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diff --git a/share/doc/gcc/Labels-as-Values.html b/share/doc/gcc/Labels-as-Values.html new file mode 100644 index 0000000..f7c2f70 --- /dev/null +++ b/share/doc/gcc/Labels-as-Values.html @@ -0,0 +1,176 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd"> +<html> +<!-- This file documents the use of the GNU compilers. + +Copyright (C) 1988-2023 Free Software Foundation, Inc. + +Permission is granted to copy, distribute and/or modify this document +under the terms of the GNU Free Documentation License, Version 1.3 or +any later version published by the Free Software Foundation; with the +Invariant Sections being "Funding Free Software", the Front-Cover +Texts being (a) (see below), and with the Back-Cover Texts being (b) +(see below). A copy of the license is included in the section entitled +"GNU Free Documentation License". + +(a) The FSF's Front-Cover Text is: + +A GNU Manual + +(b) The FSF's Back-Cover Text is: + +You have freedom to copy and modify this GNU Manual, like GNU + software. Copies published by the Free Software Foundation raise + funds for GNU development. --> +<!-- Created by GNU Texinfo 5.1, http://www.gnu.org/software/texinfo/ --> +<head> +<title>Using the GNU Compiler Collection (GCC): Labels as Values</title> + +<meta name="description" content="Using the GNU Compiler Collection (GCC): Labels as Values"> +<meta name="keywords" content="Using the GNU Compiler Collection (GCC): Labels as Values"> +<meta name="resource-type" content="document"> +<meta name="distribution" content="global"> +<meta name="Generator" content="makeinfo"> +<meta http-equiv="Content-Type" content="text/html; charset=utf-8"> +<link href="index.html#Top" rel="start" title="Top"> +<link href="Indices.html#Indices" rel="index" title="Indices"> +<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents"> +<link href="C-Extensions.html#C-Extensions" rel="up" title="C Extensions"> +<link href="Nested-Functions.html#Nested-Functions" rel="next" title="Nested Functions"> +<link href="Local-Labels.html#Local-Labels" rel="previous" title="Local Labels"> +<style type="text/css"> +<!-- +a.summary-letter {text-decoration: none} +blockquote.smallquotation {font-size: smaller} +div.display {margin-left: 3.2em} +div.example {margin-left: 3.2em} +div.indentedblock {margin-left: 3.2em} +div.lisp {margin-left: 3.2em} +div.smalldisplay {margin-left: 3.2em} +div.smallexample {margin-left: 3.2em} +div.smallindentedblock {margin-left: 3.2em; font-size: smaller} +div.smalllisp {margin-left: 3.2em} +kbd {font-style:oblique} +pre.display {font-family: inherit} +pre.format {font-family: inherit} +pre.menu-comment {font-family: serif} +pre.menu-preformatted {font-family: serif} +pre.smalldisplay {font-family: inherit; font-size: smaller} +pre.smallexample {font-size: smaller} +pre.smallformat {font-family: inherit; font-size: smaller} +pre.smalllisp {font-size: smaller} +span.nocodebreak {white-space:nowrap} +span.nolinebreak {white-space:nowrap} +span.roman {font-family:serif; font-weight:normal} +span.sansserif {font-family:sans-serif; font-weight:normal} +ul.no-bullet {list-style: none} +--> +</style> + + +</head> + +<body lang="en_US" bgcolor="#FFFFFF" text="#000000" link="#0000FF" vlink="#800080" alink="#FF0000"> +<a name="Labels-as-Values"></a> +<div class="header"> +<p> +Next: <a href="Nested-Functions.html#Nested-Functions" accesskey="n" rel="next">Nested Functions</a>, Previous: <a href="Local-Labels.html#Local-Labels" accesskey="p" rel="previous">Local Labels</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html#Indices" title="Index" rel="index">Index</a>]</p> +</div> +<hr> +<a name="Labels-as-Values-1"></a> +<h3 class="section">6.3 Labels as Values</h3> +<a name="index-labels-as-values"></a> +<a name="index-computed-gotos"></a> +<a name="index-goto-with-computed-label"></a> +<a name="index-address-of-a-label"></a> + +<p>You can get the address of a label defined in the current function +(or a containing function) with the unary operator ‘<samp>&&</samp>’. The +value has type <code>void *</code>. This value is a constant and can be used +wherever a constant of that type is valid. For example: +</p> +<div class="smallexample"> +<pre class="smallexample">void *ptr; +/* <span class="roman">…</span> */ +ptr = &&foo; +</pre></div> + +<p>To use these values, you need to be able to jump to one. This is done +with the computed goto statement<a name="DOCF6" href="#FOOT6"><sup>6</sup></a>, <code>goto *<var>exp</var>;</code>. For example, +</p> +<div class="smallexample"> +<pre class="smallexample">goto *ptr; +</pre></div> + +<p>Any expression of type <code>void *</code> is allowed. +</p> +<p>One way of using these constants is in initializing a static array that +serves as a jump table: +</p> +<div class="smallexample"> +<pre class="smallexample">static void *array[] = { &&foo, &&bar, &&hack }; +</pre></div> + +<p>Then you can select a label with indexing, like this: +</p> +<div class="smallexample"> +<pre class="smallexample">goto *array[i]; +</pre></div> + +<p>Note that this does not check whether the subscript is in bounds—array +indexing in C never does that. +</p> +<p>Such an array of label values serves a purpose much like that of the +<code>switch</code> statement. The <code>switch</code> statement is cleaner, so +use that rather than an array unless the problem does not fit a +<code>switch</code> statement very well. +</p> +<p>Another use of label values is in an interpreter for threaded code. +The labels within the interpreter function can be stored in the +threaded code for super-fast dispatching. +</p> +<p>You may not use this mechanism to jump to code in a different function. +If you do that, totally unpredictable things happen. The best way to +avoid this is to store the label address only in automatic variables and +never pass it as an argument. +</p> +<p>An alternate way to write the above example is +</p> +<div class="smallexample"> +<pre class="smallexample">static const int array[] = { &&foo - &&foo, &&bar - &&foo, + &&hack - &&foo }; +goto *(&&foo + array[i]); +</pre></div> + +<p>This is more friendly to code living in shared libraries, as it reduces +the number of dynamic relocations that are needed, and by consequence, +allows the data to be read-only. +This alternative with label differences is not supported for the AVR target, +please use the first approach for AVR programs. +</p> +<p>The <code>&&foo</code> expressions for the same label might have different +values if the containing function is inlined or cloned. If a program +relies on them being always the same, +<code>__attribute__((__noinline__,__noclone__))</code> should be used to +prevent inlining and cloning. If <code>&&foo</code> is used in a static +variable initializer, inlining and cloning is forbidden. +</p> +<div class="footnote"> +<hr> +<h4 class="footnotes-heading">Footnotes</h4> + +<h3><a name="FOOT6" href="#DOCF6">(6)</a></h3> +<p>The analogous feature in +Fortran is called an assigned goto, but that name seems inappropriate in +C, where one can do more than simply store label addresses in label +variables.</p> +</div> +<hr> +<div class="header"> +<p> +Next: <a href="Nested-Functions.html#Nested-Functions" accesskey="n" rel="next">Nested Functions</a>, Previous: <a href="Local-Labels.html#Local-Labels" accesskey="p" rel="previous">Local Labels</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Indices.html#Indices" title="Index" rel="index">Index</a>]</p> +</div> + + + +</body> +</html> |