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<li class="toctree-l2"><a class="reference internal" href="../chapter4/3sv39-implementation-1.html">实现 SV39 多级页表机制（上）</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter7/2cmdargs-and-redirection.html">命令行参数与标准 I/O 重定向</a></li>
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<li class="toctree-l2"><a class="reference internal" href="../chapter8/1thread-kernel.html">内核态的线程管理</a></li>
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<li class="toctree-l1"><a class="reference internal" href="../appendix-d/index.html">附录 D：RISC-V相关信息</a></li>
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<div class="section" id="term-print-userminienv">
<span id="id1"></span><h1>构建用户态执行环境<a class="headerlink" href="#term-print-userminienv" title="永久链接至标题"></a></h1>
<div class="toctree-wrapper compound">
</div>
<div class="admonition note">
<p class="admonition-title">注解</p>
<p>前三小节的用户态程序案例代码在 <a class="reference external" href="https://github.com/LearningOS/rCore-Tutorial-Book-2021Autumn/tree/ch2-U-nostd">此处</a> 获取。</p>
</div>
<div class="section" id="id3">
<h2>用户态最小化执行环境<a class="headerlink" href="#id3" title="永久链接至标题"></a></h2>
<div class="section" id="id4">
<h3>执行环境初始化<a class="headerlink" href="#id4" title="永久链接至标题"></a></h3>
<p>首先我们要给 Rust 编译器编译器提供入口函数 <code class="docutils literal notranslate"><span class="pre">_start()</span></code> ，
在 <code class="docutils literal notranslate"><span class="pre">main.rs</span></code> 中添加如下内容：</p>
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="c1">// os/src/main.rs</span>
<span class="cp">#[no_mangle]</span><span class="w"></span>
<span class="k">extern</span><span class="w"> </span><span class="s">"C"</span><span class="w"> </span><span class="k">fn</span> <span class="nf">_start</span><span class="p">()</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="k">loop</span><span class="p">{};</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
</pre></div>
</div>
<p>对上述代码重新编译，再用分析工具分析：</p>
<div class="highlight-console notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>cargo build
<span class="go"> Compiling os v0.1.0 (/home/shinbokuow/workspace/v3/rCore-Tutorial-v3/os)</span>
<span class="go"> Finished dev [unoptimized + debuginfo] target(s) in 0.06s</span>
<span class="go">[反汇编导出汇编程序]</span>
<span class="gp">$ </span>rust-objdump -S target/riscv64gc-unknown-none-elf/debug/os
<span class="go"> target/riscv64gc-unknown-none-elf/debug/os: file format elf64-littleriscv</span>
<span class="go"> Disassembly of section .text:</span>
<span class="go"> 0000000000011120 &lt;_start&gt;:</span>
<span class="go"> ; loop {}</span>
<span class="go"> 11120: 09 a0 j 2 &lt;_start+0x2&gt;</span>
<span class="go"> 11122: 01 a0 j 0 &lt;_start+0x2&gt;</span>
</pre></div>
</div>
<p>反汇编出的两条指令就是一个死循环，
这说明编译器生成的已经是一个合理的程序了。
用 <code class="docutils literal notranslate"><span class="pre">qemu-riscv64</span> <span class="pre">target/riscv64gc-unknown-none-elf/debug/os</span></code> 命令可以执行这个程序。</p>
</div>
<div class="section" id="id5">
<h3>程序正常退出<a class="headerlink" href="#id5" title="永久链接至标题"></a></h3>
<p>我们把 <code class="docutils literal notranslate"><span class="pre">_start()</span></code> 函数中的循环语句注释掉，重新编译并分析，看到其汇编代码是：</p>
<div class="highlight-console notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>rust-objdump -S target/riscv64gc-unknown-none-elf/debug/os
<span class="go"> target/riscv64gc-unknown-none-elf/debug/os: file format elf64-littleriscv</span>
<span class="go"> Disassembly of section .text:</span>
<span class="go"> 0000000000011120 &lt;_start&gt;:</span>
<span class="go"> ; }</span>
<span class="go"> 11120: 82 80 ret</span>
</pre></div>
</div>
<p>看起来是合法的执行程序。但如果我们执行它，会引发问题：</p>
<div class="highlight-console notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>qemu-riscv64 target/riscv64gc-unknown-none-elf/debug/os
<span class="go"> 段错误 (核心已转储)</span>
</pre></div>
</div>
<p>这个简单的程序导致 <code class="docutils literal notranslate"><span class="pre">qemu-riscv64</span></code> 崩溃了！为什么会这样？</p>
<div class="admonition note">
<p class="admonition-title">注解</p>
<p>QEMU有两种运行模式：</p>
<p><code class="docutils literal notranslate"><span class="pre">User</span> <span class="pre">mode</span></code> 模式，即用户态模拟，如 <code class="docutils literal notranslate"><span class="pre">qemu-riscv64</span></code> 程序，
能够模拟不同处理器的用户态指令的执行，并可以直接解析ELF可执行文件，
加载运行那些为不同处理器编译的用户级Linux应用程序。</p>
<p><code class="docutils literal notranslate"><span class="pre">System</span> <span class="pre">mode</span></code> 模式，即系统态模式，如 <code class="docutils literal notranslate"><span class="pre">qemu-system-riscv64</span></code> 程序，
