First, the overall outline
Second, the production shared libraries
1. Naming:
lib + name + .so
2. Production steps:
1) independent of the position code generate (generate position-independent .o)
2) .o packaged into the shared library (DLL)
3. Publish and use of shared libraries:
4. solve the problem of dynamic libraries can not be loaded when the program is executed:
1) into the library catalog system - are not allowed
2) temporary test
Environment variables: LD_LIBRARY_PATH = DLL path is provided to the variable
value is set into the system environment variable: export LD_LIBRARY_PATH
when the terminal is closed, setting fail
3) is not commonly used method (permanent setting):
.Bashrc file in the home directory to add one sentence: export LD_LIBRARY_PATH = absolute path of a dynamic library catalog
.bashrc changes are complete, need to restart the terminal
4) modify the configuration file
. a need to find dynamic linker configuration files - /etc/ld.so.conf
. b path dynamic library written to the configuration file - absolute path
c update -. sudo ldconfig -v
5. Advantages and disadvantages:
Third, the production example of dynamic library
1. Production steps
(1) regardless of the position of compiled code number (FIG shared library), generates .o object files, key parameters -fPIC
Process virtual address space
(2) the .o file is packaged, the key parameters -shared
(3) will be released together with the library header files
- Generates .o object files
And the contents of the file path as above, into the src directory and generates .o object files:
[root@centos1 src]# gcc -fPIC -c *.c -I ../include/ [root@centos1 src]# ll 总用量 16 -rw-r--r--. 1 root root 63 4月 20 14:53 add.c -rw-r--r--. 1 root root 1240 4月 20 15:20 add.o -rw-r--r--. 1 root root 63 4月 20 14:24 sub.c -rw-r--r--. 1 root root 1240 4月 20 15:20 sub.o
- 将.o文件打包生成动态库
[root@centos1 src]# gcc -shared -o libCalc.so *.o [root@centos1 src]# ll 总用量 24 -rw-r--r--. 1 root root 63 4月 20 14:53 add.c -rw-r--r--. 1 root root 1240 4月 20 15:20 add.o -rwxr-xr-x. 1 root root 5817 4月 20 15:21 libCalc.so -rw-r--r--. 1 root root 63 4月 20 14:24 sub.c -rw-r--r--. 1 root root 1240 4月 20 15:20 sub.o
2. 使用
- 使用生成的动态库，-L 指定动态库路径 -l 指定库名（gcc main.c -o main -I include/ -L ./lib -lCalc）
退出到calc目录下，将动态库拷贝到lib目录下并编译mian.c并执行main：
[root@centos1 calc]# gcc main.c -o main -I include/ -L ./lib -lCalc [root@centos1 calc]# ll 总用量 24 drwxr-xr-x. 2 root root 4096 4月 20 15:07 include drwxr-xr-x. 2 root root 4096 4月 20 15:29 lib -rwxr-xr-x. 1 root root 7066 4月 20 15:30 main -rw-r--r--. 1 root root 179 4月 20 14:57 main.c drwxr-xr-x. 2 root root 4096 4月 20 15:29 src [root@centos1 calc]# ./main ./main: error while loading shared libraries: libCalc.so: cannot open shared object file: No such file or directory
无法找到动态库目标文件，解决方法有三个：
（1）拷贝到/lib下（不允许，容易与别人库命名重合）
（2）将库路径增加到环境变量LD_LIBRARY_PATH中（不是特别推荐）
[root@centos1 calc]# export LD_LIBRARY_PATH=/home/xuejiale/src/calc/lib/:$LD_LIBRARY_PATH [root@centos1 calc]# ./main 10 + 10 is 20 10 - 10 is 0
（3）配置/etc/ld.so.conf文件，增加 /home/xuejiale/src/calc/lib/ 路径，执行 ldconfig -v，并执行main
/home/xuejiale/src/calc/lib/
[root@centos1 calc]# ./main 10 + 10 is 20 10 - 10 is 0
3. 优缺点总结
缺点：
- 执行时需要加载动态库，相对而言，比静态库慢
- 发布应用时需要同步发布动态库
优点：
- 执行程序体积小
- 库变更时，一般不需要重新编译应用
动静态库总结：
静态库特点：
- 库的代码会编译进程序里面，所以静态库编译的程序比较大。
- 由静态库编译的程序不用依赖于系统的环境变量，所以环境变量有没有这个库文件，也可以运行。
动态库特点：
- 库的代码不会编译进程序里面，所以动态库编译的程序比较小。
- 由动态库编译的程序依赖于系统的环境变量有没有这个库文件，没有则运行不了。