聊聊Linux下的静态库与动态库的生成与使用
一句话总结：静态库在链接阶段集成于程序中，程序生成可删除；动态库在程序运行时在/usr/lib/目录下查找调用，删除后错误，编译链接均需加上-fPIC。
实验代码
fun.h
#ifndef _FUN_H_
#define _FUN_H_
#define ver 2.0
void print();
#endif#include <iostream>
#include "fun.h"
using namespace std;
void print()
{
cout << "print fun" << endl;
}main.cpp
#include <iostream>
#include "fun.h"
using namespace std;
int main()
{
print();
return 0;
}1、静态库
[email protected]:~/libaray$ ls
fun.cpp fun.h main.cpp
[email protected]:~/libaray$ g++ -c fun.cpp
[email protected]:~/libaray$ ls
fun.cpp fun.h fun.o main.cpp
[email protected]:~/libaray$ ar -cr libfun.a fun.o
[email protected]:~/libaray$ ls
fun.cpp fun.h fun.o libfun.a main.cpp
[email protected]:~/libaray$ g++ main.cpp -lfun
/usr/bin/ld: cannot find -lfun
collect2: error: ld returned 1 exit status
[email protected]:~/libaray$ g++ main.cpp -L. -lfun
[email protected]:~/libaray$ ls
a.out fun.cpp fun.h fun.o libfun.a main.cpp
[email protected]:~/libaray$ ./a.out
print fun
[email protected]:~/libaray$ g++ -o main main.cpp -L. -lfun
[email protected]:~/libaray$ ls
a.out fun.cpp fun.h fun.o libfun.a main main.cpp
[email protected]:~/libaray$ ./main
print fun
[email protected]:~/libaray$
ar是用来创建静态库的命令
ar - create, modify, and extract from archives
2、动态库
[email protected]:~/libaray$ ls
fun.cpp fun.h main.cpp
[email protected]:~/libaray$ g++ -c -fPIC fun.cpp
[email protected]:~/libaray$ ls
fun.cpp fun.h fun.o main.cpp
[email protected]:~/libaray$ g++ -shared -fPIC -o libfun.so fun.o
[email protected]:~/libaray$ ls
fun.cpp fun.h fun.o libfun.so main.cpp
[email protected]:~/libaray$ g++ main.cpp -lfun
/usr/bin/ld: cannot find -lfun
collect2: error: ld returned 1 exit status
[email protected]:~/libaray$ g++ main.cpp -lfun -L.
[email protected]:~/libaray$ ls
a.out fun.cpp fun.h fun.o libfun.so main.cpp
[email protected]:~/libaray$ ./a.out
./a.out: error while loading shared libraries: libfun.so: cannot open shared object file: No such file or directory
[email protected]:~/libaray$ g++ -o main main.cpp -L. -lfun
[email protected]:~/libaray$ ls
a.out fun.cpp fun.h fun.o libfun.so main main.cpp
[email protected]:~/libaray$ ./main
./main: error while loading shared libraries: libfun.so: cannot open shared object file: No such file or directory
[email protected]:~/libaray$ sudo cp libfun.so /usr/lib/
[sudo] password for zjy:
[email protected]:~/libaray$ ./a.out
print fun
[email protected]:~/libaray$ ./main
print fun
[email protected]:~/libaray$ sudo rm /usr/lib/libfun.so
[email protected]:~/libaray$ ./a.out
./a.out: error while loading shared libraries: libfun.so: cannot open shared object file: No such file or directory
[email protected]:~/libaray$ ./main
./main: error while loading shared libraries: libfun.so: cannot open shared object file: No such file or directory
[email protected]:~/libaray$
-shared 该选项指定生成动态连接库（让连接器生成T类型的导出符号表，有时候也生成弱连接W类型的导出符号），不用该标志外部程序无法连接。相当于一个可执行文件
-fPIC：表示编译为位置独立的代码，不用此选项的话编译后的代码是位置相关的所以动态载入时是通过代码拷贝的方式来满足不同进程的需要，而不能达到真正代码段共享的目的。（实验时Ubuntu下面编译的时候不带这个选项链接时会发生错误）
[email protected]:~/libaray$ g++ -shared -fPIC -o libfun.so fun.o
[email protected]:~/libaray$ g++ -c fun.cpp
[email protected]:~/libaray$ g++ -shared -fPIC -o libfun.so fun.o
/usr/bin/ld: fun.o: relocation R_X86_64_32 against `.rodata' can not be used when making a shared object; recompile with -fPIC
fun.o: error adding symbols: Bad value
collect2: error: ld returned 1 exit status
[email protected]:~/libaray$