能够模拟一个完整的基于不同CPU的硬件系统，包括处理器、内存及其他外部设备，支持运行完整的操作系统。</p>
</div>
<p>目前的执行环境还缺了一个退出机制，我们需要操作系统提供的 <code class="docutils literal notranslate"><span class="pre">exit</span></code> 系统调用来退出程序。这里先给出代码：</p>
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="c1">// os/src/main.rs</span>
<span class="k">const</span><span class="w"> </span><span class="n">SYSCALL_EXIT</span>: <span class="kt">usize</span> <span class="o">=</span><span class="w"> </span><span class="mi">93</span><span class="p">;</span><span class="w"></span>
<span class="k">fn</span> <span class="nf">syscall</span><span class="p">(</span><span class="n">id</span>: <span class="kt">usize</span><span class="p">,</span><span class="w"> </span><span class="n">args</span>: <span class="p">[</span><span class="kt">usize</span><span class="p">;</span><span class="w"> </span><span class="mi">3</span><span class="p">])</span><span class="w"> </span>-&gt; <span class="kt">isize</span> <span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="kd">let</span><span class="w"> </span><span class="k">mut</span><span class="w"> </span><span class="n">ret</span><span class="p">;</span><span class="w"></span>
<span class="w"> </span><span class="k">unsafe</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">core</span>::<span class="n">arch</span>::<span class="fm">asm!</span><span class="p">(</span><span class="w"></span>
<span class="w"> </span><span class="s">"ecall"</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="n">inlateout</span><span class="p">(</span><span class="s">"x10"</span><span class="p">)</span><span class="w"> </span><span class="n">args</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span><span class="w"> </span><span class="o">=&gt;</span><span class="w"> </span><span class="n">ret</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="k">in</span><span class="p">(</span><span class="s">"x11"</span><span class="p">)</span><span class="w"> </span><span class="n">args</span><span class="p">[</span><span class="mi">1</span><span class="p">],</span><span class="w"></span>
<span class="w"> </span><span class="k">in</span><span class="p">(</span><span class="s">"x12"</span><span class="p">)</span><span class="w"> </span><span class="n">args</span><span class="p">[</span><span class="mi">2</span><span class="p">],</span><span class="w"></span>
<span class="w"> </span><span class="k">in</span><span class="p">(</span><span class="s">"x17"</span><span class="p">)</span><span class="w"> </span><span class="n">id</span><span class="p">,</span><span class="w"></span>
<span class="w"> </span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="w"> </span><span class="n">ret</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
<span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">sys_exit</span><span class="p">(</span><span class="n">xstate</span>: <span class="kt">i32</span><span class="p">)</span><span class="w"> </span>-&gt; <span class="kt">isize</span> <span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">syscall</span><span class="p">(</span><span class="n">SYSCALL_EXIT</span><span class="p">,</span><span class="w"> </span><span class="p">[</span><span class="n">xstate</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="kt">usize</span><span class="p">,</span><span class="w"> </span><span class="mi">0</span><span class="p">,</span><span class="w"> </span><span class="mi">0</span><span class="p">])</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
<span class="cp">#[no_mangle]</span><span class="w"></span>
<span class="k">extern</span><span class="w"> </span><span class="s">"C"</span><span class="w"> </span><span class="k">fn</span> <span class="nf">_start</span><span class="p">()</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">sys_exit</span><span class="p">(</span><span class="mi">9</span><span class="p">);</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
</pre></div>
</div>
<p><code class="docutils literal notranslate"><span class="pre">main.rs</span></code> 增加的内容不多，但还是有点与一般的应用程序有所不同，因为它引入了汇编和系统调用。
第二章的第二节 <a class="reference internal" href="../chapter2/2application.html"><span class="doc">实现应用程序</span></a> 会详细介绍上述代码的含义。
这里读者只需要知道 <code class="docutils literal notranslate"><span class="pre">_start</span></code> 函数调用了一个 <code class="docutils literal notranslate"><span class="pre">sys_exit</span></code> 函数，
向操作系统发出了退出的系统调用请求，退出码为 <code class="docutils literal notranslate"><span class="pre">9</span></code> 。</p>
<p>我们编译执行以下修改后的程序：</p>
