| 1 | <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
|---|
| 2 | <html>
|
|---|
| 3 | <!-- Copyright (C) 2022 Richard Stallman and Free Software Foundation, Inc.
|
|---|
| 4 |
|
|---|
| 5 | (The work of Trevis Rothwell and Nelson Beebe has been assigned or
|
|---|
| 6 | licensed to the FSF.)
|
|---|
| 7 |
|
|---|
| 8 | Permission is granted to copy, distribute and/or modify this document
|
|---|
| 9 | under the terms of the GNU Free Documentation License, Version 1.3 or
|
|---|
| 10 | any later version published by the Free Software Foundation; with the
|
|---|
| 11 | Invariant Sections being "GNU General Public License," with the
|
|---|
| 12 | Front-Cover Texts being "A GNU Manual," and with the Back-Cover
|
|---|
| 13 | Texts as in (a) below. A copy of the license is included in the
|
|---|
| 14 | section entitled "GNU Free Documentation License."
|
|---|
| 15 |
|
|---|
| 16 | (a) The FSF's Back-Cover Text is: "You have the freedom to copy and
|
|---|
| 17 | modify this GNU manual. Buying copies from the FSF supports it in
|
|---|
| 18 | developing GNU and promoting software freedom." -->
|
|---|
| 19 | <!-- Created by GNU Texinfo 6.7, http://www.gnu.org/software/texinfo/ -->
|
|---|
| 20 | <head>
|
|---|
| 21 | <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
|---|
| 22 | <title>Binary Operator Grammar (GNU C Language Manual)</title>
|
|---|
| 23 |
|
|---|
| 24 | <meta name="description" content="Binary Operator Grammar (GNU C Language Manual)">
|
|---|
| 25 | <meta name="keywords" content="Binary Operator Grammar (GNU C Language Manual)">
|
|---|
| 26 | <meta name="resource-type" content="document">
|
|---|
| 27 | <meta name="distribution" content="global">
|
|---|
| 28 | <meta name="Generator" content="makeinfo">
|
|---|
| 29 | <link href="index.html" rel="start" title="Top">
|
|---|
| 30 | <link href="Symbol-Index.html" rel="index" title="Symbol Index">
|
|---|
| 31 | <link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
|---|
| 32 | <link href="index.html" rel="up" title="Top">
|
|---|
| 33 | <link href="Order-of-Execution.html" rel="next" title="Order of Execution">
|
|---|
| 34 | <link href="Avoid-Comma.html" rel="prev" title="Avoid Comma">
|
|---|
| 35 | <style type="text/css">
|
|---|
| 36 | <!--
|
|---|
| 37 | a.summary-letter {text-decoration: none}
|
|---|
| 38 | blockquote.indentedblock {margin-right: 0em}
|
|---|
| 39 | div.display {margin-left: 3.2em}
|
|---|
| 40 | div.example {margin-left: 3.2em}
|
|---|
| 41 | div.lisp {margin-left: 3.2em}
|
|---|
| 42 | kbd {font-style: oblique}
|
|---|
| 43 | pre.display {font-family: inherit}
|
|---|
| 44 | pre.format {font-family: inherit}
|
|---|
| 45 | pre.menu-comment {font-family: serif}
|
|---|
| 46 | pre.menu-preformatted {font-family: serif}
|
|---|
| 47 | span.nolinebreak {white-space: nowrap}
|
|---|
| 48 | span.roman {font-family: initial; font-weight: normal}
|
|---|
| 49 | span.sansserif {font-family: sans-serif; font-weight: normal}
|
|---|
| 50 | ul.no-bullet {list-style: none}
|
|---|
| 51 | -->
|
|---|
| 52 | </style>
|
|---|
| 53 |
|
|---|
| 54 |
|
|---|
| 55 | </head>
|
|---|
| 56 |
|
|---|
| 57 | <body lang="en">
|
|---|
| 58 | <span id="Binary-Operator-Grammar"></span><div class="header">
|
|---|
| 59 | <p>
|
|---|
| 60 | Next: <a href="Order-of-Execution.html" accesskey="n" rel="next">Order of Execution</a>, Previous: <a href="Execution-Control-Expressions.html" accesskey="p" rel="prev">Execution Control Expressions</a>, Up: <a href="index.html" accesskey="u" rel="up">Top</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>
|
|---|
| 61 | </div>
|
|---|
| 62 | <hr>
|
|---|
| 63 | <span id="Binary-Operator-Grammar-1"></span><h2 class="chapter">9 Binary Operator Grammar</h2>
|
|---|
| 64 | <span id="index-binary-operator-grammar"></span>
|
|---|
| 65 | <span id="index-grammar_002c-binary-operator"></span>
|
|---|
| 66 | <span id="index-operator-precedence"></span>
|
|---|
| 67 | <span id="index-precedence_002c-operator"></span>
|
|---|
| 68 | <span id="index-left_002dassociative"></span>
|
|---|
| 69 |
|
|---|
| 70 | <p><em>Binary operators</em> are those that take two operands, one
|
|---|
| 71 | on the left and one on the right.
