linux dynamic lib

1 // test1.h
2 int a = 10;
3 struct AA
4 {
5     int a,b:
6 };　AA b(5,6);
7 int ball();

 1 // test1.cpp
 2 # include"test1.h"
 3
 4 // the following line is invalid for it will not executed( if it‘s initialization of a var, it‘s valid)
 5 // a = 11;
 6
 7 int ball()
 8 {
 9     a =13;
10     b = AA(3,4);
11     return a+4;
12 }

build a lib with command line : g++ -fPIC -shared test1.cpp -o libtt.so

# include<iostream>
using namespace std;

extern int a;
extern int b;
int ball();

int main()
{
    cout << " call ball() " << ball() <<endl;
    cout << "value of a " << a <<endl;
    cout << "value of b" << b << endl;
}

to build an executable with command line: g++ test-main.cpp -L./ -ltt

Notice:

• in test-main.cpp, we can‘t use " int a" instead of "extern int a", because "int a" is a declaration, which also can be a defination.
• wahile we can use " int ball()" instread of "extern int ball()" because " int ball() " will never be seen as a defination.
• in test1.h, dont define "int a" with prefix ‘ extern "C" ‘, because in the symbol table of it‘s lib, the name of variable is the symbol
• in test-main.cpp, we assume b is of type int and succeeds, because the symbol of b is "b", has no other information
• C or C++ excutable can get the value of a/b; but if C wants to call ball() from a C++ lib, we should add the prifix " extern "C" " to "int ball()"