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| 22 | <title>Pointer Arithmetic (GNU C Language Manual)</title>
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Pointers.html" rel="up" title="Pointers">
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| 33 | <link href="Pointers-and-Arrays.html" rel="next" title="Pointers and Arrays">
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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="Pointer-Arithmetic"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Pointers-and-Arrays.html" accesskey="n" rel="next">Pointers and Arrays</a>, Previous: <a href="Pointer-Comparison.html" accesskey="p" rel="prev">Pointer Comparison</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>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="Pointer-Arithmetic-1"></span><h3 class="section">14.10 Pointer Arithmetic</h3>
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| 64 | <span id="index-pointer-arithmetic"></span>
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| 65 | <span id="index-arithmetic_002c-pointer"></span>
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| 66 |
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| 67 | <p>Adding an integer (positive or negative) to a pointer is valid in C.
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| 68 | It assumes that the pointer points to an element in an array, and
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| 69 | advances or retracts the pointer across as many array elements as the
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| 70 | integer specifies. Here is an example, in which adding a positive
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| 71 | integer advances the pointer to a later element in the same array.
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| 72 | </p>
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| 73 | <div class="example">
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| 74 | <pre class="example">void
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| 75 | incrementing_pointers ()
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| 76 | {
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| 77 | int array[5] = { 45, 29, 104, -3, 123456 };
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| 78 | int elt0, elt1, elt4;
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| 79 |
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| 80 | int *p = &array[0];
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| 81 | /* <span class="roman">Now <code>p</code> points at element 0. Fetch it.</span> */
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| 82 | elt0 = *p;
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| 83 |
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| 84 | ++p;
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| 85 | /* <span class="roman">Now <code>p</code> points at element 1. Fetch it.</span> */
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| 86 | elt1 = *p;
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| 87 |
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| 88 | p += 3;
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| 89 | /* <span class="roman">Now <code>p</code> points at element 4 (the last). Fetch it.</span> */
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| 90 | elt4 = *p;
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| 91 |
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| 92 | printf ("elt0 %d elt1 %d elt4 %d.\n",
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| 93 | elt0, elt1, elt4);
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| 94 | /* <span class="roman">Prints elt0 45 elt1 29 elt4 123456.</span> */
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| 95 | }
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| 96 | </pre></div>
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| 97 |
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| 98 | <p>Here’s an example where adding a negative integer retracts the pointer
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| 99 | to an earlier element in the same array.
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| 100 | </p>
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| 101 | <div class="example">
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| 102 | <pre class="example">void
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| 103 | decrementing_pointers ()
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| 104 | {
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| 105 | int array[5] = { 45, 29, 104, -3, 123456 };
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| 106 | int elt0, elt3, elt4;
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| 107 |
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| 108 | int *p = &array[4];
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| 109 | /* <span class="roman">Now <code>p</code> points at element 4 (the last). Fetch it.</span> */
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| 110 | elt4 = *p;
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| 111 |
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| 112 | --p;
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| 113 | /* <span class="roman">Now <code>p</code> points at element 3. Fetch it.</span> */
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| 114 | elt3 = *p;
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| 115 |
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| 116 | p -= 3;
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| 117 | /* <span class="roman">Now <code>p</code> points at element 0. Fetch it.</span> */
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| 118 | elt0 = *p;
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| 119 |
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| 120 | printf ("elt0 %d elt3 %d elt4 %d.\n",
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| 121 | elt0, elt3, elt4);
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| 122 | /* <span class="roman">Prints elt0 45 elt3 -3 elt4 123456.</span> */
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| 123 | }
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| 124 | </pre></div>
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| 125 |
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| 126 | <p>If one pointer value was made by adding an integer to another
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| 127 | pointer value, it should be possible to subtract the pointer values
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| 128 | and recover that integer. That works too in C.
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| 129 | </p>
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| 130 | <div class="example">
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| 131 | <pre class="example">void
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| 132 | subtract_pointers ()
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| 133 | {
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| 134 | int array[5] = { 45, 29, 104, -3, 123456 };
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| 135 | int *p0, *p3, *p4;
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| 136 |
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| 137 | int *p = &array[4];
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| 138 | /* <span class="roman">Now <code>p</code> points at element 4 (the last). Save the value.</span> */
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| 139 | p4 = p;
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| 140 |
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| 141 | --p;
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| 142 | /* <span class="roman">Now <code>p</code> points at element 3. Save the value.</span> */
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| 143 | p3 = p;
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| 144 |
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| 145 | p -= 3;
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| 146 | /* <span class="roman">Now <code>p</code> points at element 0. Save the value.</span> */
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| 147 | p0 = p;
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| 148 |
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| 149 | printf ("%d, %d, %d, %d\n",
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| 150 | p4 - p0, p0 - p0, p3 - p0, p0 - p3);
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| 151 | /* <span class="roman">Prints 4, 0, 3, -3.</span> */
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| 152 | }
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| 153 | </pre></div>
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| 154 |
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| 155 | <p>The addition operation does not know where arrays are. All it does is
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| 156 | add the integer (multiplied by object size) to the value of the
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| 157 | pointer. When the initial pointer and the result point into a single
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| 158 | array, the result is well-defined.
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| 159 | </p>
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| 160 | <p><strong>Warning:</strong> Only experts should do pointer arithmetic involving pointers
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| 161 | into different memory objects.
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| 162 | </p>
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| 163 | <p>The difference between two pointers has type <code>int</code>, or
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| 164 | <code>long</code> if necessary (see <a href="Integer-Types.html">Integer Types</a>). The clean way to
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| 165 | declare it is to use the typedef name <code>ptrdiff_t</code> defined in the
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| 166 | file <samp>stddef.h</samp>.
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| 167 | </p>
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| 168 | <p>This definition of pointer subtraction is consistent with
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| 169 | pointer-integer addition, in that <code>(p3 - p1) + p1</code> equals
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| 170 | <code>p3</code>, as in ordinary algebra.
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| 171 | </p>
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| 172 | <p>In standard C, addition and subtraction are not allowed on <code>void
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| 173 | *</code>, since the target type’s size is not defined in that case.
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| 174 | Likewise, they are not allowed on pointers to function types.
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| 175 | However, these operations work in GNU C, and the “size of the target
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| 176 | type” is taken as 1.
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| 177 | </p>
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| 178 | <hr>
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| 179 | <div class="header">
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| 180 | <p>
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| 181 | Next: <a href="Pointers-and-Arrays.html" accesskey="n" rel="next">Pointers and Arrays</a>, Previous: <a href="Pointer-Comparison.html" accesskey="p" rel="prev">Pointer Comparison</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>
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| 182 | </div>
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| 183 |
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| 184 |
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| 185 |
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