什么是信号量？ [英] What is a semaphore?

问题描述

信号量是一种经常用于解决多线程问题的编程概念。我对社群的问题：

A semaphore is a programming concept that is frequently used to solve multi-threading problems. My question to the community:

什么是信号量，如何使用？

What is a semaphore and how do you use it?

推荐答案

将信号灯视为夜总会的保镖。有一个专门的人数，在俱乐部允许一次。如果俱乐部满员，则不允许任何人进入，但只要一个人离开，另一个人就可以进入。

Think of semaphores as bouncers at a nightclub. There are a dedicated number of people that are allowed in the club at once. If the club is full no one is allowed to enter, but as soon as one person leaves another person might enter.

这只是一种限制消费者数量的方法一个特定的资源。例如，限制同时调用应用程序中数据库的次数。

It's simply a way to limit the number of consumers for a specific resource. For example, to limit the number of simultaneous calls to a database in an application.

这是C＃中的一个非常教育学的例子： - ）

Here is a very pedagogic example in C# :-)

using System;
using System.Collections.Generic;
using System.Text;
using System.Threading;

namespace TheNightclub
{
    public class Program
    {
        public static Semaphore Bouncer { get; set; }

        public static void Main(string[] args)
        {
            // Create the semaphore with 3 slots, where 3 are available.
            Bouncer = new Semaphore(3, 3);

            // Open the nightclub.
            OpenNightclub();
        }

        public static void OpenNightclub()
        {
            for (int i = 1; i <= 50; i++)
            {
                // Let each guest enter on an own thread.
                Thread thread = new Thread(new ParameterizedThreadStart(Guest));
                thread.Start(i);
            }
        }

        public static void Guest(object args)
        {
            // Wait to enter the nightclub (a semaphore to be released).
            Console.WriteLine("Guest {0} is waiting to entering nightclub.", args);
            Bouncer.WaitOne();          

            // Do some dancing.
            Console.WriteLine("Guest {0} is doing some dancing.", args);
            Thread.Sleep(500);

            // Let one guest out (release one semaphore).
            Console.WriteLine("Guest {0} is leaving the nightclub.", args);
            Bouncer.Release(1);
        }
    }
}

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