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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-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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<p>由于后续实验需要 <code class="docutils literal notranslate"><span class="pre">rustc</span></code> 编译器缺省生成RISC-V 64的目标代码，所以我们首先要给 <code class="docutils literal notranslate"><span class="pre">rustc</span></code> 添加一个target : <code class="docutils literal notranslate"><span class="pre">riscv64gc-unknown-none-elf</span></code> 。这可通过如下命令来完成：</p>
<div class="highlight-bash notranslate"><div class="highlight"><pre><span></span>$ rustup target add riscv64gc-unknown-none-elf
</pre></div>
</div>
<p>然后在 <code class="docutils literal notranslate"><span class="pre">os</span></code> 目录下新建 <code class="docutils literal notranslate"><span class="pre">.cargo</span></code> 目录，并在这个目录下创建 <code class="docutils literal notranslate"><span class="pre">config</span></code> 文件，输入如下内容：</p>
<div class="highlight-toml notranslate"><div class="highlight"><pre><span></span><span class="c1"># os/.cargo/config</span>
<span class="k">[build]</span>
<span class="n">target</span> <span class="o">=</span> <span class="s">"riscv64gc-unknown-none-elf"</span>
</pre></div>
</div>
<p>这将使 cargo 工具在 os 目录下默认会使用 riscv64gc-unknown-none-elf 作为目标平台。
这种编译器运行的平台（x86_64）与可执行文件运行的目标平台不同的情况，称为 <strong>交叉编译</strong> (Cross Compile)。</p>
<div class="section" id="println">
<h2>移除 println! 宏<a class="headerlink" href="#println" title="永久链接至标题"></a></h2>
<p>我们在 <code class="docutils literal notranslate"><span class="pre">main.rs</span></code> 的开头加上一行 <code class="docutils literal notranslate"><span class="pre">#![no_std]</span></code>，
告诉 Rust 编译器不使用 Rust 标准库 std 转而使用核心库 core。重新编译，报错如下：</p>
<div class="admonition error">
<p class="admonition-title">错误</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">error: cannot find macro `println` in this scope</span>
<span class="go">--&gt; src/main.rs:4:5</span>
<span class="go"> |</span>
<span class="go">4 | println!("Hello, world!");</span>
<span class="go"> | ^^^^^^^</span>
</pre></div>
</div>
</div>
<p>println! 宏是由标准库 std 提供的，且会使用到一个名为 write 的系统调用。
无论如何，我们先将这行代码注释掉。</p>
</div>
<div class="section" id="panic-handler">
<h2>提供语义项 panic_handler<a class="headerlink" href="#panic-handler" title="永久链接至标题"></a></h2>
<div class="admonition error">
<p class="admonition-title">错误</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">error: `#[panic_handler]` function required, but not found</span>
</pre></div>
</div>
</div>
<p>标准库 std 提供了 Rust 错误处理函数 <code class="docutils literal notranslate"><span class="pre">#[panic_handler]</span></code>，其大致功能是打印出错位置和原因并杀死当前应用。
但核心库 core 并没有提供这项功能，得靠我们自己实现。</p>
<p>新建一个子模块 <code class="docutils literal notranslate"><span class="pre">lang_items.rs</span></code>，在里面编写 panic 处理函数，通过标记 <code class="docutils literal notranslate"><span class="pre">#[panic_handler]</span></code> 告知编译器采用我们的实现：</p>
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="c1">// os/src/lang_items.rs</span>
<span class="k">use</span><span class="w"> </span><span class="n">core</span>::<span class="n">panic</span>::<span class="n">PanicInfo</span><span class="p">;</span><span class="w"></span>
<span class="cp">#[panic_handler]</span><span class="w"></span>
<span class="k">fn</span> <span class="nf">panic</span><span class="p">(</span><span class="n">_info</span>: <span class="kp">&amp;</span><span class="nc">PanicInfo</span><span class="p">)</span><span class="w"> </span>-&gt; <span class="o">!</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="k">loop</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>在把 <code class="docutils literal notranslate"><span class="pre">panic_handler</span></code> 配置在单独的文件 <code class="docutils literal notranslate"><span class="pre">os/src/lang_items.rs</span></code> 后，需要在os/src/main.rs文件中添加以下内容才能正常编译整个软件：</p>
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="c1">// os/src/main.rs</span>
<span class="cp">#![no_std]</span><span class="w"></span>
<span class="k">mod</span> <span class="nn">lang_items</span><span class="p">;</span><span class="w"></span>
<span class="c1">// ... other code</span>
</pre></div>
</div>
<p>注意，panic 处理函数的函数签名需要一个 <code class="docutils literal notranslate"><span class="pre">PanicInfo</span></code> 的不可变借用作为输入参数，它在核心库中得以保留，这也是我们第一次与核心库打交道。之后我们会从 <code class="docutils literal notranslate"><span class="pre">PanicInfo</span></code> 解析出错位置并打印出来，然后杀死应用程序。但目前编译出的简单OS在运行时如果遇到错误什么都不做，只在原地 <code class="docutils literal notranslate"><span class="pre">loop</span></code> 。</p>
</div>
<div class="section" id="main">
<h2>移除 main 函数<a class="headerlink" href="#main" title="永久链接至标题"></a></h2>
<p>重新编译，又有了新错误：</p>
<div class="admonition error">
<p class="admonition-title">错误</p>
