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| 58 | <span id="Unsigned-Overflow"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Signed-Overflow.html" accesskey="n" rel="next">Signed Overflow</a>, Up: <a href="Integer-Overflow.html" accesskey="u" rel="up">Integer Overflow</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="Overflow-with-Unsigned-Integers"></span><h4 class="subsection">6.3.1 Overflow with Unsigned Integers</h4>
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| 64 |
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| 65 | <p>Unsigned arithmetic in C ignores overflow; it produces the true result
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| 66 | modulo the <var>n</var>th power of 2, where <var>n</var> is the number of bits
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| 67 | in the data type. We say it “truncates” the true result to the
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| 68 | lowest <var>n</var> bits.
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| 69 | </p>
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| 70 | <p>A true result that is negative, when taken modulo the <var>n</var>th power
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| 71 | of 2, yields a positive number. For instance,
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| 72 | </p>
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| 73 | <div class="example">
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| 74 | <pre class="example">unsigned int x = 1;
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| 75 | unsigned int y;
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| 76 |
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| 77 | y = -x;
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| 78 | </pre></div>
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| 79 |
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| 80 | <p>causes overflow because the negative number -1 can’t be stored
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| 81 | in an unsigned type. The actual result, which is -1 modulo the
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| 82 | <var>n</var>th power of 2, is one less than the <var>n</var>th power of 2. That
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| 83 | is the largest value that the unsigned data type can store. For a
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| 84 | 32-bit <code>unsigned int</code>, the value is 4,294,967,295. See <a href="Maximum-and-Minimum-Values.html">Maximum and Minimum Values</a>.
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| 85 | </p>
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| 86 | <p>Adding that number to itself, as here,
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| 87 | </p>
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| 88 | <div class="example">
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| 89 | <pre class="example">unsigned int z;
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| 90 |
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| 91 | z = y + y;
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| 92 | </pre></div>
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| 93 |
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| 94 | <p>ought to yield 8,489,934,590; however, that is again too large to fit,
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| 95 | so overflow truncates the value to 4,294,967,294. If that were a
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| 96 | signed integer, it would mean -2, which (not by coincidence)
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| 97 | equals -1 + -1.
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| 98 | </p>
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| 99 |
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| 100 |
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| 101 |
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| 103 | </html>
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