多個(gè)線程同時(shí)調(diào)用某個(gè)函數(shù)，std::call_once可以保證多線程對(duì)該函數(shù)只調(diào)用一次

#include <iostream>
#include <thread>
#include <mutex>
#include <unistd.h>

std::once_flag flag;

void func() {
    std::call_once(flag, [](){
        std::cout << "hello world" << std::endl;
        sleep(1);
    });
}

int main() {
    std::thread t1(func);
    std::thread t2(func);
    std::thread t3(func);

    t1.join();
    t2.join();
    t3.join();
}

pthread_once實(shí)現(xiàn)

#include <iostream>
#include <pthread.h>
#include <unistd.h>

pthread_once_t once = PTHREAD_ONCE_INIT;

void print() {
    std::cout << "hello world" << std::endl;
    sleep(1);
}

void* func(void*) {
    pthread_once(&once, print);
}

int main() {
    pthread_t tid1, tid2, tid3;
    pthread_create(&tid1, NULL, func, NULL);
    pthread_create(&tid2, NULL, func, NULL);
    pthread_create(&tid3, NULL, func, NULL);

    pthread_join(tid1, NULL);
    pthread_join(tid2, NULL);
    pthread_join(tid3, NULL);
}