<div class="highlight-default notranslate"><div class="highlight"><pre><span></span>$ cargo build
Compiling os v0.1.0 (/home/shinbokuow/workspace/v3/rCore-Tutorial-v3/os)
error: requires `start` lang_item
</pre></div>
</div>
</div>
<p>编译器提醒我们缺少一个名为 <code class="docutils literal notranslate"><span class="pre">start</span></code> 的语义项。
<code class="docutils literal notranslate"><span class="pre">start</span></code> 语义项代表了标准库 std 在执行应用程序之前需要进行的一些初始化工作。由于我们禁用了标准库，编译器也就找不到这项功能的实现了。</p>
<p>在 <code class="docutils literal notranslate"><span class="pre">main.rs</span></code> 的开头加入设置 <code class="docutils literal notranslate"><span class="pre">#![no_main]</span></code> 告诉编译器我们没有一般意义上的 <code class="docutils literal notranslate"><span class="pre">main</span></code> 函数，
并将原来的 <code class="docutils literal notranslate"><span class="pre">main</span></code> 函数删除。这样编译器也就不需要考虑初始化工作了。</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>
</pre></div>
</div>
<p>至此，我们终于移除了所有标准库依赖。目前的主要代码包括 <code class="docutils literal notranslate"><span class="pre">main.rs</span></code> 和 <code class="docutils literal notranslate"><span class="pre">lang_items.rs</span></code> ，大致内容如下：</p>
<div class="highlight-rust notranslate"><div class="highlight"><pre><span></span><span class="c1">// os/src/main.rs</span>
<span class="cp">#![no_main]</span><span class="w"></span>
<span class="cp">#![no_std]</span><span class="w"></span>
<span class="k">mod</span> <span class="nn">lang_items</span><span class="p">;</span><span class="w"></span>
<span class="c1">// ... other code</span>
<span class="c1">// os/src/lang_items.rs</span>
<span class="k">use</span><span class="w"> </span><span class="n">core</span>::<span class="n">panic</span>::<span class="n">PanicInfo</span><span class="p">;</span><span class="w"></span>
<span class="cp">#[panic_handler]</span><span class="w"></span>
<span class="k">fn</span> <span class="nf">panic</span><span class="p">(</span><span class="n">_info</span>: <span class="kp">&amp;</span><span class="nc">PanicInfo</span><span class="p">)</span><span class="w"> </span>-&gt; <span class="o">!</span><span class="w"> </span><span class="p">{</span><span class="w"></span>
<span class="w"> </span><span class="k">loop</span><span class="w"> </span><span class="p">{}</span><span class="w"></span>
<span class="p">}</span><span class="w"></span>
</pre></div>
</div>
</div>
<div class="section" id="id2">
<h2>分析被移除标准库的程序<a class="headerlink" href="#id2" title="永久链接至标题"></a></h2>
<p>首先安装 cargo-binutils 工具集：</p>
<p>我们可以通过各种工具来分析目前的程序：</p>
<div class="highlight-console notranslate"><div class="highlight"><pre><span></span><span class="go">[文件格式]</span>
<span class="gp">$ </span>file target/riscv64gc-unknown-none-elf/debug/os
<span class="go">target/riscv64gc-unknown-none-elf/debug/os: ELF 64-bit LSB executable, UCB RISC-V, ......</span>
<span class="go">[文件头信息]</span>
<span class="gp">$ </span>rust-readobj -h target/riscv64gc-unknown-none-elf/debug/os
<span class="go"> File: target/riscv64gc-unknown-none-elf/debug/os</span>
<span class="go"> Format: elf64-littleriscv</span>
<span class="go"> Arch: riscv64</span>
<span class="go"> AddressSize: 64bit</span>
<span class="go"> ......</span>
<span class="go"> Type: Executable (0x2)</span>
<span class="go"> Machine: EM_RISCV (0xF3)</span>
<span class="go"> Version: 1</span>
<span class="go"> Entry: 0x0</span>
<span class="go"> ......</span>
<span class="go"> }</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>
</pre></div>
</div>
<p>通过 <code class="docutils literal notranslate"><span class="pre">file</span></code> 工具对二进制程序 <code class="docutils literal notranslate"><span class="pre">os</span></code> 的分析可以看到，它好像是一个合法的 RV64 执行程序，
但 <code class="docutils literal notranslate"><span class="pre">rust-readobj</span></code> 工具告诉我们它的入口地址 Entry 是 <code class="docutils literal notranslate"><span class="pre">0</span></code>。
再通过 <code class="docutils literal notranslate"><span class="pre">rust-objdump</span></code> 工具把它反汇编，没有生成任何汇编代码。
可见，这个二进制程序虽然合法，但它是一个空程序，原因是缺少了编译器规定的入口函数 <code class="docutils literal notranslate"><span class="pre">_start</span></code> 。</p>
<p>从下一节开始，我们将着手实现本节移除的、由用户态执行环境提供的功能。</p>
<div class="admonition note">
<p class="admonition-title">注解</p>
<p>本节内容部分参考自 <a class="reference external" href="https://os.phil-opp.com/freestanding-rust-binary/">BlogOS 的相关章节</a> 。</p>
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<ul>
<li><a class="reference internal" href="#">移除标准库依赖</a><ul>
<li><a class="reference internal" href="#println">移除 println! 宏</a></li>
<li><a class="reference internal" href="#panic-handler">提供语义项 panic_handler</a></li>
<li><a class="reference internal" href="#main">移除 main 函数</a></li>
<li><a class="reference internal" href="#id2">分析被移除标准库的程序</a></li>
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