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| 22 | <title>Compatible Types (GNU C Language Manual)</title>
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| 57 | <body lang="en">
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| 58 | <span id="Compatible-Types"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Type-Conversions.html" accesskey="n" rel="next">Type Conversions</a>, Previous: <a href="Functions.html" accesskey="p" rel="prev">Functions</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>
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| 61 | </div>
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| 62 | <hr>
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| 63 | <span id="Compatible-Types-1"></span><h2 class="chapter">23 Compatible Types</h2>
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| 64 | <span id="index-compatible-types"></span>
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| 65 | <span id="index-types_002c-compatible"></span>
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| 66 |
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| 67 | <p>Declaring a function or variable twice is valid in C only if the two
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| 68 | declarations specify <em>compatible</em> types. In addition, some
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| 69 | operations on pointers require operands to have compatible target
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| 70 | types.
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| 71 | </p>
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| 72 | <p>In C, two different primitive types are never compatible. Likewise for
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| 73 | the defined types <code>struct</code>, <code>union</code> and <code>enum</code>: two
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| 74 | separately defined types are incompatible unless they are defined
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| 75 | exactly the same way.
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| 76 | </p>
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| 77 | <p>However, there are a few cases where different types can be
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| 78 | compatible:
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| 79 | </p>
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| 80 | <ul>
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| 81 | <li> Every enumeration type is compatible with some integer type. In GNU
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| 82 | C, the choice of integer type depends on the largest enumeration
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| 83 | value.
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| 84 |
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| 85 |
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| 86 | </li><li> Array types are compatible if the element types are compatible
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| 87 | and the sizes (when specified) match.
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| 88 |
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| 89 | </li><li> Pointer types are compatible if the pointer target types are
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| 90 | compatible.
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| 91 |
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| 92 | </li><li> Function types that specify argument types are compatible if the
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| 93 | return types are compatible and the argument types are compatible,
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| 94 | argument by argument. In addition, they must all agree in whether
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| 95 | they use <code>...</code> to allow additional arguments.
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| 96 |
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| 97 | </li><li> Function types that don’t specify argument types are compatible if the
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| 98 | return types are.
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| 99 |
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| 100 | </li><li> Function types that specify the argument types are compatible with
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| 101 | function types that omit them, if the return types are compatible and
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| 102 | the specified argument types are unaltered by the argument promotions
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| 103 | (see <a href="Argument-Promotions.html">Argument Promotions</a>).
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| 104 | </li></ul>
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| 105 |
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| 106 | <p>In order for types to be compatible, they must agree in their type
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| 107 | qualifiers. Thus, <code>const int</code> and <code>int</code> are incompatible.
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| 108 | It follows that <code>const int *</code> and <code>int *</code> are incompatible
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| 109 | too (they are pointers to types that are not compatible).
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| 110 | </p>
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| 111 | <p>If two types are compatible ignoring the qualifiers, we call them
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| 112 | <em>nearly compatible</em>. (If they are array types, we ignore
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| 113 | qualifiers on the element types.<a id="DOCF7" href="#FOOT7"><sup>7</sup></a>)
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| 114 | Comparison of pointers is valid if the pointers’ target types are
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| 115 | nearly compatible. Likewise, the two branches of a conditional
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| 116 | expression may be pointers to nearly compatible target types.
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| 117 | </p>
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| 118 | <p>If two types are compatible ignoring the qualifiers, and the first
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| 119 | type has all the qualifiers of the second type, we say the first is
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| 120 | <em>upward compatible</em> with the second. Assignment of pointers
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| 121 | requires the assigned pointer’s target type to be upward compatible
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| 122 | with the right operand (the new value)’s target type.
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| 123 | </p>
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| 124 | <div class="footnote">
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| 125 | <hr>
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| 126 | <h4 class="footnotes-heading">Footnotes</h4>
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| 127 |
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| 128 | <h5><a id="FOOT7" href="#DOCF7">(7)</a></h3>
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| 129 | <p>This is a GNU C extension.</p>
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| 130 | </div>
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| 131 | <hr>
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| 132 | <div class="header">
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| 133 | <p>
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| 134 | Next: <a href="Type-Conversions.html" accesskey="n" rel="next">Type Conversions</a>, Previous: <a href="Functions.html" accesskey="p" rel="prev">Functions</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>
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