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| 22 | <title>Limitations of C Arrays (GNU C Language Manual)</title>
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| 32 | <link href="Arrays.html" rel="up" title="Arrays">
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| 57 | <body lang="en">
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| 58 | <span id="Limitations-of-C-Arrays"></span><div class="header">
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| 59 | <p>
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| 60 | Next: <a href="Multidimensional-Arrays.html" accesskey="n" rel="next">Multidimensional Arrays</a>, Previous: <a href="Incomplete-Array-Types.html" accesskey="p" rel="prev">Incomplete Array Types</a>, Up: <a href="Arrays.html" accesskey="u" rel="up">Arrays</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="Limitations-of-C-Arrays-1"></span><h3 class="section">16.6 Limitations of C Arrays</h3>
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| 64 | <span id="index-limitations-of-C-arrays"></span>
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| 65 | <span id="index-first_002dclass-object"></span>
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| 66 |
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| 67 | <p>Arrays have quirks in C because they are not “first-class objects”:
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| 68 | there is no way in C to operate on an array as a unit.
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| 69 | </p>
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| 70 | <p>The other composite objects in C, structures and unions, are
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| 71 | first-class objects: a C program can copy a structure or union value
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| 72 | in an assignment, or pass one as an argument to a function, or make a
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| 73 | function return one. You can’t do those things with an array in C.
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| 74 | That is because a value you can operate on never has an array type.
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| 75 | </p>
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| 76 | <p>An expression in C can have an array type, but that doesn’t produce
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| 77 | the array as a value. Instead it is converted automatically to a
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| 78 | pointer to the array’s element at index zero. The code can operate
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| 79 | on the pointer, and through that on individual elements of the array,
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| 80 | but it can’t get and operate on the array as a unit.
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| 81 | </p>
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| 82 | <p>There are three exceptions to this conversion rule, but none of them
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| 83 | offers a way to operate on the array as a whole.
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| 84 | </p>
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| 85 | <p>First, ‘<samp>&</samp>’ applied to an expression with array type gives you the
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| 86 | address of the array, as an array type. However, you can’t operate on the
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| 87 | whole array that way—if you apply ‘<samp>*</samp>’ to get the array back,
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| 88 | that expression converts, as usual, to a pointer to its zeroth
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| 89 | element.
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| 90 | </p>
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| 91 | <p>Second, the operators <code>sizeof</code>, <code>_Alignof</code>, and
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| 92 | <code>typeof</code> do not convert the array to a pointer; they leave it as
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| 93 | an array. But they don’t operate on the array’s data—they only give
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| 94 | information about its type.
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| 95 | </p>
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| 96 | <p>Third, a string constant used as an initializer for an array is not
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| 97 | converted to a pointer—rather, the declaration copies the
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| 98 | <em>contents</em> of that string in that one special case.
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| 99 | </p>
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| 100 | <p>You <em>can</em> copy the contents of an array, just not with an
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| 101 | assignment operator. You can do it by calling the library function
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| 102 | <code>memcpy</code> or <code>memmove</code> (see <a href="https://www.gnu.org/software/libc/manual/html_node/Copying-and-Concatenation.html#Copying-and-Concatenation">The
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| 103 | GNU C Library</a> in <cite>The GNU C Library Reference Manual</cite>). Also,
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| 104 | when a structure contains just an array, you can copy that structure.
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| 105 | </p>
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| 106 | <p>An array itself is an lvalue if it is a declared variable, or part of
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| 107 | a structure or union that is an lvalue. When you construct an array
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| 108 | from elements (see <a href="Constructing-Array-Values.html">Constructing Array Values</a>), that array is not
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| 109 | an lvalue.
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| 110 | </p>
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| 111 | <hr>
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| 112 | <div class="header">
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| 113 | <p>
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| 114 | Next: <a href="Multidimensional-Arrays.html" accesskey="n" rel="next">Multidimensional Arrays</a>, Previous: <a href="Incomplete-Array-Types.html" accesskey="p" rel="prev">Incomplete Array Types</a>, Up: <a href="Arrays.html" accesskey="u" rel="up">Arrays</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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