在生成單例時加鎖，生成結束后釋放鎖。
注意兩點：volatile 和 double-check

#include <stdio.h>
#include <pthread.h>
#include <unistd.h>
#include <stdlib.h>

class singleton{
private:
    static volatile singleton *p;
    static pthread_mutex_t mtx;
    singleton(){}
public:
    static singleton * getInstance();
};

singleton * singleton::p = NULL;
pthread_mutex_t singleton::mtx;

singleton * singleton::getInstance(){
    if (p == NULL){
        pthread_mutex_lock(&mtx);
        if (p == NULL)  p = new singleton;
        pthread_mutex_unlock(&mtx);
    }
    return p;
}

int main(){
    singleton * ptr = singleton::getInstance();
    singleton * ptr1 = singleton::getInstance();
    return 0;
}