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<ol class="chapter"><li class="chapter-item expanded affix "><a href="index.html">简明 zCore 教程</a></li><li class="chapter-item expanded affix "><a href="zcore-intro.html">🚧 zCore 整体结构和设计模式</a></li><li class="chapter-item expanded affix "><a href="fuchsia.html">🚧 Fuchsia OS 和 Zircon 微内核</a></li><li class="chapter-item expanded "><a href="ch01-00-object.html"><strong aria-hidden="true">1.</strong> 内核对象</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch01-01-kernel-object.html"><strong aria-hidden="true">1.1.</strong> ✅ 初识内核对象</a></li><li class="chapter-item expanded "><a href="ch01-02-process-object.html"><strong aria-hidden="true">1.2.</strong> 🚧 对象管理器：Process 对象</a></li><li class="chapter-item expanded "><a href="ch01-03-channel-object.html" class="active"><strong aria-hidden="true">1.3.</strong> 🚧 对象传送器：Channel 对象</a></li></ol></li><li class="chapter-item expanded "><a href="ch02-00-task.html"><strong aria-hidden="true">2.</strong> 任务管理</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch02-01-zircon-task.html"><strong aria-hidden="true">2.1.</strong> 🚧 Zircon 任务管理体系</a></li><li class="chapter-item expanded "><a href="ch02-02-process-job-object.html"><strong aria-hidden="true">2.2.</strong> 🚧 进程管理：Process 与 Job 对象</a></li><li class="chapter-item expanded "><a href="ch02-03-thread-object.html"><strong aria-hidden="true">2.3.</strong> 🚧 线程管理：Thread 对象</a></li></ol></li><li class="chapter-item expanded "><a href="ch03-00-memory.html"><strong aria-hidden="true">3.</strong> 内存管理</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch03-01-zircon-memory.html"><strong aria-hidden="true">3.1.</strong> 🚧 Zircon 内存管理模型</a></li><li class="chapter-item expanded "><a href="ch03-02-vmo.html"><strong aria-hidden="true">3.2.</strong> 🚧 物理内存：VMO 对象</a></li><li class="chapter-item expanded "><a href="ch03-03-vmo-paged.html"><strong aria-hidden="true">3.3.</strong> 🚧 物理内存：按页分配的 VMO</a></li><li class="chapter-item expanded "><a href="ch03-04-vmar.html"><strong aria-hidden="true">3.4.</strong> 🚧 虚拟内存：VMAR 对象</a></li></ol></li><li class="chapter-item expanded "><a href="ch04-00-userspace.html"><strong aria-hidden="true">4.</strong> 用户程序</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch04-01-user-program.html"><strong aria-hidden="true">4.1.</strong> 🚧 Zircon 用户程序</a></li><li class="chapter-item expanded "><a href="ch04-02-context-switch.html"><strong aria-hidden="true">4.2.</strong> 🚧 上下文切换</a></li><li class="chapter-item expanded "><a href="ch04-03-syscall.html"><strong aria-hidden="true">4.3.</strong> 🚧 系统调用</a></li></ol></li><li class="chapter-item expanded "><a href="ch05-00-signal-and-waiting.html"><strong aria-hidden="true">5.</strong> 信号和等待</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch05-01-wait-signal.html"><strong aria-hidden="true">5.1.