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| 22 | <title>Swallowing the Semicolon (GNU C Language Manual)</title>
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| 32 | <link href="Macro-Pitfalls.html" rel="up" title="Macro Pitfalls">
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| 58 | <span id="Swallowing-the-Semicolon"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Duplication-of-Side-Effects.html" accesskey="n" rel="next">Duplication of Side Effects</a>, Previous: <a href="Operator-Precedence-Problems.html" accesskey="p" rel="prev">Operator Precedence Problems</a>, Up: <a href="Macro-Pitfalls.html" accesskey="u" rel="up">Macro Pitfalls</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="Swallowing-the-Semicolon-1"></span><h4 class="subsubsection">26.5.10.3 Swallowing the Semicolon</h4>
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| 64 | <span id="index-semicolons-_0028after-macro-calls_0029"></span>
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| 65 |
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| 66 | <p>Often it is desirable to define a macro that expands into a compound
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| 67 | statement. Consider, for example, the following macro, that advances a
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| 68 | pointer (the parameter <code>p</code> says where to find it) across whitespace
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| 69 | characters:
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| 70 | </p>
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| 71 | <div class="example">
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| 72 | <pre class="example">#define SKIP_SPACES(p, limit) \
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| 73 | { char *lim = (limit); \
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| 74 | while (p < lim) { \
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| 75 | if (*p++ != ' ') { \
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| 76 | p--; break; }}}
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| 77 | </pre></div>
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| 78 |
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| 79 | <p>Here backslash-newline is used to split the macro definition, which must
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| 80 | be a single logical line, so that it resembles the way such code would
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| 81 | be laid out if not part of a macro definition.
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| 82 | </p>
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| 83 | <p>A call to this macro might be <code>SKIP_SPACES (p, lim)</code>. Strictly
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| 84 | speaking, the call expands to a compound statement, which is a complete
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| 85 | statement with no need for a semicolon to end it. However, since it
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| 86 | looks like a function call, it minimizes confusion if you can use it
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| 87 | like a function call, writing a semicolon afterward, as in
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| 88 | <code>SKIP_SPACES (p, lim);</code>
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| 89 | </p>
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| 90 | <p>This can cause trouble before <code>else</code> statements, because the
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| 91 | semicolon is actually a null statement. Suppose you write
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| 92 | </p>
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| 93 | <div class="example">
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| 94 | <pre class="example">if (*p != 0)
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| 95 | SKIP_SPACES (p, lim);
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| 96 | else /* <span class="roman">…</span> */
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| 97 | </pre></div>
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| 98 |
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| 99 | <p>The presence of two statements—the compound statement and a null
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| 100 | statement—in between the <code>if</code> condition and the <code>else</code>
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| 101 | makes invalid C code.
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| 102 | </p>
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| 103 | <p>The definition of the macro <code>SKIP_SPACES</code> can be altered to solve
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| 104 | this problem, using a <code>do <span class="roman">…</span> while</code> statement. Here is how:
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| 105 | </p>
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| 106 | <div class="example">
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| 107 | <pre class="example">#define SKIP_SPACES(p, limit) \
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| 108 | do { char *lim = (limit); \
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| 109 | while (p < lim) { \
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| 110 | if (*p++ != ' ') { \
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| 111 | p--; break; }}} \
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| 112 | while (0)
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| 113 | </pre></div>
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| 114 |
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| 115 | <p>Now <code>SKIP_SPACES (p, lim);</code> expands into
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| 116 | </p>
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| 117 | <div class="example">
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| 118 | <pre class="example">do { /* <span class="roman">…</span> */ } while (0);
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| 119 | </pre></div>
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| 120 |
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| 121 | <p>which is one statement. The loop executes exactly once; most compilers
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| 122 | generate no extra code for it.
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| 123 | </p>
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| 124 | <hr>
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| 125 | <div class="header">
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| 126 | <p>
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| 127 | Next: <a href="Duplication-of-Side-Effects.html" accesskey="n" rel="next">Duplication of Side Effects</a>, Previous: <a href="Operator-Precedence-Problems.html" accesskey="p" rel="prev">Operator Precedence Problems</a>, Up: <a href="Macro-Pitfalls.html" accesskey="u" rel="up">Macro Pitfalls</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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