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<html lang="en"><head><title>Using the GNU Compiler Collection (GCC)</title><meta http-equiv="Content-Type" content="text/html"><meta name="description" content="Using the GNU Compiler Collection (GCC)"><meta name="generator" content="makeinfo 4.6"><!--Copyright &copy; 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998,1999, 2000, 2001, 2002, 2003, 2004 Free Software Foundation, Inc.   <p>Permission is granted to copy, distribute and/or modify this documentunder the terms of the GNU Free Documentation License, Version 1.2 orany later version published by the Free Software Foundation; with theInvariant Sections being "GNU General Public License" and "FundingFree Software", the Front-Cover texts being (a) (see below), and withthe Back-Cover Texts being (b) (see below).  A copy of the license isincluded in the section entitled "GNU Free Documentation License".   <p>(a) The FSF's Front-Cover Text is:   <p>A GNU Manual   <p>(b) The FSF's Back-Cover Text is:   <p>You have freedom to copy and modify this GNU Manual, like GNU     software.  Copies published by the Free Software Foundation raise     funds for GNU development.--><meta http-equiv="Content-Style-Type" content="text/css"><style type="text/css"><!--  pre.display { font-family:inherit }  pre.format  { font-family:inherit }  pre.smalldisplay { font-family:inherit; font-size:smaller }  pre.smallformat  { font-family:inherit; font-size:smaller }  pre.smallexample { font-size:smaller }  pre.smalllisp    { font-size:smaller }--></style></head><body><div class="node"><p>Node:&nbsp;<a name="Zero%20Length">Zero Length</a>,Next:&nbsp;<a rel="next" accesskey="n" href="Variable-Length.html#Variable%20Length">Variable Length</a>,Previous:&nbsp;<a rel="previous" accesskey="p" href="Hex-Floats.html#Hex%20Floats">Hex Floats</a>,Up:&nbsp;<a rel="up" accesskey="u" href="C-Extensions.html#C%20Extensions">C Extensions</a><hr><br></div><h3 class="section">Arrays of Length Zero</h3><p>Zero-length arrays are allowed in GNU C.  They are very useful as thelast element of a structure which is really a header for a variable-lengthobject:<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>   <p>In ISO C90, you would have to give <code>contents</code> a length of 1, whichmeans 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 isslightly different in syntax and semantics:     <ul><li>Flexible array members are written as <code>contents[]</code> withoutthe <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 implementationof 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 containingsuch a structure (possibly recursively), may not be a member of astructure or an element of an array.  (However, these uses arepermitted by GCC as extensions.) </ul>   <p>GCC versions before 3.0 allowed zero-length arrays to be staticallyinitialized, as if they were flexible arrays.  In addition to thosecases that were useful, it also allowed initializations in situationsthat would corrupt later data.  Non-empty initialization of zero-lengtharrays is now treated like any case where there are more initializerelements than the array holds, in that a suitable warning about "excesselements in array" is given, and the excess elements (all of them, inthis case) are ignored.   <p>Instead GCC allows static initialization of flexible array members. This is equivalent to defining a new structure containing the originalstructure 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 declaredlike <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>The convenience of this extension is that <code>f1</code> has the desiredtype, eliminating the need to consistently refer to <code>f2.f1</code>.   <p>This has symmetry with normal static arrays, in that an array ofunknown size is also written with <code>[]</code>.   <p>Of course, this extension only makes sense if the extra data comes atthe end of a top-level object, as otherwise we would be overwritingdata at subsequent offsets.  To avoid undue complication and confusionwith initialization of deeply nested arrays, we simply disallow anynon-empty initialization except when the structure is the top-levelobject.  For example:<pre class="smallexample">     struct foo { int x; int y[]; };     struct bar { struct foo z; };          struct foo a = { 1, { 2, 3, 4 } };        // Valid.     struct bar b = { { 1, { 2, 3, 4 } } };    // Invalid.     struct bar c = { { 1, { } } };            // Valid.     struct foo d[1] = { { 1 { 2, 3, 4 } } };  // Invalid.     </pre>   </body></html>
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