</strong> 🚧 等待内核对象的信号</a></li><li class="chapter-item expanded "><a href="ch05-02-port-object.html"><strong aria-hidden="true">5.2.</strong> 🚧 同时等待多个信号：Port 对象</a></li><li class="chapter-item expanded "><a href="ch05-03-more-signal-objects.html"><strong aria-hidden="true">5.3.</strong> 🚧 实现更多：EventPair, Timer 对象</a></li><li class="chapter-item expanded "><a href="ch05-04-futex-object.html"><strong aria-hidden="true">5.4.</strong> 🚧 用户态同步互斥：Futex 对象</a></li></ol></li><li class="chapter-item expanded "><a href="ch06-00-hal.html"><strong aria-hidden="true">6.</strong> 硬件抽象层</a></li><li><ol class="section"><li class="chapter-item expanded "><a href="ch06-01-zcore-hal-unix.html"><strong aria-hidden="true">6.1.</strong> ✅ UNIX硬件抽象层</a></li></ol></li></ol> </div>
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<main>
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<h1 id="对象传送器channel-对象"><a class="header" href="#对象传送器channel-对象">对象传送器：Channel 对象</a></h1>
|
|
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<h2 id="概要"><a class="header" href="#概要">概要</a></h2>
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<p>通道（Channel）是由一定数量的字节数据和一定数量的句柄组成的双向消息传输。</p>
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<h2 id="用于ipc的内核对象"><a class="header" href="#用于ipc的内核对象">用于IPC的内核对象</a></h2>
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<p>Zircon中用于IPC的内核对象主要有Channel、Socket和FIFO。这里我们主要介绍一下前两个。</p>
|
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<blockquote>
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<p><strong>进程间通信</strong>（<strong>IPC</strong>，<em>Inter-Process Communication</em>），指至少两个进程或线程间传送数据或信号的一些技术或方法。进程是计算机系统分配资源的最小单位(进程是分配资源最小的单位，而线程是调度的最小单位，线程共用进程资源)。每个进程都有自己的一部分独立的系统资源，彼此是隔离的。为了能使不同的进程互相访问资源并进行协调工作，才有了进程间通信。举一个典型的例子，使用进程间通信的两个应用可以被分类为客户端和服务器，客户端进程请求数据，服务端回复客户端的数据请求。有一些应用本身既是服务器又是客户端，这在分布式计算中，时常可以见到。这些进程可以运行在同一计算机上或网络连接的不同计算机上。</p>
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</blockquote>
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<p><code>Socket</code>和<code>Channel</code>都是双向和双端的IPC相关的<code>Object</code>。创建<code>Socket</code>或<code>Channel</code>将返回两个不同的<code>Handle</code>，分别指向<code>Socket</code>或<code>Channel</code>的两端。与channel的不同之处在于，socket仅能传输数据（而不移动句柄），而channel可以传递句柄。</p>
|
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<ul>
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<li><code>Socket</code>是面向流的对象，可以通过它读取或写入以一个或多个字节为单位的数据。</li>
|
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<li><code>Channel</code>是面向数据包的对象，并限制消息的大小最多为64K（如果有改变，可能会更小），以及最多1024个<code>Handle</code>挂载到同一消息上（如果有改变，同样可能会更小）。</li>
|
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</ul>
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<p>当<code>Handle</code>被写入到<code>Channel</code>中时，在发送端<code>Process</code>中将会移除这些<code>Handle</code>。同时携带<code>Handle</code>的消息从<code>Channel</code>中被读取时，该<code>Handle</code>也将被加入到接收端<code>Process</code>中。在这两个时间点之间时，<code>Handle</code>将同时存在于两端（以保证它们指向的<code>Object</code>继续存在而不被销毁），除非<code>Channel</code>写入方向一端被关闭，这种情况下，指向该端点的正在发送的消息将被丢弃，并且它们包含的任何句柄都将被关闭。</p>