<div class="highlight-console notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>cargo build --target riscv64gc-unknown-none-elf
<span class="go"> Compiling os v0.1.0 (/media/chyyuu/ca8c7ba6-51b7-41fc-8430-e29e31e5328f/thecode/rust/os_kernel_lab/os)</span>
<span class="go"> Finished dev [unoptimized + debuginfo] target(s) in 0.26s</span>
<span class="go">[打印程序的返回值]</span>
<span class="gp">$ </span>qemu-riscv64 target/riscv64gc-unknown-none-elf/debug/os<span class="p">;</span> <span class="nb">echo</span> <span class="nv">$?</span>
<span class="go">9</span>
</pre></div>
</div>
<p>可以看到，返回的结果确实是 <code class="docutils literal notranslate"><span class="pre">9</span></code> 。这样，我们勉强完成了一个简陋的用户态最小化执行环境。</p>
</div>
</div>
<div class="section" id="id6">
<h2>有显示支持的用户态执行环境<a class="headerlink" href="#id6" title="永久链接至标题"></a></h2>
<p>没有 <code class="docutils literal notranslate"><span class="pre">println</span></code> 输出信息，终究觉得缺了点啥。</p>
<p>Rust 的 core 库内建了以一系列帮助实现显示字符的基本 Trait 和数据结构，函数等，我们可以对其中的关键部分进行扩展，就可以实现定制的 <code class="docutils literal notranslate"><span class="pre">println!</span></code> 功能。</p>
<div class="section" id="id7">
<h3>实现输出字符串的相关函数<a class="headerlink" href="#id7" title="永久链接至标题"></a></h3>
<div class="admonition attention">
<p class="admonition-title">注意</p>
<p>如果你觉得理解 Rust 宏有困难，把它当成黑盒就好！</p>
</div>
<p>首先封装一下对 <code class="docutils literal notranslate"><span class="pre">SYSCALL_WRITE</span></code> 系统调用。</p>
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="k">const</span><span class="w"> </span><span class="n">SYSCALL_WRITE</span>: <span class="kt">usize</span> <span class="o">=</span><span class="w"> </span><span class="mi">64</span><span class="p">;</span><span class="w"></span>
<span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">sys_write</span><span class="p">(</span><span class="n">fd</span>: <span class="kt">usize</span><span class="p">,</span><span class="w"> </span><span class="n">buffer</span>: <span class="kp">&amp;</span><span class="p">[</span><span class="kt">u8</span><span class="p">])</span><span class="w"> </span>-&gt; <span class="kt">isize</span> <span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">syscall</span><span class="p">(</span><span class="n">SYSCALL_WRITE</span><span class="p">,</span><span class="w"> </span><span class="p">[</span><span class="n">fd</span><span class="p">,</span><span class="w"> </span><span class="n">buffer</span><span class="p">.</span><span class="n">as_ptr</span><span class="p">()</span><span class="w"> </span><span class="k">as</span><span class="w"> </span><span class="kt">usize</span><span class="p">,</span><span class="w"> </span><span class="n">buffer</span><span class="p">.</span><span class="n">len</span><span class="p">()])</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
</pre></div>
</div>
<p>然后实现基于 <code class="docutils literal notranslate"><span class="pre">Write</span></code> Trait 的数据结构，并完成 <code class="docutils literal notranslate"><span class="pre">Write</span></code> Trait 所需要的 <code class="docutils literal notranslate"><span class="pre">write_str</span></code> 函数，并用 <code class="docutils literal notranslate"><span class="pre">print</span></code> 函数进行包装。</p>
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="k">struct</span> <span class="nc">Stdout</span><span class="p">;</span><span class="w"></span>
<span class="k">impl</span><span class="w"> </span><span class="n">Write</span><span class="w"> </span><span class="k">for</span><span class="w"> </span><span class="n">Stdout</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="k">fn</span> <span class="nf">write_str</span><span class="p">(</span><span class="o">&amp;</span><span class="k">mut</span><span class="w"> </span><span class="bp">self</span><span class="p">,</span><span class="w"> </span><span class="n">s</span>: <span class="kp">&amp;</span><span class="kt">str</span><span class="p">)</span><span class="w"> </span>-&gt; <span class="nc">fmt</span>::<span class="nb">Result</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">sys_write</span><span class="p">(</span><span class="mi">1</span><span class="p">,</span><span class="w"> </span><span class="n">s</span><span class="p">.</span><span class="n">as_bytes</span><span class="p">());</span><span class="w"></span>
<span class="w"> </span><span class="nb">Ok</span><span class="p">(())</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
