并发性是使程序在同一时间运行一个以上的线程。并发程序的一个例子是在Web服务器响应多个客户端在同一时间。并发是容易与消息传递却很难，如果它们是基于数据共享的写入。

传递线程之间的数据被称为消息。消息可以由任何类型和任意数量的变量。每个线程都有一个ID，它是用于指定邮件的收件人。即启动另一个线程的任何线程被称为新线程的所有者。即启动另一个线程的任何线程被称为新线程的所有者。

启动线程

spawn() 接受一个函数指针作为参数，并从该函数启动一个新线程。正在开展的功能，包括它可能调用其他函数的任何操作，将在新的线程中执行。owner和worker开始独立执行的，好像他们是独立的程序：

import std.stdio;
import std.stdio;
import std.concurrency;
import core.thread;

void worker(int a)
{
   foreach (i; 0 .. 4)
   {
      Thread.sleep(1);
      writeln("Worker Thread ",a + i);
   }
}

void main()
{
   foreach (i; 1 .. 4)
   {
      Thread.sleep(2);
      writeln("Main Thread ",i);
      spawn(&worker, i * 5);
    }

    writeln("main is done.");

}

Main Thread 1
Worker Thread 5
Main Thread 2
Worker Thread 6
Worker Thread 10
Main Thread 3
main is done.
Worker Thread 7
Worker Thread 11
Worker Thread 15
Worker Thread 8
Worker Thread 12
Worker Thread 16
Worker Thread 13
Worker Thread 17
Worker Thread 18

线程标识符

thisTid变量是全局可用在模块级始终是当前线程的id。也可以收到重生时被调用threadid。一个例子如下所示。

import std.stdio;
import std.concurrency;

void printTid(string tag)
{
   writefln("%s: %s, address: %s", tag, thisTid, &thisTid);
}

void worker()
{
   printTid("Worker");
}

void main()
{
   Tid myWorker = spawn(&worker);

   printTid("Owner ");

   writeln(myWorker);
}

Owner : Tid(std.concurrency.MessageBox), address: 10C71A59C
Worker: Tid(std.concurrency.MessageBox), address: 10C71A59C
Tid(std.concurrency.MessageBox)

消息传递

send() 发送的消息和receiveOnly()等待一个特定类型的消息。还有prioritySend()，receive()和receiveTimeout()，这将在后面进行说明。在下面的程序的所有者将其工作者int类型的消息，并等待来自double类型的工人消息。线程继续发送邮件来回，直到车主发出了一个负的int。

import std.stdio;
import std.concurrency;
import core.thread;
import std.conv;

void workerFunc(Tid tid)
{
   int value = 0;

   while (value >= 0) 
   {
      value = receiveOnly!int();
      auto result = to!double(value) * 5;
      tid.send(result);
   } 

}

void main()
{
   Tid worker = spawn(&workerFunc,thisTid);
   
   foreach (value; 5 .. 10) {
      worker.send(value);
      auto result = receiveOnly!double();
      writefln("sent: %s, received: %s", value, result);
   }   
 
   worker.send(-1);
}

sent: 5, received: 25
sent: 6, received: 30
sent: 7, received: 35
sent: 8, received: 40
sent: 9, received: 45

消息等待传递

一个简单的例子与传递与等待消息如下所示。

import std.stdio;
import std.concurrency;
import core.thread;
import std.conv;

void workerFunc(Tid tid)
{
   Thread.sleep(dur!("msecs")( 500 ),);
   tid.send("hello");
}

void main()
{
   spawn(&workerFunc,thisTid);

   writeln("Waiting for a message");

   bool received = false;

   while (!received) 
   {
      received = receiveTimeout(dur!("msecs")( 100 ),
      (string message){
         writeln("received: ", message);
      });

      if (!received) {
         writeln("... no message yet");
      } 
   }
}

Waiting for a message
... no message yet
... no message yet
... no message yet
... no message yet
received: hello