多线程

1114. 按序打印

要按顺序打印，并且保证不会出现思索。

#include <semaphore.h>
class Foo {
protected:
    sem_t firstJobDone;//两个信号量
    sem_t secondJobDone;
public:
    Foo() {
        sem_init(&firstJobDone, 0, 0);//初始值为0
        sem_init(&secondJobDone, 0, 0);
    }
    void first(function<void()> printFirst) {
        // printFirst() outputs "first".
        printFirst();//因为执行first不需要解锁，所以先执行
        sem_post(&firstJobDone);//解锁
    }
    void second(function<void()> printSecond) {
        sem_wait(&firstJobDone);//加锁
        // printSecond() outputs "second".
        printSecond();
        sem_post(&secondJobDone);
    }
    void third(function<void()> printThird) {
        sem_wait(&secondJobDone);
        // printThird() outputs "third".
        printThird();
    }
};