diff options
Diffstat (limited to 'share/doc/gcc/Variable-Length.html')
| -rw-r--r-- | share/doc/gcc/Variable-Length.html | 180 |
1 files changed, 180 insertions, 0 deletions
diff --git a/share/doc/gcc/Variable-Length.html b/share/doc/gcc/Variable-Length.html new file mode 100644 index 0000000..be406fd --- /dev/null +++ b/share/doc/gcc/Variable-Length.html @@ -0,0 +1,180 @@ +<!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): Variable Length</title> + +<meta name="description" content="Using the GNU Compiler Collection (GCC): Variable Length"> +<meta name="keywords" content="Using the GNU Compiler Collection (GCC): Variable Length"> +<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="Variadic-Macros.html#Variadic-Macros" rel="next" title="Variadic Macros"> +<link href="Empty-Structures.html#Empty-Structures" rel="previous" title="Empty Structures"> +<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="Variable-Length"></a> +<div class="header"> +<p> +Next: <a href="Variadic-Macros.html#Variadic-Macros" accesskey="n" rel="next">Variadic Macros</a>, Previous: <a href="Empty-Structures.html#Empty-Structures" accesskey="p" rel="previous">Empty Structures</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="Arrays-of-Variable-Length"></a> +<h3 class="section">6.20 Arrays of Variable Length</h3> +<a name="index-variable_002dlength-arrays"></a> +<a name="index-arrays-of-variable-length"></a> +<a name="index-VLAs"></a> + +<p>Variable-length automatic arrays are allowed in ISO C99, and as an +extension GCC accepts them in C90 mode and in C++. These arrays are +declared like any other automatic arrays, but with a length that is not +a constant expression. The storage is allocated at the point of +declaration and deallocated when the block scope containing the declaration +exits. For +example: +</p> +<div class="smallexample"> +<pre class="smallexample">FILE * +concat_fopen (char *s1, char *s2, char *mode) +{ + char str[strlen (s1) + strlen (s2) + 1]; + strcpy (str, s1); + strcat (str, s2); + return fopen (str, mode); +} +</pre></div> + +<a name="index-scope-of-a-variable-length-array"></a> +<a name="index-variable_002dlength-array-scope"></a> +<a name="index-deallocating-variable-length-arrays"></a> +<p>Jumping or breaking out of the scope of the array name deallocates the +storage. Jumping into the scope is not allowed; you get an error +message for it. +</p> +<a name="index-variable_002dlength-array-in-a-structure"></a> +<p>As an extension, GCC accepts variable-length arrays as a member of +a structure or a union. For example: +</p> +<div class="smallexample"> +<pre class="smallexample">void +foo (int n) +{ + struct S { int x[n]; }; +} +</pre></div> + +<a name="index-alloca-vs-variable_002dlength-arrays"></a> +<p>You can use the function <code>alloca</code> to get an effect much like +variable-length arrays. The function <code>alloca</code> is available in +many other C implementations (but not in all). On the other hand, +variable-length arrays are more elegant. +</p> +<p>There are other differences between these two methods. Space allocated +with <code>alloca</code> exists until the containing <em>function</em> returns. +The space for a variable-length array is deallocated as soon as the array +name’s scope ends, unless you also use <code>alloca</code> in this scope. +</p> +<p>You can also use variable-length arrays as arguments to functions: +</p> +<div class="smallexample"> +<pre class="smallexample">struct entry +tester (int len, char data[len][len]) +{ + /* <span class="roman">…</span> */ +} +</pre></div> + +<p>The length of an array is computed once when the storage is allocated +and is remembered for the scope of the array in case you access it with +<code>sizeof</code>. +</p> +<p>If you want to pass the array first and the length afterward, you can +use a forward declaration in the parameter list—another GNU extension. +</p> +<div class="smallexample"> +<pre class="smallexample">struct entry +tester (int len; char data[len][len], int len) +{ + /* <span class="roman">…</span> */ +} +</pre></div> + +<a name="index-parameter-forward-declaration"></a> +<p>The ‘<samp>int len</samp>’ before the semicolon is a <em>parameter forward +declaration</em>, and it serves the purpose of making the name <code>len</code> +known when the declaration of <code>data</code> is parsed. +</p> +<p>You can write any number of such parameter forward declarations in the +parameter list. They can be separated by commas or semicolons, but the +last one must end with a semicolon, which is followed by the “real” +parameter declarations. Each forward declaration must match a “real” +declaration in parameter name and data type. ISO C99 does not support +parameter forward declarations. +</p> +<hr> +<div class="header"> +<p> +Next: <a href="Variadic-Macros.html#Variadic-Macros" accesskey="n" rel="next">Variadic Macros</a>, Previous: <a href="Empty-Structures.html#Empty-Structures" accesskey="p" rel="previous">Empty Structures</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> |