<span class="k">pub</span><span class="w"> </span><span class="k">fn</span> <span class="nf">print</span><span class="p">(</span><span class="n">args</span>: <span class="nc">fmt</span>::<span class="n">Arguments</span><span class="p">)</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">Stdout</span><span class="p">.</span><span class="n">write_fmt</span><span class="p">(</span><span class="n">args</span><span class="p">).</span><span class="n">unwrap</span><span class="p">();</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
</pre></div>
</div>
<p>最后，实现基于 <code class="docutils literal notranslate"><span class="pre">print</span></code> 函数，实现Rust语言 <strong>格式化宏</strong> ( <a class="reference external" href="https://doc.rust-lang.org/std/fmt/#related-macros">formatting macros</a> )。</p>
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="cp">#[macro_export]</span><span class="w"></span>
<span class="fm">macro_rules!</span><span class="w"> </span><span class="n">print</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="p">(</span><span class="cp">$fmt</span>: <span class="nc">literal</span><span class="w"> </span><span class="cp">$(,</span><span class="w"> </span><span class="cp">$($arg</span>: <span class="nc">tt</span><span class="p">)</span><span class="o">+</span><span class="p">)</span><span class="o">?</span><span class="p">)</span><span class="w"> </span><span class="o">=&gt;</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="cp">$crate</span>::<span class="n">console</span>::<span class="n">print</span><span class="p">(</span><span class="fm">format_args!</span><span class="p">(</span><span class="cp">$fmt</span><span class="w"> </span><span class="cp">$(,</span><span class="w"> </span><span class="cp">$($arg</span><span class="p">)</span><span class="o">+</span><span class="p">)</span><span class="o">?</span><span class="p">));</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
<span class="cp">#[macro_export]</span><span class="w"></span>
<span class="fm">macro_rules!</span><span class="w"> </span><span class="n">println</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="p">(</span><span class="cp">$fmt</span>: <span class="nc">literal</span><span class="w"> </span><span class="cp">$(,</span><span class="w"> </span><span class="cp">$($arg</span>: <span class="nc">tt</span><span class="p">)</span><span class="o">+</span><span class="p">)</span><span class="o">?</span><span class="p">)</span><span class="w"> </span><span class="o">=&gt;</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="n">print</span><span class="p">(</span><span class="fm">format_args!</span><span class="p">(</span><span class="fm">concat!</span><span class="p">(</span><span class="cp">$fmt</span><span class="p">,</span><span class="w"> </span><span class="s">"</span><span class="se">\n</span><span class="s">"</span><span class="p">)</span><span class="w"> </span><span class="cp">$(,</span><span class="w"> </span><span class="cp">$($arg</span><span class="p">)</span><span class="o">+</span><span class="p">)</span><span class="o">?</span><span class="p">));</span><span class="w"></span>
<span class="w"> </span><span class="p">}</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
</pre></div>
</div>
<p>接下来，我们调整一下应用程序，让它发出显示字符串和退出的请求：</p>
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="cp">#[no_mangle]</span><span class="w"></span>
<span class="k">extern</span><span class="w"> </span><span class="s">"C"</span><span class="w"> </span><span class="k">fn</span> <span class="nf">_start</span><span class="p">()</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="fm">println!</span><span class="p">(</span><span class="s">"Hello, world!"</span><span class="p">);</span><span class="w"></span>
<span class="w"> </span><span class="n">sys_exit</span><span class="p">(</span><span class="mi">9</span><span class="p">);</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
</pre></div>
</div>
<p>现在，我们编译并执行一下，可以看到正确的字符串输出，且程序也能正确退出！</p>
<div class="highlight-console notranslate"><div class="highlight"><pre><span></span><span class="gp">$ </span>cargo build --target riscv64gc-unknown-none-elf
<span class="go"> Compiling os v0.1.0 (/media/chyyuu/ca8c7ba6-51b7-41fc-8430-e29e31e5328f/thecode/rust/os_kernel_lab/os)</span>
<span class="go"> Finished dev [unoptimized + debuginfo] target(s) in 0.61s</span>
<span class="gp">$ </span>qemu-riscv64 target/riscv64gc-unknown-none-elf/debug/os<span class="p">;</span> <span class="nb">echo</span> <span class="nv">$?</span>
<span class="go"> Hello, world!</span>
<span class="go"> 9</span>
</pre></div>
</div>
<span class="target" id="start"></span></div>
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