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| 22 | <title>volatile (GNU C Language Manual)</title>
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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="volatile"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="restrict-Pointers.html" accesskey="n" rel="next">restrict Pointers</a>, Previous: <a href="const.html" accesskey="p" rel="prev">const</a>, Up: <a href="Type-Qualifiers.html" accesskey="u" rel="up">Type Qualifiers</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="volatile-Variables-and-Fields"></span><h3 class="section">21.2 <code>volatile</code> Variables and Fields</h3>
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| 64 | <span id="index-volatile-variables-and-fields"></span>
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| 65 | <span id="index-variables_002c-volatile"></span>
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| 66 | <span id="index-volatile"></span>
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| 67 |
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| 68 | <p>The GNU C compiler often performs optimizations that eliminate the
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| 69 | need to write or read a variable. For instance,
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| 70 | </p>
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| 71 | <div class="example">
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| 72 | <pre class="example">int foo;
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| 73 | foo = 1;
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| 74 | foo++;
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| 75 | </pre></div>
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| 76 |
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| 77 | <p>might simply store the value 2 into <code>foo</code>, without ever storing 1.
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| 78 | These optimizations can also apply to structure fields in some cases.
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| 79 | </p>
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| 80 | <p>If the memory containing <code>foo</code> is shared with another program,
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| 81 | or if it is examined asynchronously by hardware, such optimizations
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| 82 | could confuse the communication. Using <code>volatile</code> is one way
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| 83 | to prevent them.
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| 84 | </p>
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| 85 | <p>Writing <code>volatile</code> with the type in a variable or field declaration
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| 86 | says that the value may be examined or changed for reasons outside the
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| 87 | control of the program at any moment. Therefore, the program must
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| 88 | execute in a careful way to assure correct interaction with those
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| 89 | accesses, whenever they may occur.
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| 90 | </p>
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| 91 | <p>The simplest use looks like this:
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| 92 | </p>
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| 93 | <div class="example">
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| 94 | <pre class="example">volatile int lock;
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| 95 | </pre></div>
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| 96 |
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| 97 | <p>This directs the compiler not to do certain common optimizations on
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| 98 | use of the variable <code>lock</code>. All the reads and writes for a volatile
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| 99 | variable or field are really done, and done in the order specified
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| 100 | by the source code. Thus, this code:
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| 101 | </p>
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| 102 | <div class="example">
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| 103 | <pre class="example">lock = 1;
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| 104 | list = list->next;
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| 105 | if (lock)
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| 106 | lock_broken (&lock);
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| 107 | lock = 0;
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| 108 | </pre></div>
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| 109 |
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| 110 | <p>really stores the value 1 in <code>lock</code>, even though there is no
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| 111 | sign it is really used, and the <code>if</code> statement reads and
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| 112 | checks the value of <code>lock</code>, rather than assuming it is still 1.
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| 113 | </p>
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| 114 | <p>A limited amount of optimization can be done, in principle, on
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| 115 | <code>volatile</code> variables and fields: multiple references between two
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| 116 | sequence points (see <a href="Sequence-Points.html">Sequence Points</a>) can be simplified together.
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| 117 | </p>
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| 118 | <p>Use of <code>volatile</code> does not eliminate the flexibility in ordering
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| 119 | the computation of the operands of most operators. For instance, in
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| 120 | <code>lock + foo ()</code>, the order of accessing <code>lock</code> and calling
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| 121 | <code>foo</code> is not specified, so they may be done in either order; the
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| 122 | fact that <code>lock</code> is <code>volatile</code> has no effect on that.
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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="restrict-Pointers.html" accesskey="n" rel="next">restrict Pointers</a>, Previous: <a href="const.html" accesskey="p" rel="prev">const</a>, Up: <a href="Type-Qualifiers.html" accesskey="u" rel="up">Type Qualifiers</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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