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| 22 | <title>Shift Caveats (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="Shift-Operations.html" rel="up" title="Shift Operations">
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| 33 | <link href="Shift-Hacks.html" rel="next" title="Shift Hacks">
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| 55 | </head>
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| 57 | <body lang="en">
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| 58 | <span id="Shift-Caveats"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Shift-Hacks.html" accesskey="n" rel="next">Shift Hacks</a>, Previous: <a href="Bits-Shifted-In.html" accesskey="p" rel="prev">Bits Shifted In</a>, Up: <a href="Shift-Operations.html" accesskey="u" rel="up">Shift Operations</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="Caveats-for-Shift-Operations"></span><h4 class="subsection">6.7.2 Caveats for Shift Operations</h4>
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| 64 |
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| 65 | <p><strong>Warning:</strong> If the shift count is greater than or equal to the
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| 66 | width in bits of the first operand, the results are machine-dependent.
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| 67 | Logically speaking, the “correct” value would be either -1 (for
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| 68 | right shift of a negative number) or 0 (in all other cases), but what
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| 69 | it really generates is whatever the machine’s shift instruction does in
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| 70 | that case. So unless you can prove that the second operand is not too
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| 71 | large, write code to check it at run time.
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| 72 | </p>
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| 73 | <p><strong>Warning:</strong> Never rely on how the shift operators relate in
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| 74 | precedence to other arithmetic binary operators. Programmers don’t
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| 75 | remember these precedences, and won’t understand the code. Always use
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| 76 | parentheses to explicitly specify the nesting, like this:
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| 77 | </p>
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| 78 | <div class="example">
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| 79 | <pre class="example">a + (b << 5) /* <span class="roman">Shift first, then add.</span> */
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| 80 | (a + b) << 5 /* <span class="roman">Add first, then shift.</span> */
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| 81 | </pre></div>
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| 82 |
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| 83 | <p>Note: according to the C standard, shifting of signed values isn’t
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| 84 | guaranteed to work properly when the value shifted is negative, or
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| 85 | becomes negative during the operation of shifting left. However, only
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| 86 | pedants have a reason to be concerned about this; only computers with
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| 87 | strange shift instructions could plausibly do this wrong. In GNU C,
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| 88 | the operation always works as expected,
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| 89 | </p>
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| 90 |
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| 91 |
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| 92 |
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| 93 | </body>
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| 94 | </html>
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