|
|---|
| 72 | </p>
|
|---|
| 73 | <p>All the binary operators in C are syntactically left-associative.
|
|---|
| 74 | This means that <code>a <var>op</var> b <var>op</var> c</code><!-- /@w --> means <code>(a <var>op</var> b) <var>op</var> c</code><!-- /@w -->. However, you should only write repeated
|
|---|
| 75 | operators without parentheses using ‘<samp>+</samp>’, ‘<samp>-</samp>’, ‘<samp>*</samp>’ and
|
|---|
| 76 | ‘<samp>/</samp>’, because those cases are clear from algebra. So it is ok to
|
|---|
| 77 | write <code>a + b + c</code> or <code>a - b - c</code>, but never <code>a == b ==
|
|---|
| 78 | c</code> or <code>a % b % c</code>.
|
|---|
| 79 | </p>
|
|---|
| 80 | <p>Each C operator has a <em>precedence</em>, which is its rank in the
|
|---|
| 81 | grammatical order of the various operators. The operators with the
|
|---|
| 82 | highest precedence grab adjoining operands first; these expressions
|
|---|
| 83 | then become operands for operators of lower precedence.
|
|---|
| 84 | </p>
|
|---|
| 85 | <p>The precedence order of operators in C is fully specified, so any
|
|---|
| 86 | combination of operations leads to a well-defined nesting. We state
|
|---|
| 87 | only part of the full precedence ordering here because it is bad
|
|---|
| 88 | practice for C code to depend on the other cases. For cases not
|
|---|
| 89 | specified in this chapter, always use parentheses to make the nesting
|
|---|
| 90 | explicit.<a id="DOCF2" href="#FOOT2"><sup>2</sup></a>
|
|---|
| 91 | </p>
|
|---|
| 92 | <p>You can depend on this subsequence of the precedence ordering
|
|---|
| 93 | (stated from highest precedence to lowest):
|
|---|
| 94 | </p>
|
|---|
| 95 | <ol>
|
|---|
| 96 | <li> Component access (‘<samp>.</samp>’ and ‘<samp>-></samp>’).
|
|---|
| 97 |
|
|---|
| 98 | </li><li> Unary prefix operators.
|
|---|
| 99 |
|
|---|
| 100 | </li><li> Unary postfix operators.
|
|---|
| 101 |
|
|---|
| 102 | </li><li> Multiplication, division, and remainder (they have the same precedence).
|
|---|
| 103 |
|
|---|
| 104 | </li><li> Addition and subtraction (they have the same precedence).
|
|---|
| 105 |
|
|---|
| 106 | </li><li> Comparisons—but watch out!
|
|---|
| 107 |
|
|---|
| 108 | </li><li> Logical operators ‘<samp>&&</samp>’ and ‘<samp>||</samp>’—but watch out!
