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58<span id="restrict-Pointers"></span><div class="header">
59<p>
60Next: <a href="restrict-Pointer-Example.html" accesskey="n" rel="next">restrict Pointer Example</a>, Previous: <a href="volatile.html" accesskey="p" rel="prev">volatile</a>, Up: <a href="Type-Qualifiers.html" accesskey="u" rel="up">Type Qualifiers</a> &nbsp; [<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>
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62<hr>
63<span id="restrict_002dQualified-Pointers"></span><h3 class="section">21.3 <code>restrict</code>-Qualified Pointers</h3>
64<span id="index-restrict-pointers"></span>
65<span id="index-pointers_002c-restrict_002dqualified"></span>
66<span id="index-restrict"></span>
67
68<p>You can declare a pointer as &ldquo;restricted&rdquo; using the <code>restrict</code>
69type qualifier, like this:
70</p>
71<div class="example">
72<pre class="example">int *restrict p = x;
73</pre></div>
74
75<p>This enables better optimization of code that uses the pointer.
76</p>
77<p>If <code>p</code> is declared with <code>restrict</code>, and then the code
78references the object that <code>p</code> points to (using <code>*p</code> or
79<code>p[<var>i</var>]</code>), the <code>restrict</code> declaration promises that the
80code will not access that object in any other way&mdash;only through
81<code>p</code>.
82</p>
83<p>For instance, it means the code must not use another pointer
84to access the same space, as shown here:
85</p>
86<div class="example">
87<pre class="example">int *restrict p = <var>whatever</var>;
88int *q = p;
89foo (*p, *q);
90</pre></div>
91
92<p>That contradicts the <code>restrict</code> promise by accessing the object
93that <code>p</code> points to using <code>q</code>, which bypasses <code>p</code>.
94Likewise, it must not do this:
95</p>
96<div class="example">
97<pre class="example">int *restrict p = <var>whatever</var>;
98struct { int *a, *b; } s;
99s.a = p;
100foo (*p, *s.a);
101</pre></div>
102
103<p>This example uses a structure field instead of the variable <code>q</code>
104to hold the other pointer, and that contradicts the promise just the
105same.
106</p>
107<p>The keyword <code>restrict</code> also promises that <code>p</code> won&rsquo;t point to
108the allocated space of any automatic or static variable. So the code
109must not do this:
110</p>
111<div class="example">
112<pre class="example">int a;
113int *restrict p = &amp;a;
114foo (*p, a);
115</pre></div>
116
117<p>because that does direct access to the object (<code>a</code>) that <code>p</code>
118points to, which bypasses <code>p</code>.
119</p>
120<p>If the code makes such promises with <code>restrict</code> then breaks them,
121execution is unpredictable.
122</p>
123
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