| 1 | <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
|---|
| 2 | <html>
|
|---|
| 3 | <!-- Copyright (C) 2022 Richard Stallman and Free Software Foundation, Inc.
|
|---|
| 4 |
|
|---|
| 5 | (The work of Trevis Rothwell and Nelson Beebe has been assigned or
|
|---|
| 6 | licensed to the FSF.)
|
|---|
| 7 |
|
|---|
| 8 | Permission is granted to copy, distribute and/or modify this document
|
|---|
| 9 | under the terms of the GNU Free Documentation License, Version 1.3 or
|
|---|
| 10 | any later version published by the Free Software Foundation; with the
|
|---|
| 11 | Invariant Sections being "GNU General Public License," with the
|
|---|
| 12 | Front-Cover Texts being "A GNU Manual," and with the Back-Cover
|
|---|
| 13 | Texts as in (a) below. A copy of the license is included in the
|
|---|
| 14 | section entitled "GNU Free Documentation License."
|
|---|
| 15 |
|
|---|
| 16 | (a) The FSF's Back-Cover Text is: "You have the freedom to copy and
|
|---|
| 17 | modify this GNU manual. Buying copies from the FSF supports it in
|
|---|
| 18 | developing GNU and promoting software freedom." -->
|
|---|
| 19 | <!-- Created by GNU Texinfo 6.7, http://www.gnu.org/software/texinfo/ -->
|
|---|
| 20 | <head>
|
|---|
| 21 | <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
|---|
| 22 | <title>Pointer Comparison (GNU C Language Manual)</title>
|
|---|
| 23 |
|
|---|
| 24 | <meta name="description" content="Pointer Comparison (GNU C Language Manual)">
|
|---|
| 25 | <meta name="keywords" content="Pointer Comparison (GNU C Language Manual)">
|
|---|
| 26 | <meta name="resource-type" content="document">
|
|---|
| 27 | <meta name="distribution" content="global">
|
|---|
| 28 | <meta name="Generator" content="makeinfo">
|
|---|
| 29 | <link href="index.html" rel="start" title="Top">
|
|---|
| 30 | <link href="Symbol-Index.html" rel="index" title="Symbol Index">
|
|---|
| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
|---|
| 32 | <link href="Pointers.html" rel="up" title="Pointers">
|
|---|
| 33 | <link href="Pointer-Arithmetic.html" rel="next" title="Pointer Arithmetic">
|
|---|
| 34 | <link href="Void-Pointers.html" rel="prev" title="Void Pointers">
|
|---|
| 35 | <style type="text/css">
|
|---|
| 36 | <!--
|
|---|
| 37 | a.summary-letter {text-decoration: none}
|
|---|
| 38 | blockquote.indentedblock {margin-right: 0em}
|
|---|
| 39 | div.display {margin-left: 3.2em}
|
|---|
| 40 | div.example {margin-left: 3.2em}
|
|---|
| 41 | div.lisp {margin-left: 3.2em}
|
|---|
| 42 | kbd {font-style: oblique}
|
|---|
| 43 | pre.display {font-family: inherit}
|
|---|
| 44 | pre.format {font-family: inherit}
|
|---|
| 45 | pre.menu-comment {font-family: serif}
|
|---|
| 46 | pre.menu-preformatted {font-family: serif}
|
|---|
| 47 | span.nolinebreak {white-space: nowrap}
|
|---|
| 48 | span.roman {font-family: initial; font-weight: normal}
|
|---|
| 49 | span.sansserif {font-family: sans-serif; font-weight: normal}
|
|---|
| 50 | ul.no-bullet {list-style: none}
|
|---|
| 51 | -->
|
|---|
| 52 | </style>
|
|---|
| 53 |
|
|---|
| 54 |
|
|---|
| 55 | </head>
|
|---|
| 56 |
|
|---|
| 57 | <body lang="en">
|
|---|
| 58 | <span id="Pointer-Comparison"></span><div class="header">
|
|---|
| 59 | <p>
|
|---|
| 60 | Next: <a href="Pointer-Arithmetic.html" accesskey="n" rel="next">Pointer Arithmetic</a>, Previous: <a href="Void-Pointers.html" accesskey="p" rel="prev">Void Pointers</a>, Up: <a href="Pointers.html" accesskey="u" rel="up">Pointers</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Symbol-Index.html" title="Index" rel="index">Index</a>]</p>
|
|---|
| 61 | </div>
|
|---|
| 62 | <hr>
|
|---|
| 63 | <span id="Pointer-Comparison-1"></span><h3 class="section">14.9 Pointer Comparison</h3>
|
|---|
| 64 | <span id="index-pointer-comparison"></span>
|
|---|
| 65 | <span id="index-comparison_002c-pointer"></span>
|
|---|
| 66 |
|
|---|
| 67 | <p>Two pointer values are equal if they point to the same location, or if
|
|---|
| 68 | they are both null. You can test for this with <code>==</code> and
|
|---|
| 69 | <code>!=</code>. Here’s a trivial example:
|
|---|
| 70 | </p>
|
|---|
| 71 | <div class="example">
|
|---|
| 72 | <pre class="example">{
|
|---|
| 73 | int i;
|
|---|
| 74 | int *p, *q;
|
|---|
| 75 |
|
|---|
| 76 | p = &i;
|
|---|
| 77 | q = &i;
|
|---|
| 78 | if (p == q)
|
|---|
| 79 | printf ("This will be printed.\n");
|
|---|
| 80 | if (p != q)
|
|---|
| 81 | printf ("This won't be printed.\n");
|
|---|
| 82 | }
|
|---|
| 83 | </pre></div>
|
|---|
| 84 |
|
|---|
| 85 | <p>Ordering comparisons such as <code>></code> and <code>>=</code> operate on
|
|---|
| 86 | pointers by converting them to unsigned integers. The C standard says
|
|---|
| 87 | the two pointers must point within the same object in memory, but on
|
|---|
| 88 | GNU/Linux systems these operations simply compare the numeric values
|
|---|
| 89 | of the pointers.
|
|---|
| 90 | </p>
|
|---|
| 91 | <p>The pointer values to be compared should in principle have the same type, but
|
|---|
| 92 | they are allowed to differ in limited cases. First of all, if the two
|
|---|
| 93 | pointers’ target types are nearly compatible (see <a href="Compatible-Types.html">Compatible Types</a>), the comparison is allowed.
|
|---|
| 94 | </p>
|
|---|
| 95 | <p>If one of the operands is <code>void *</code> (see <a href="Void-Pointers.html">Void Pointers</a>) and
|
|---|
| 96 | the other is another pointer type, the comparison operator converts
|
|---|
| 97 | the <code>void *</code> pointer to the other type so as to compare them.
|
|---|
| 98 | (In standard C, this is not allowed if the other type is a function
|
|---|
| 99 | pointer type, but that works in GNU C.)
|
|---|
| 100 | </p>
|
|---|
| 101 | <p>Comparison operators also allow comparing the integer 0 with a pointer
|
|---|
| 102 | value. Thus works by converting 0 to a null pointer of the same type
|
|---|
| 103 | as the other operand.
|
|---|
| 104 | </p>
|
|---|
| 105 |
|
|---|
| 106 |
|
|---|
| 107 |
|
|---|
| 108 | </body>
|
|---|
| 109 | </html>
|
|---|
