如何让多个 POSIX 线程等待另一个线程开始

Question

我正在开发一个使用多线程来模拟人和电梯的程序。一部电梯，多人乘坐。

到目前为止，我正在尝试为每个人创建一个线程，为电梯创建一个线程。但是人需要等到电梯创建好之后才能开始执行动作，而电梯需要等到所有的人都创建好之后才能开始移动。

我查看了 pthread_join()，但它看起来像是在等待一个线程完成，这不是我想要的。

Answer 1

你可以使用障碍。

来自 randu.org's pthread tutorial 的 pthread Barriers 部分：

pthreads can participate in a barrier to synchronize to some point in time. Barrier objects are initialized like mutexes or condition variables, except there is one additional parameter, count. The count variable defines the number threads that must join the barrier for the barrier to reach completion and unblock all threads waiting at the barrier.

换句话说，您可以使用 n 创建屏障，任何调用 pthread_barrier_wait 的线程都将等待，直到对 pthread_barrier_wait 的 n 次调用完成。

下面是一个简单的示例，其中所有三个线程将在任何线程打印 "After".

之前打印 "Before"

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

pthread_barrier_t barrier; 

void* foo(void* msg) {
    printf("%s: before\n", (char*)msg);

    // No thread will move on until all three threads have reached this point
    pthread_barrier_wait(&barrier);

    printf("%s: after\n", (char*)msg);
}

int main() {

    // Declare three threads
    int count = 3;
    pthread_t person_A, person_B, elevator;

    // Create a barrier that waits for three threads
    pthread_barrier_init(&barrier, NULL, count); 

    // Create three threads
    pthread_create(&person_A, NULL, foo, "personA");
    pthread_create(&person_B, NULL, foo, "personB");
    pthread_create(&elevator, NULL, foo, "elevator");

    pthread_join(person_A, NULL);
    pthread_join(person_B, NULL);
    pthread_join(elevator, NULL);
    printf("end\n");
}

运行代码here

如何让多个 POSIX 线程等待另一个线程开始

How to make multiple POSIX threads wait for another to begin

c

multithreading

posix

pthreads