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<h2 id="channel"><a class="header" href="#channel">Channel</a></h2>
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<p>Channel是唯一一个能传递handle的IPC，其他只能传递消息。通道有两个端点<code>endpoints</code>，对于代码实现来说，<strong>通道是虚拟的，我们实际上是用通道的两个端点来描述一个通道</strong>。两个端点各自要维护一个消息队列，在一个端点写消息，实际上是把消息写入<strong>另一个端点</strong>的消息队列队尾；在一个端点读消息，实际上是从<strong>当前端点</strong>的消息队列的队头读出一个消息。</p>
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<p>消息通常含有<code>data</code>和<code>handles</code>两部分，我们这里将消息封装为<code>MessagePacket</code>结构体，结构体中含有上述两个字段：</p>
|
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<pre><code class="language-rust noplaypen">#[derive(Default)]
|
|
|
pub struct MessagePacket {
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|
/// message packet携带的数据data
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|
|
pub data: Vec<u8>,
|
|
|
/// message packet携带的句柄Handle
|
|
|
pub handles: Vec<Handle>,
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|
|
}
|
|
|
</code></pre>
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|
<h3 id="实现空的channel对象"><a class="header" href="#实现空的channel对象">实现空的Channel对象</a></h3>
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|
|
<p>在<code>src</code>目录下创建一个<code>ipc</code>目录，在<code>ipc</code>模块下定义一个子模块<code>channel</code>：</p>
|
|
|
<pre><code class="language-rust noplaypen">// src/ipc/mod.rs
|
|
|
use super::*;
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|
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|
mod channel;
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|
pub use self::channel::*;
|
|
|
</code></pre>
|
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|
<p>在<code>ipc.rs</code>中引入<code>crate</code>：</p>
|
|
|
<pre><code class="language-rust noplaypen">// src/ipc/channel.rs
|
|
|
|
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|
use {
|
|
|
super::*,
|
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|
crate::error::*,
|
|
|
crate::object::*,
|
|
|
alloc::collections::VecDeque,
|
|
|
alloc::sync::{Arc, Weak},
|
|
|
spin::Mutex,
|
|
|
};
|
|
|
</code></pre>
|
|
|
<p>把在上面提到的<code>MessagePacket</code>结构体添加到该文件中。</p>
|
|
|
<p>下面我们添加Channel结构体：</p>
|
|
|
<pre><code class="language-rust noplaypen">// src/ipc/channel.rs
|
|
|
pub struct Channel {
|
|
|
base: KObjectBase,
|
|
|
peer: Weak<Channel>,
|
|
|
recv_queue: Mutex<VecDeque<T>>,
|
|
|
}
|
|
|
|
|
|
type T = MessagePacket;
|
|
|
</code></pre>
|
|
|
<p><code>peer</code>代表当前端点所在管道的另一个端点，两端的结构体分别持有对方的<code>Weak</code>引用，并且两端的结构体将分别通过<code>Arc</code>引用，作为内核对象而被内核中的其他数据结构引用，这一部分我们将在创建Channel实例时提到。</p>
|
|
|
<p><code>recv_queue</code>代表当前端点维护的消息队列，它使用<code>VecDeque</code>来存放<code>MessagePacket</code>，可以通过<code>pop_front()</code>、<code>push_back</code>等方法在队头弹出数据和在队尾压入数据。</p>
