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22 | <title>restrict Pointers (GNU C Language Manual)</title>
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30 | <link href="Symbol-Index.html" rel="index" title="Symbol Index">
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31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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32 | <link href="Type-Qualifiers.html" rel="up" title="Type Qualifiers">
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33 | <link href="restrict-Pointer-Example.html" rel="next" title="restrict Pointer Example">
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55 | </head>
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56 |
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57 | <body lang="en">
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58 | <span id="restrict-Pointers"></span><div class="header">
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59 | <p>
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60 | Next: <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> [<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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61 | </div>
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62 | <hr>
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63 | <span id="restrict_002dQualified-Pointers"></span><h3 class="section">21.3 <code>restrict</code>-Qualified Pointers</h3>
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64 | <span id="index-restrict-pointers"></span>
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65 | <span id="index-pointers_002c-restrict_002dqualified"></span>
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66 | <span id="index-restrict"></span>
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67 |
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68 | <p>You can declare a pointer as “restricted” using the <code>restrict</code>
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69 | type qualifier, like this:
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70 | </p>
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71 | <div class="example">
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72 | <pre class="example">int *restrict p = x;
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73 | </pre></div>
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74 |
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75 | <p>This enables better optimization of code that uses the pointer.
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76 | </p>
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77 | <p>If <code>p</code> is declared with <code>restrict</code>, and then the code
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78 | references the object that <code>p</code> points to (using <code>*p</code> or
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79 | <code>p[<var>i</var>]</code>), the <code>restrict</code> declaration promises that the
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80 | code will not access that object in any other way—only through
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81 | <code>p</code>.
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82 | </p>
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83 | <p>For instance, it means the code must not use another pointer
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84 | to access the same space, as shown here:
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85 | </p>
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86 | <div class="example">
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87 | <pre class="example">int *restrict p = <var>whatever</var>;
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88 | int *q = p;
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89 | foo (*p, *q);
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90 | </pre></div>
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91 |
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92 | <p>That contradicts the <code>restrict</code> promise by accessing the object
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93 | that <code>p</code> points to using <code>q</code>, which bypasses <code>p</code>.
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94 | Likewise, it must not do this:
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95 | </p>
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96 | <div class="example">
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97 | <pre class="example">int *restrict p = <var>whatever</var>;
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98 | struct { int *a, *b; } s;
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99 | s.a = p;
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100 | foo (*p, *s.a);
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101 | </pre></div>
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102 |
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103 | <p>This example uses a structure field instead of the variable <code>q</code>
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104 | to hold the other pointer, and that contradicts the promise just the
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105 | same.
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106 | </p>
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107 | <p>The keyword <code>restrict</code> also promises that <code>p</code> won’t point to
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108 | the allocated space of any automatic or static variable. So the code
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109 | must not do this:
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110 | </p>
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111 | <div class="example">
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112 | <pre class="example">int a;
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113 | int *restrict p = &a;
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114 | foo (*p, a);
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115 | </pre></div>
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116 |
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117 | <p>because that does direct access to the object (<code>a</code>) that <code>p</code>
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118 | points to, which bypasses <code>p</code>.
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119 | </p>
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120 | <p>If the code makes such promises with <code>restrict</code> then breaks them,
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121 | execution is unpredictable.
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122 | </p>
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123 |
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124 |
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125 |
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