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| 20 | <head>
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| 21 | <meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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| 22 | <title>Blocks (GNU C Language Manual)</title>
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| 23 |
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| 24 | <meta name="description" content="Blocks (GNU C Language Manual)">
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| 29 | <link href="index.html" rel="start" title="Top">
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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="Statements.html" rel="up" title="Statements">
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| 33 | <link href="return-Statement.html" rel="next" title="return Statement">
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| 34 | <link href="if_002delse-Statement.html" rel="prev" title="if-else Statement">
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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="Blocks"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="return-Statement.html" accesskey="n" rel="next">return Statement</a>, Previous: <a href="if_002delse-Statement.html" accesskey="p" rel="prev">if-else Statement</a>, Up: <a href="Statements.html" accesskey="u" rel="up">Statements</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="Blocks-1"></span><h3 class="section">19.4 Blocks</h3>
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| 64 | <span id="index-block"></span>
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| 65 | <span id="index-compound-statement"></span>
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| 66 |
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| 67 | <p>A <em>block</em> is a construct that contains multiple statements of any
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| 68 | kind. It begins with ‘<samp>{</samp>’ and ends with ‘<samp>}</samp>’, and has a
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| 69 | series of statements and declarations in between. Another name for
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| 70 | blocks is <em>compound statements</em>.
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| 71 | </p>
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| 72 | <p>Is a block a statement? Yes and no. It doesn’t <em>look</em> like a
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| 73 | normal statement—it does not end with a semicolon. But you can
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| 74 | <em>use</em> it like a statement; anywhere that a statement is required
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| 75 | or allowed, you can write a block and consider that block a statement.
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| 76 | </p>
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| 77 | <p>So far it seems that a block is a kind of statement with an unusual
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| 78 | syntax. But that is not entirely true: a function body is also a
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| 79 | block, and that block is definitely not a statement. The text after a
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| 80 | function header is not treated as a statement; only a function body is
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| 81 | allowed there, and nothing else would be meaningful there.
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| 82 | </p>
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| 83 | <p>In a formal grammar we would have to choose—either a block is a kind
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| 84 | of statement or it is not. But this manual is meant for humans, not
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| 85 | for parser generators. The clearest answer for humans is, “a block
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| 86 | is a statement, in some ways.”
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| 87 | </p>
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| 88 | <span id="index-nested-block"></span>
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| 89 | <span id="index-internal-block"></span>
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| 90 | <p>A block that isn’t a function body is called an <em>internal block</em>
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| 91 | or a <em>nested block</em>. You can put a nested block directly inside
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| 92 | another block, but more often the nested block is inside some complex
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| 93 | statement, such as a <code>for</code> statement or an <code>if</code> statement.
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| 94 | </p>
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| 95 | <p>There are two uses for nested blocks in C:
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| 96 | </p>
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| 97 | <ul>
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| 98 | <li> To specify the scope for local declarations. For instance, a local
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| 99 | variable’s scope is the rest of the innermost containing block.
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| 100 |
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| 101 | </li><li> To write a series of statements where, syntactically, one statement is
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| 102 | called for. For instance, the <var>execute-if-true</var> of an <code>if</code>
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| 103 | statement is one statement. To put multiple statements there, they
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| 104 | have to be wrapped in a block, like this:
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| 105 |
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| 106 | <div class="example">
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| 107 | <pre class="example">if (x < 0)
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| 108 | {
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| 109 | printf ("x was negative\n");
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| 110 | x = -x;
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| 111 | }
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| 112 | </pre></div>
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| 113 | </li></ul>
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| 114 |
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| 115 | <p>This example (repeated from above) shows a nested block which serves
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| 116 | both purposes: it includes two statements (plus a declaration) in the
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| 117 | body of a <code>while</code> statement, and it provides the scope for the
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| 118 | declaration of <code>q</code>.
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| 119 | </p>
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| 120 | <div class="example">
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| 121 | <pre class="example">void
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| 122 | free_intlist (struct intlistlink *p)
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| 123 | {
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| 124 | while (p)
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| 125 | {
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| 126 | struct intlistlink *q = p;
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| 127 | p = p->next;
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| 128 | free (q);
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| 129 | }
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| 130 | }
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| 131 | </pre></div>
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| 132 |
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| 133 | <hr>
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| 134 | <div class="header">
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| 135 | <p>
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| 136 | Next: <a href="return-Statement.html" accesskey="n" rel="next">return Statement</a>, Previous: <a href="if_002delse-Statement.html" accesskey="p" rel="prev">if-else Statement</a>, Up: <a href="Statements.html" accesskey="u" rel="up">Statements</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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| 137 | </div>
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| 138 |
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| 139 |
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| 140 |
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| 141 | </body>
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| 142 | </html>
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