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<title>Zero Length (Using the GNU Compiler Collection (GCC))</title>

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<a name="Zero-Length"></a>
<div class="header">
<p>
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</div>
<hr>
<a name="Arrays-of-Length-Zero"></a>
<h3 class="section">6.18 Arrays of Length Zero</h3>
<a name="index-arrays-of-length-zero"></a>
<a name="index-zero_002dlength-arrays"></a>
<a name="index-length_002dzero-arrays"></a>
<a name="index-flexible-array-members"></a>

<p>Declaring zero-length arrays is allowed in GNU C as an extension.
A zero-length array can be useful as the last element of a structure
that is really a header for a variable-length object:
</p>
<div class="smallexample">
<pre class="smallexample">struct line {
  int length;
  char contents[0];
};

struct line *thisline = (struct line *)
  malloc (sizeof (struct line) + this_length);
thisline-&gt;length = this_length;
</pre></div>

<p>Although the size of a zero-length array is zero, an array member of
this kind may increase the size of the enclosing type as a result of tail
padding.  The offset of a zero-length array member from the beginning
of the enclosing structure is the same as the offset of an array with
one or more elements of the same type.  The alignment of a zero-length
array is the same as the alignment of its elements.
</p>
<p>Declaring zero-length arrays in other contexts, including as interior
members of structure objects or as non-member objects, is discouraged.
Accessing elements of zero-length arrays declared in such contexts is
undefined and may be diagnosed.
</p>
<p>In the absence of the zero-length array extension, in ISO C90
the <code>contents</code> array in the example above would typically be declared
to have a single element.  Unlike a zero-length array which only contributes
to the size of the enclosing structure for the purposes of alignment,
a one-element array always occupies at least as much space as a single
object of the type.  Although using one-element arrays this way is
discouraged, GCC handles accesses to trailing one-element array members
analogously to zero-length arrays.
</p>
<p>The preferred mechanism to declare variable-length types like
<code>struct line</code> above is the ISO C99 <em>flexible array member</em>,
with slightly different syntax and semantics:
</p>
<ul>
<li> Flexible array members are written as <code>contents[]</code> without
the <code>0</code>.

</li><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><li> Flexible array members may only appear as the last member of a
<code>struct</code> that is otherwise non-empty.

</li><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.)
</li></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 &ldquo;excess
elements in array&rdquo; is given, and the excess elements (all of them, in
this case) are ignored.
</p>
<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>.
</p>
<div class="smallexample">
<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></div>

<p>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>
<p>This has symmetry with normal static arrays, in that an array of
unknown size is also written with <code>[]</code>.
</p>
<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:
</p>
<div class="smallexample">
<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></div>

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