.bashrc文件
[jkl@host ~]$ vim .bashrc[jkl@host ~]$ source ~/.bashrc # 立即生效
注意：如果环境变量没有生效退出XShell重新登录即可。
2、执行代码
[jkl@host roommate]$ echo $LD_LIBRARY_PATH :/home/jkl/.VimForCpp/vim/bundle/YCM.so/el7.x86_64:/home/jkl/linux-learning/lib/roommate/mylib/lib[jkl@host roommate]$ ./a.out10 + 20 = 30[jkl@host roommate]$ ldd a.outlinux-vdso.so.1 => (0x00007ffcfdfe7000)libmyc.so => /home/jkl/linux-learning/lib/roommate/mylib/lib/libmyc.so (0x00007f638dac1000)libc.so.6 => /lib64/libc.so.6 (0x00007f638d6f3000)/lib64/ld-linux-x86-64.so.2 (0x00007f638dcc3000)3、将路径从文件中删除。
[jkl@host ~]$ cat .bashrc# .bashrc# Source global definitionsif [ -f /etc/bashrc ]; then. /etc/bashrcfi# Uncomment the following line if you don't like systemctl's auto-paging feature:# export SYSTEMD_PAGER=# User specific aliases and functionsalias vim='/home/jkl/.VimForCpp/nvim'export LD_LIBRARY_PATH=$LD_LIBRARY_PATH:~/.VimForCpp/vim/bundle/YCM.so/el7.x86_64解决办法五
/etc/ld.so.conf.d 新增动态库搜索的配置文件，ldconfig生效。
1、在/etc/ld.so.conf.d 目录下新增一个.conf结尾的文件，内容为动态库文件所在目录(需要root权限)
[jkl@host ~]$ cd /etc/ld.so.conf.d # 进入目录[jkl@host ld.so.conf.d]$ lsbind-export-x86_64.conf dyninst-x86_64.conf kernel-3.10.0-1160.108.1.el7.x86_64.conf mysql-x86_64.conf[jkl@host ld.so.conf.d]$ sudo touch mylib.conf # 创建文件需要提权[sudo] password for jkl: [jkl@host ld.so.conf.d]$ lsbind-export-x86_64.conf dyninst-x86_64.conf kernel-3.10.0-1160.108.1.el7.x86_64.conf mylib.conf mysql-x86_64.conf[jkl@host ld.so.conf.d]$ sudo vim mylib.conf[jkl@host ld.so.conf.d]$ sudo ldconfig # 让动态链接库为系统所共享2、执行代码
[jkl@host roommate]$ ./a.out10 + 20 = 30[jkl@host roommate]$ lddldd: missing file argumentsTry `ldd --help' for more information.[jkl@host roommate]$ ldd a.outlinux-vdso.so.1 => (0x00007fffacf78000)libmyc.so => /home/jkl/linux-learning/lib/roommate/mylib/lib/libmyc.so (0x00007f48d1437000)libc.so.6 => /lib64/libc.so.6 (0x00007f48d1069000)/lib64/ld-linux-x86-64.so.2 (0x00007f48d1639000)动静态库总结
动静态库的最终意义还是提高开发效率。
2、外部库使用
系统中其实有很多库，它们通常由一组互相关联的用来完成某项常见工作的函数构成。
比如用来处理屏幕显示情况的函数（ncurses库）
#include <math.h>#include <stdio.h>int main(){ double x = pow(2.0, 3.0); printf("The cubed is %f\n", x); return 0;}使用外部库链接
[jkl@host lib]$ gcc -Wall calc.c -o calc -lm[jkl@host lib]$ ldd calclinux-vdso.so.1 => (0x00007ffdf00a1000)libm.so.6 => /lib64/libm.so.6 (0x00007fb17991b000)libc.so.6 => /lib64/libc.so.6 (0x00007fb17954d000)/lib64/ld-linux-x86-64.so.2 (0x00007fb179c1d000)[jkl@host lib]$ ./calcThe cubed is 8.000000有链接到外部库。
lm表示要链接libm.so或者libm.a库文件库文件名称和引入库的名称
如：libc.so -> c库，去掉前缀lib，去掉后缀.so,.a直接编译
[jkl@host lib]$ gcc calc.c[jkl@host lib]$ ./a.outThe cubed is 8.000000[jkl@host lib]$ ldd a.outlinux-vdso.so.1 => (0x00007fffb4359000)libc.so.6 => /lib64/libc.so.6 (0x00007f2081cb1000)/lib64/ld-linux-x86-64.so.2 (0x00007f208207f000)可执行程序的结果是一样的，只是链接到的库有差别。