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| <h3 class="section">6.16 Arrays of Length Zero</h3> |
| |
| <p><a name="index-arrays-of-length-zero-2284"></a><a name="index-zero_002dlength-arrays-2285"></a><a name="index-length_002dzero-arrays-2286"></a><a name="index-flexible-array-members-2287"></a> |
| Zero-length arrays are allowed in GNU C. They are very useful as the |
| last element of a structure which 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>GCC versions before 3.0 allowed zero-length arrays to be statically |
| initialized, as if they were flexible arrays. In addition to those |
| cases that were useful, it also allowed initializations in situations |
| that would corrupt later data. Non-empty initialization of zero-length |
| arrays is now 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>Instead 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. |
| I.e. 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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