linux dynamic lib

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

// test1.cpp
# include"test1.h"

// the following line is invalid for it will not executed( if it's initialization of a var, it's valid)
// a = 11;

int ball()
{
    a =13;
    b = AA(3,4);
    return a+4;
}

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()"