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| 22 | <title>Signed Overflow (GNU C Language Manual)</title>
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| 23 |
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| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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| 32 | <link href="Integer-Overflow.html" rel="up" title="Integer Overflow">
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| 33 | <link href="Mixed-Mode.html" rel="next" title="Mixed Mode">
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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="Signed-Overflow"></span><div class="header">
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| 59 | <p>
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| 60 | Previous: <a href="Unsigned-Overflow.html" accesskey="p" rel="prev">Unsigned 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-Signed-Integers"></span><h4 class="subsection">6.3.2 Overflow with Signed Integers</h4>
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| 64 | <span id="index-compiler-options-for-integer-overflow"></span>
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| 65 | <span id="index-integer-overflow_002c-compiler-options"></span>
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| 66 | <span id="index-overflow_002c-compiler-options"></span>
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| 67 |
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| 68 | <p>For signed integers, the result of overflow in C is <em>in
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| 69 | principle</em> undefined, meaning that anything whatsoever could happen.
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| 70 | Therefore, C compilers can do optimizations that treat the overflow
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| 71 | case with total unconcern. (Since the result of overflow is undefined
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| 72 | in principle, one cannot claim that these optimizations are
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| 73 | erroneous.)
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| 74 | </p>
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| 75 | <p><strong>Watch out:</strong> These optimizations can do surprising things. For
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| 76 | instance,
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| 77 | </p>
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| 78 | <div class="example">
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| 79 | <pre class="example">int i;
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| 80 | <span class="roman">…</span>
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| 81 | if (i < i + 1)
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| 82 | x = 5;
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| 83 | </pre></div>
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| 84 |
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| 85 | <p>could be optimized to do the assignment unconditionally, because the
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| 86 | <code>if</code>-condition is always true if <code>i + 1</code> does not overflow.
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| 87 | </p>
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| 88 | <p>GCC offers compiler options to control handling signed integer
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| 89 | overflow. These options operate per module; that is, each module
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| 90 | behaves according to the options it was compiled with.
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| 91 | </p>
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| 92 | <p>These two options specify particular ways to handle signed integer
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| 93 | overflow, other than the default way:
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| 94 | </p>
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| 95 | <dl compact="compact">
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| 96 | <dt><samp>-fwrapv</samp></dt>
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| 97 | <dd><p>Make signed integer operations well-defined, like unsigned integer
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| 98 | operations: they produce the <var>n</var> low-order bits of the true
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| 99 | result. The highest of those <var>n</var> bits is the sign bit of the
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| 100 | result. With <samp>-fwrapv</samp>, these out-of-range operations are not
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| 101 | considered overflow, so (strictly speaking) integer overflow never
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| 102 | happens.
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| 103 | </p>
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| 104 | <p>The option <samp>-fwrapv</samp> enables some optimizations based on the
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| 105 | defined values of out-of-range results. In GCC 8, it disables
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| 106 | optimizations that are based on assuming signed integer operations
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| 107 | will not overflow.
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| 108 | </p>
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| 109 | </dd>
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| 110 | <dt><samp>-ftrapv</samp></dt>
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| 111 | <dd><p>Generate a signal <code>SIGFPE</code> when signed integer overflow occurs.
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| 112 | This terminates the program unless the program handles the signal.
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| 113 | See <a href="Signals.html">Signals</a>.
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| 114 | </p></dd>
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| 115 | </dl>
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| 116 |
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| 117 | <p>One other option is useful for finding where overflow occurs:
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| 118 | </p>
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| 119 |
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| 120 | <dl compact="compact">
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| 121 | <dt><samp>-fsanitize=signed-integer-overflow</samp></dt>
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| 122 | <dd><p>Output a warning message at run time when signed integer overflow
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| 123 | occurs. This checks the ‘<samp>+</samp>’, ‘<samp>*</samp>’, and ‘<samp>-</samp>’ operators.
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| 124 | This takes priority over <samp>-ftrapv</samp>.
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| 125 | </p></dd>
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| 126 | </dl>
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| 127 |
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| 128 | <hr>
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| 129 | <div class="header">
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| 130 | <p>
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| 131 | Previous: <a href="Unsigned-Overflow.html" accesskey="p" rel="prev">Unsigned 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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| 132 | </div>
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| 134 |
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| 135 |
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