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233 lines
8.5 KiB
HTML
233 lines
8.5 KiB
HTML
<!DOCTYPE html>
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<html lang="en">
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<head>
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<meta charset="UTF-8" />
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<meta name="viewport" content="width=device-width, initial-scale=1.0" />
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<link rel="stylesheet" href="index.css" />
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<title>Womangling</title>
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</head>
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<body>
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<main class="main">
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<div class="content" id="content-area">
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<h1>
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<a class="root-link" href=".">Learn C++ Itanium Symbol Mangling</a>
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</h1>
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<h2>Lesson 1: Basics</h2>
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<noscript>
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<p>
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Warning: You have JavaScript disabled While the content is still
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viewable, interactive exercises will not work. Consider enabling
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JavaScript for this website.
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</p>
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</noscript>
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<section data-step="0" class="step">
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<p>
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After getting an understanding of how this guide works and learning
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about the not-mangling of C identifiers, we are ready to dive into
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C++.
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</p>
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<p>
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Every C++ mangled symbol is prefixed with the string
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<code>_Z</code>. This signifies that this is a mangled C++ symbol.
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<code>_Z</code> starts with an underscore followed by an uppercase
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letter. All symbols of that structures are reserved by the C
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standard and cannot be used by programs. This ensures that there are
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no name collisions with normal C functions and mangled C++
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functions.
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</p>
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<p>
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After that, the name of the entity is stored. For now, we will only
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look at functions. For functions, the function type is appended to
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the name to get the full symbol.
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</p>
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<pre class="code">
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void f() {}
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</pre>
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<p>
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This empty function will be mangled to <code>_Z1fv</code>. The
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<code>1f</code> signifies the name (we will look at this in more
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detail later in this lesson) and the <code>v</code> signifies the
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function type.
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</p>
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<p>
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We will see the <code>v</code> function type a lot in the rest of
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this guide. It stands for a function that takes no arguments and
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returns <code>void</code>.
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</p>
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<div class="quiz-section">
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<p>
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Which of these symbols cannot possibly be a mangled C++ symbol?
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Answer with the name of the symbol.
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</p>
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<ul>
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<li><code>_ZN3FooIA4_iE3barE</code></li>
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<li><code>_ZN6System5Sound4beepEv</code></li>
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<li><code>_RN3FooIA4_iE3barE</code></li>
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</ul>
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<form
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data-challenge="1"
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data-answer="_RN3FooIA4_iE3barE"
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data-hint="Look at the prefix"
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>
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<input class="quiz-input" />
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<button
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data-challenge-submit="1"
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class="submit-challenge"
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type="submit"
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>
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Answer
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</button>
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<div class="error"></div>
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</form>
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</div>
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</section>
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<section data-step="1" class="step">
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<p>
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For names, there are two cases to consider for now. Either the name
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is in the global scope, or it is in a namespace.
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</p>
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<p>For global names, we just prefix the name with its length.</p>
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<pre class="code">
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void hello_world() {}
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</pre>
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<p>
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This will therefore get mangled as <code>_Z11hello_worldv</code>.
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The length of <code>hello_world</code> is 11, so we concatenate
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<code>11</code> and <code>hello_world</code>. This entire thing is
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then appended to the previously mentioned prefix <code>_Z</code> and
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then we add the type, which is just <code>v</code> here, at the end.
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</p>
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<div class="quiz-section">
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<p>What is the mangling of the following identifier?</p>
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<pre class="code">
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void meow() {}
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</pre>
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<form
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data-challenge="2"
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data-answer="_Z4meowv"
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data-hint="Remember the prefix and function type"
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>
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<input class="quiz-input" />
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<button
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data-challenge-submit="2"
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class="submit-challenge"
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type="submit"
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>
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Answer
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</button>
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<div class="error"></div>
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</form>
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</div>
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</section>
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<section data-step="2" class="step">
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<p>
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Functions that are declared in a namespace get a bit more
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complicated. They are referred to as <i>nested names</i>, because
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they are <i>nested</i> in a namespace. They can also be nested in
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multiple namespaces, the encoding is the same.
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</p>
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<p>
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Nested names start with an <code>N</code> and end with an
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<code>E</code>. Between those two letters, the hierarchy of the
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namespace is represented by putting on namespace name after another,
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with the function name last. Every name has the leading length and
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then the name itself, just like with global names.
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</p>
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<pre class="code">
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namespace outer {
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void inner() {}
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}
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</pre>
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<p>
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That means that this function will be mangled as
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<code>_ZN5outer5innerEv</code>. We can decode this into the
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following structure
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</p>
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<ul>
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<li><code>_Z</code>: Prefix</li>
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<li><code>N</code>: Start of nested name</li>
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<li>
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<code>5outer</code>: Outer namespace, name prefixed by length
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</li>
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<li>
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<code>5inner</code>: Inner function, name prefixed by length
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</li>
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<li><code>E</code>: End of nested name</li>
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<li><code>v</code>: Function type</li>
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</ul>
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<p>Nested namespaces follow the same structure.</p>
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<pre class="code">
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namespace a {
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namespace b {
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namespace c {
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void inner() {}
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}
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}
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}
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</pre>
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<p>
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This function will mangle as <code>_ZN1a1b1c5innerEv</code>. We get
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all the concatinated names as <code>1a1b1c5inner</code>, with the
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previously mentioned characters around them.
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</p>
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<div class="quiz-section">
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<p>What is the mangling of the following identifier?</p>
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<pre class="code">
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namespace cats {
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namespace like {
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void meow() {}
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}
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}
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</pre>
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<form
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data-challenge="3"
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data-answer="_ZN4cats4like4meowEv"
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data-hint="Remember the prefix and function type, and don't forget to wrap it in the nested start and end"
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>
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<input class="quiz-input" />
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<button
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data-challenge-submit="3"
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class="submit-challenge"
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type="submit"
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>
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Answer
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</button>
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<div class="error"></div>
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</form>
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</div>
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</section>
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<section data-step="3" class="step">
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<p>
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Good job! You have successfully answered all the question and now
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know the basic makeup of an Itanium-mangled C++ symbol.
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</p>
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<p>
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In the next lesson, we will use this knowledge to look at basic
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function types beyond <code>v</code>. Mangling function types is
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important for function overloading, but I don't want to overload you
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with information, so feel free to take a break and let the previous
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knowledge sink in.
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</p>
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<p>
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If you want to try out more code and look at its mangling, I
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recommend using Compiler Explorer on
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<a href="https://godbolt.org">godbolt.org</a>. Under "Output", you
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can uncheck the box to demangle identifiers to see the mangled
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identifiers for any C++ code you enter on the left.
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</p>
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<div class="center">
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<a href="lesson-2.html" class="action-button">
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Lesson 2: something that has not been written yet.
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</a>
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</div>
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</section>
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</div>
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</main>
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<script>
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window.LESSON = 1;
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</script>
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<script type="module" src="lessons.js"></script>
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</html>
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