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