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| 57 | <body lang="en">
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| 58 | <span id="Static-Assertions"></span><div class="header">
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| 59 | <p>
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| 60 | Previous: <a href="Pragmas.html" accesskey="p" rel="prev">Pragmas</a>, Up: <a href="Directing-Compilation.html" accesskey="u" rel="up">Directing Compilation</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="Static-Assertions-1"></span><h3 class="section">30.2 Static Assertions</h3>
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| 64 | <span id="index-static-assertions"></span>
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| 65 | <span id="index-_005fStatic_005fassert"></span>
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| 66 |
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| 67 | <p>You can add compiler-time tests for necessary conditions into your
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| 68 | code using <code>_Static_assert</code>. This can be useful, for example, to
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| 69 | check that the compilation target platform supports the type sizes
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| 70 | that the code expects. For example,
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| 71 | </p>
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| 72 | <div class="example">
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| 73 | <pre class="example">_Static_assert ((sizeof (long int) >= 8),
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| 74 | "long int needs to be at least 8 bytes");
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| 75 | </pre></div>
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| 76 |
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| 77 | <p>reports a compile-time error if compiled on a system with long
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| 78 | integers smaller than 8 bytes, with ‘<samp>long int needs to be at
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| 79 | least 8 bytes</samp>’ as the error message.
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| 80 | </p>
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| 81 | <p>Since calls <code>_Static_assert</code> are processed at compile time, the
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| 82 | expression must be computable at compile time and the error message
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| 83 | must be a literal string. The expression can refer to the sizes of
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| 84 | variables, but can’t refer to their values. For example, the
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| 85 | following static assertion is invalid for two reasons:
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| 86 | </p>
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| 87 | <div class="example">
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| 88 | <pre class="example">char *error_message
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| 89 | = "long int needs to be at least 8 bytes";
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| 90 | int size_of_long_int = sizeof (long int);
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| 91 |
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| 92 | _Static_assert (size_of_long_int == 8, error_message);
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| 93 | </pre></div>
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| 94 |
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| 95 | <p>The expression <code>size_of_long_int == 8</code> isn’t computable at
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| 96 | compile time, and the error message isn’t a literal string.
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| 97 | </p>
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| 98 | <p>You can, though, use preprocessor definition values with
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| 99 | <code>_Static_assert</code>:
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| 100 | </p>
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| 101 | <div class="example">
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| 102 | <pre class="example">#define LONG_INT_ERROR_MESSAGE "long int needs to be \
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| 103 | at least 8 bytes"
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| 104 |
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| 105 | _Static_assert ((sizeof (long int) == 8),
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| 106 | LONG_INT_ERROR_MESSAGE);
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| 107 | </pre></div>
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| 108 |
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| 109 | <p>Static assertions are permitted wherever a statement or declaration is
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| 110 | permitted, including at top level in the file, and also inside the
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| 111 | definition of a type.
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| 112 | </p>
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| 113 | <div class="example">
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| 114 | <pre class="example">union y
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| 115 | {
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| 116 | int i;
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| 117 | int *ptr;
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| 118 | _Static_assert (sizeof (int *) == sizeof (int),
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| 119 | "Pointer and int not same size");
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| 120 | };
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| 121 | </pre></div>
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| 122 |
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