|
|---|
| 109 |
|
|---|
| 110 | </li><li> Conditional expression with ‘<samp>?</samp>’ and ‘<samp>:</samp>’.
|
|---|
| 111 |
|
|---|
| 112 | </li><li> Assignments.
|
|---|
| 113 |
|
|---|
| 114 | </li><li> Sequential execution (the comma operator, ‘<samp>,</samp>’).
|
|---|
| 115 | </li></ol>
|
|---|
| 116 |
|
|---|
| 117 | <p>Two of the lines in the above list say “but watch out!” That means
|
|---|
| 118 | that the line covers operators with subtly different precedence.
|
|---|
| 119 | Never depend on the grammar of C to decide how two comparisons nest;
|
|---|
| 120 | instead, always use parentheses to specify their nesting.
|
|---|
| 121 | </p>
|
|---|
| 122 | <p>You can let several ‘<samp>&&</samp>’ operators associate, or several
|
|---|
| 123 | ‘<samp>||</samp>’ operators, but always use parentheses to show how ‘<samp>&&</samp>’
|
|---|
| 124 | and ‘<samp>||</samp>’ nest with each other. See <a href="Logical-Operators.html">Logical Operators</a>.
|
|---|
| 125 | </p>
|
|---|
| 126 | <p>There is one other precedence ordering that code can depend on:
|
|---|
| 127 | </p>
|
|---|
| 128 | <ol>
|
|---|
| 129 | <li> Unary postfix operators.
|
|---|
| 130 |
|
|---|
| 131 | </li><li> Bitwise and shift operators—but watch out!
|
|---|
| 132 |
|
|---|
| 133 | </li><li> Conditional expression with ‘<samp>?</samp>’ and ‘<samp>:</samp>’.
|
|---|
| 134 | </li></ol>
|
|---|
| 135 |
|
|---|
| 136 | <p>The caveat for bitwise and shift operators is like that for logical
|
|---|
| 137 | operators: you can let multiple uses of one bitwise operator
|
|---|
| 138 | associate, but always use parentheses to control nesting of dissimilar
|
|---|
| 139 | operators.
|
|---|
| 140 | </p>
|
|---|
| 141 | <p>These lists do not specify any precedence ordering between the bitwise
|
|---|
| 142 | and shift operators of the second list and the binary operators above
|
|---|
| 143 | conditional expressions in the first list. When they come together,
|
|---|
| 144 | parenthesize them. See <a href="Bitwise-Operations.html">Bitwise Operations</a>.
|
|---|
| 145 | </p>
|
|---|
| 146 | <div class="footnote">
|
|---|
| 147 | <hr>
|
|---|
| 148 | <h4 class="footnotes-heading">Footnotes</h4>
|
|---|
| 149 |
|
|---|
| 150 | <h5><a id="FOOT2" href="#DOCF2">(2)</a></h3>
|
|---|
| 151 | <p>Personal note from Richard Stallman: I wrote GCC without
|
|---|
| 152 | remembering anything about the C precedence order beyond what’s stated
|
|---|
| 153 | here. I studied the full precedence table to write the parser, and
|
|---|
| 154 | promptly forgot it again. If you need to look up the full precedence order
|
|---|
| 155 | to understand some C code, fix the code with parentheses so nobody else
|
|---|
| 156 | needs to do that.</p>
|
|---|
| 157 | </div>
|
|---|
| 158 | <hr>
|
|---|
| 159 | <div class="header">
|
|---|
| 160 | <p>
|
|---|
| 161 | Next: <a href="Order-of-Execution.html" accesskey="n" rel="next">Order of Execution</a>, Previous: <a href="Execution-Control-Expressions.html" accesskey="p" rel="prev">Execution Control Expressions</a>, Up: <a href="index.html" accesskey="u" rel="up">Top</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>
|
|---|
| 162 | </div>
|
|---|
| 163 |
|
|---|
| 164 |
|
|---|
| 165 |
|
|---|
| 166 | </body>
|
|---|
| 167 | </html>
|
|---|