|
|
|
<p>用使用宏自动实现 <code>KernelObject</code> trait ，使用channel类型名，并添加两个函数。</p>
|
|
|
<pre><code class="language-rust noplaypen">impl_kobject!(Channel
|
|
|
fn peer(&self) -> ZxResult<Arc<dyn KernelObject>> {
|
|
|
let peer = self.peer.upgrade().ok_or(ZxError::PEER_CLOSED)?;
|
|
|
Ok(peer)
|
|
|
}
|
|
|
fn related_koid(&self) -> KoID {
|
|
|
self.peer.upgrade().map(|p| p.id()).unwrap_or(0)
|
|
|
}
|
|
|
);
|
|
|
</code></pre>
|
|
|
<h3 id="实现创建channel的方法"><a class="header" href="#实现创建channel的方法">实现创建Channel的方法</a></h3>
|
|
|
<p>下面我们来实现创建一个<code>Channel</code>的方法：</p>
|
|
|
<pre><code class="language-rust noplaypen">impl Channel {
|
|
|
|
|
|
#[allow(unsafe_code)]
|
|
|
pub fn create() -> (Arc<Self>, Arc<Self>) {
|
|
|
let mut channel0 = Arc::new(Channel {
|
|
|
base: KObjectBase::default(),
|
|
|
peer: Weak::default(),
|
|
|
recv_queue: Default::default(),
|
|
|
});
|
|
|
let channel1 = Arc::new(Channel {
|
|
|
base: KObjectBase::default(),
|
|
|
peer: Arc::downgrade(&channel0),
|
|
|
recv_queue: Default::default(),
|
|
|
});
|
|
|
// no other reference of `channel0`
|
|
|
unsafe {
|
|
|
Arc::get_mut_unchecked(&mut channel0).peer = Arc::downgrade(&channel1);
|
|
|
}
|
|
|
(channel0, channel1)
|
|
|
}
|
|
|
</code></pre>
|
|
|
<p>该方法的返回值是两端点结构体（Channel）的<code>Arc</code>引用，这将作为内核对象被内核中的其他数据结构引用。两个端点互相持有对方<code>Weak</code>指针，这是因为一个端点无需引用计数为0，只要<code>strong_count</code>为0就可以被清理掉，即使另一个端点指向它。</p>
|
|
|
<blockquote>
|
|
|
<p>rust 语言并没有提供垃圾回收 (GC, Garbage Collection ) 的功能， 不过它提供了最简单的引用计数包装类型 <code>Rc</code>，这种引用计数功能也是早期 GC 常用的方法， 但是引用计数不能解决循环引用。那么如何 fix 这个循环引用呢？答案是 <code>Weak</code> 指针，只增加引用逻辑，不共享所有权，即不增加 strong reference count。由于 <code>Weak</code> 指针指向的对象可能析构了，所以不能直接解引用，要模式匹配，再 upgrade。</p>
|
|
|
</blockquote>
|
|
|
<p>下面我们来分析一下这个<code>unsafe</code>代码块：</p>
|
|
|
<pre><code class="language-rust noplaypen">unsafe {
|
|
|
Arc::get_mut_unchecked(&mut channel0).peer = Arc::downgrade(&channel1);
|
|
|
}
|
|
|
</code></pre>
|
|
|
<p>由于两端的结构体将分别通过 <code>Arc</code> 引用，作为内核对象而被内核中的其他数据结构使用。因此，在同时初始化两端的同时，将必须对某一端的 Arc 指针进行获取可变引用的操作，即<code>get_mut_unchecked</code>接口。当 <code>Arc</code> 指针的引用计数不为 <code>1</code> 时，这一接口是非常不安全的，但是在当前情境下，我们使用这一接口进行<code>IPC</code> 对象的初始化，安全性是可以保证的。</p>
|
|
|
<h3 id="单元测试"><a class="header" href="#单元测试">单元测试</a></h3>
|
|
|
<p>下面我们写一个单元测试，来验证我们写的<code>create</code>方法：</p>
|
|
|
<pre><code class="language-rust noplaypen">#[test]
|
|
|
fn test_basics() {
|
|
|
let (end0, end1) = Channel::create();
|
|
|
assert!(Arc::ptr_eq(
|
|
|
&end0.peer().unwrap().downcast_arc().unwrap(),
|
|
|
&end1
|
|
|
));
|
|
|
assert_eq!(end0.related_koid(), end1.id());
|
|
|
|
|
|
drop(end1);
|
|
|
assert_eq!(end0.peer().unwrap_err(), ZxError::PEER_CLOSED);
|
|
|
assert_eq!(end0.related_koid(), 0);
|
|
|
}
|
|
|
</code></pre>
|
|
|
<h3 id="实现数据传输"><a class="header" href="#实现数据传输">实现数据传输</a></h3>
|
|
|
