1<html lang="en"> 2<head> 3<title>Arrays and pointers implementation - Using the GNU Compiler Collection (GCC)</title> 4<meta http-equiv="Content-Type" content="text/html"> 5<meta name="description" content="Using the GNU Compiler Collection (GCC)"> 6<meta name="generator" content="makeinfo 4.13"> 7<link title="Top" rel="start" href="index.html#Top"> 8<link rel="up" href="C-Implementation.html#C-Implementation" title="C Implementation"> 9<link rel="prev" href="Floating-point-implementation.html#Floating-point-implementation" title="Floating point implementation"> 10<link rel="next" href="Hints-implementation.html#Hints-implementation" title="Hints implementation"> 11<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> 12<!-- 13Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 141998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 152010 Free Software Foundation, Inc. 16 17Permission is granted to copy, distribute and/or modify this document 18under the terms of the GNU Free Documentation License, Version 1.3 or 19any later version published by the Free Software Foundation; with the 20Invariant Sections being ``Funding Free Software'', the Front-Cover 21Texts being (a) (see below), and with the Back-Cover Texts being (b) 22(see below). A copy of the license is included in the section entitled 23``GNU Free Documentation License''. 24 25(a) The FSF's Front-Cover Text is: 26 27 A GNU Manual 28 29(b) The FSF's Back-Cover Text is: 30 31 You have freedom to copy and modify this GNU Manual, like GNU 32 software. Copies published by the Free Software Foundation raise 33 funds for GNU development.--> 34<meta http-equiv="Content-Style-Type" content="text/css"> 35<style type="text/css"><!-- 36 pre.display { font-family:inherit } 37 pre.format { font-family:inherit } 38 pre.smalldisplay { font-family:inherit; font-size:smaller } 39 pre.smallformat { font-family:inherit; font-size:smaller } 40 pre.smallexample { font-size:smaller } 41 pre.smalllisp { font-size:smaller } 42 span.sc { font-variant:small-caps } 43 span.roman { font-family:serif; font-weight:normal; } 44 span.sansserif { font-family:sans-serif; font-weight:normal; } 45--></style> 46<link rel="stylesheet" type="text/css" href="../cs.css"> 47</head> 48<body> 49<div class="node"> 50<a name="Arrays-and-pointers-implementation"></a> 51<p> 52Next: <a rel="next" accesskey="n" href="Hints-implementation.html#Hints-implementation">Hints implementation</a>, 53Previous: <a rel="previous" accesskey="p" href="Floating-point-implementation.html#Floating-point-implementation">Floating point implementation</a>, 54Up: <a rel="up" accesskey="u" href="C-Implementation.html#C-Implementation">C Implementation</a> 55<hr> 56</div> 57 58<h3 class="section">4.7 Arrays and pointers</h3> 59 60 <ul> 61<li><cite>The result of converting a pointer to an integer or 62vice versa (C90 6.3.4, C99 6.3.2.3).</cite> 63 64 <p>A cast from pointer to integer discards most-significant bits if the 65pointer representation is larger than the integer type, 66sign-extends<a rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a> 67if the pointer representation is smaller than the integer type, otherwise 68the bits are unchanged. 69<!-- ??? We've always claimed that pointers were unsigned entities. --> 70<!-- Shouldn't we therefore be doing zero-extension? If so, the bug --> 71<!-- is in convert_to_integer, where we call type_for_size and request --> 72<!-- a signed integral type. On the other hand, it might be most useful --> 73<!-- for the target if we extend according to POINTERS_EXTEND_UNSIGNED. --> 74 75 <p>A cast from integer to pointer discards most-significant bits if the 76pointer representation is smaller than the integer type, extends according 77to the signedness of the integer type if the pointer representation 78is larger than the integer type, otherwise the bits are unchanged. 79 80 <p>When casting from pointer to integer and back again, the resulting 81pointer must reference the same object as the original pointer, otherwise 82the behavior is undefined. That is, one may not use integer arithmetic to 83avoid the undefined behavior of pointer arithmetic as proscribed in 84C99 6.5.6/8. 85 86 <li><cite>The size of the result of subtracting two pointers to elements 87of the same array (C90 6.3.6, C99 6.5.6).</cite> 88 89 <p>The value is as specified in the standard and the type is determined 90by the ABI. 91 92</ul> 93 94 <div class="footnote"> 95<hr> 96<h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> Future versions of GCC may zero-extend, or use 97a target-defined <code>ptr_extend</code> pattern. Do not rely on sign extension.</p> 98 99 <hr></div> 100 101 </body></html> 102 103