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137 lines
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<title>Zero Length - Using the GNU Compiler Collection (GCC)</title>
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<a name="Zero-Length"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="Empty-Structures.html#Empty-Structures">Empty Structures</a>,
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Previous: <a rel="previous" accesskey="p" href="Named-Address-Spaces.html#Named-Address-Spaces">Named Address Spaces</a>,
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Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a>
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<hr>
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</div>
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<h3 class="section">6.17 Arrays of Length Zero</h3>
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<p><a name="index-arrays-of-length-zero-2983"></a><a name="index-zero_002dlength-arrays-2984"></a><a name="index-length_002dzero-arrays-2985"></a><a name="index-flexible-array-members-2986"></a>
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Zero-length arrays are allowed in GNU C. They are very useful as the
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last element of a structure that is really a header for a variable-length
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object:
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<pre class="smallexample"> struct line {
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int length;
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char contents[0];
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};
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struct line *thisline = (struct line *)
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malloc (sizeof (struct line) + this_length);
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thisline->length = this_length;
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</pre>
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<p>In ISO C90, you would have to give <code>contents</code> a length of 1, which
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means either you waste space or complicate the argument to <code>malloc</code>.
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<p>In ISO C99, you would use a <dfn>flexible array member</dfn>, which is
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slightly different in syntax and semantics:
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<ul>
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<li>Flexible array members are written as <code>contents[]</code> without
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the <code>0</code>.
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<li>Flexible array members have incomplete type, and so the <code>sizeof</code>
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operator may not be applied. As a quirk of the original implementation
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of zero-length arrays, <code>sizeof</code> evaluates to zero.
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<li>Flexible array members may only appear as the last member of a
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<code>struct</code> that is otherwise non-empty.
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<li>A structure containing a flexible array member, or a union containing
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such a structure (possibly recursively), may not be a member of a
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structure or an element of an array. (However, these uses are
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permitted by GCC as extensions.)
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</ul>
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<p>Non-empty initialization of zero-length
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arrays is treated like any case where there are more initializer
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elements than the array holds, in that a suitable warning about “excess
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elements in array” is given, and the excess elements (all of them, in
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this case) are ignored.
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<p>GCC allows static initialization of flexible array members.
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This is equivalent to defining a new structure containing the original
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structure followed by an array of sufficient size to contain the data.
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E.g. in the following, <code>f1</code> is constructed as if it were declared
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like <code>f2</code>.
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<pre class="smallexample"> struct f1 {
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int x; int y[];
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} f1 = { 1, { 2, 3, 4 } };
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struct f2 {
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struct f1 f1; int data[3];
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} f2 = { { 1 }, { 2, 3, 4 } };
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</pre>
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<p class="noindent">The convenience of this extension is that <code>f1</code> has the desired
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type, eliminating the need to consistently refer to <code>f2.f1</code>.
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<p>This has symmetry with normal static arrays, in that an array of
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unknown size is also written with <code>[]</code>.
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<p>Of course, this extension only makes sense if the extra data comes at
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the end of a top-level object, as otherwise we would be overwriting
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data at subsequent offsets. To avoid undue complication and confusion
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with initialization of deeply nested arrays, we simply disallow any
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non-empty initialization except when the structure is the top-level
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object. For example:
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<pre class="smallexample"> struct foo { int x; int y[]; };
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struct bar { struct foo z; };
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struct foo a = { 1, { 2, 3, 4 } }; // <span class="roman">Valid.</span>
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struct bar b = { { 1, { 2, 3, 4 } } }; // <span class="roman">Invalid.</span>
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struct bar c = { { 1, { } } }; // <span class="roman">Valid.</span>
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struct foo d[1] = { { 1, { 2, 3, 4 } } }; // <span class="roman">Invalid.</span>
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</pre>
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</body></html>
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