<p>Channel中的数据传输，可以理解为<code>MessagePacket</code>在两个端点之间的传输，那么谁可以读写消息呢？</p>
|
|
|
<p>有一个句柄与通道端点相关联，持有该句柄的进程被视为所有者（owner）。所以是（持有与通道端点关联句柄的）进程可以读取或写入消息，或将通道端点发送到另一个进程。</p>
|
|
|
<p>当<code>MessagePacket</code>被写入通道时，它们会从发送进程中删除。当从通道读取<code>MessagePacket</code>时，<code>MessagePacket</code>的句柄被添加到接收进程中。</p>
|
|
|
<h4 id="read"><a class="header" href="#read">read</a></h4>
|
|
|
<p>获取当前端点的<code>recv_queue</code>，从队头中读取一条消息，如果能读取到消息，返回<code>Ok</code>，否则返回错误信息。</p>
|
|
|
<pre><code class="language-rust noplaypen">pub fn read(&self) -> ZxResult<T> {
|
|
|
let mut recv_queue = self.recv_queue.lock();
|
|
|
if let Some(_msg) = recv_queue.front() {
|
|
|
let msg = recv_queue.pop_front().unwrap();
|
|
|
return Ok(msg);
|
|
|
}
|
|
|
if self.peer_closed() {
|
|
|
Err(ZxError::PEER_CLOSED)
|
|
|
} else {
|
|
|
Err(ZxError::SHOULD_WAIT)
|
|
|
}
|
|
|
}
|
|
|
</code></pre>
|
|
|
<h4 id="write"><a class="header" href="#write">write</a></h4>
|
|
|
<p>先获取当前端点对应的另一个端点的<code>Weak</code>指针，通过<code>upgrade</code>接口升级为<code>Arc</code>指针，从而获取到对应的结构体对象。在它的<code>recv_queue</code>队尾push一个<code>MessagePacket</code>。</p>
|
|
|
<pre><code class="language-rust noplaypen">pub fn write(&self, msg: T) -> ZxResult {
|
|
|
let peer = self.peer.upgrade().ok_or(ZxError::PEER_CLOSED)?;
|
|
|
peer.push_general(msg);
|
|
|
Ok(())
|
|
|
}
|
|
|
fn push_general(&self, msg: T) {
|
|
|
let mut send_queue = self.recv_queue.lock();
|
|
|
send_queue.push_back(msg);
|
|
|
}
|
|
|
</code></pre>
|
|
|
<h3 id="单元测试-1"><a class="header" href="#单元测试-1">单元测试</a></h3>
|
|
|
<p>下面我们写一个单元测试，验证我们上面写的<code>read</code>和<code>write</code>两个方法：</p>
|
|
|
<pre><code class="language-rust noplaypen">#[test]
|
|
|
fn read_write() {
|
|
|
let (channel0, channel1) = Channel::create();
|
|
|
// write a message to each other
|
|
|
channel0
|
|
|
.write(MessagePacket {
|
|
|
data: Vec::from("hello 1"),
|
|
|
handles: Vec::new(),
|
|
|
})
|
|
|
.unwrap();
|
|
|
|
|
|
channel1
|
|
|
.write(MessagePacket {
|
|
|
data: Vec::from("hello 0"),
|
|
|
handles: Vec::new(),
|
|
|
})
|
|
|
.unwrap();
|
|
|
|
|
|
// read message should success
|
|
|
let recv_msg = channel1.read().unwrap();
|
|
|
assert_eq!(recv_msg.data.as_slice(), b"hello 1");
|
|
|
assert!(recv_msg.handles.is_empty());
|
|
|
|
|
|
let recv_msg = channel0.read().unwrap();
|
|
|
assert_eq!(recv_msg.data.as_slice(), b"hello 0");
|
|
|
assert!(recv_msg.handles.is_empty());
|
|
|
|
|
|
// read more message should fail.
|
|
|
assert_eq!(channel0.read().err(), Some(ZxError::SHOULD_WAIT));
|
|
|
assert_eq!(channel1.read().err(), Some(ZxError::SHOULD_WAIT));
|
|
|
}
|
|
|
</code></pre>
|
|
|
<h2 id="总结"><a class="header" href="#总结">总结</a></h2>
|
|
|
<p>在这一节中我们实现了唯一一个可以传递句柄的对象传输器——Channel，我们先了解的Zircon中主要的IPC内核对象，再介绍了Channel如何创建和实现read和write函数的细节。</p>
|
|
|
<p>本章我们学习了中最核心的几个内核对象，在下一章中，我们将学习<code>Zircon</code>的任务管理体系和进程、线程管理的对象。</p>
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