| <html lang="en"> |
| <head> |
| <title>Alignment - 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 4.13"> |
| <link title="Top" rel="start" href="index.html#Top"> |
| <link rel="up" href="C-Extensions.html#C-Extensions" title="C Extensions"> |
| <link rel="prev" href="Type-Attributes.html#Type-Attributes" title="Type Attributes"> |
| <link rel="next" href="Inline.html#Inline" title="Inline"> |
| <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> |
| <!-- |
| Copyright (C) 1988, 1989, 1992, 1993, 1994, 1995, 1996, 1997, 1998, |
| 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, |
| 2008 Free Software Foundation, Inc. |
| |
| Permission is granted to copy, distribute and/or modify this document |
| under the terms of the GNU Free Documentation License, Version 1.2 or |
| any later version published by the Free Software Foundation; with the |
| Invariant Sections being ``Funding Free Software'', the Front-Cover |
| Texts being (a) (see below), and with the Back-Cover Texts being (b) |
| (see below). A copy of the license is included in the section entitled |
| ``GNU Free Documentation License''. |
| |
| (a) The FSF's Front-Cover Text is: |
| |
| A GNU Manual |
| |
| (b) The FSF's Back-Cover Text is: |
| |
| 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 } |
| span.sc { font-variant:small-caps } |
| span.roman { font-family:serif; font-weight:normal; } |
| span.sansserif { font-family:sans-serif; font-weight:normal; } |
| --></style> |
| <link rel="stylesheet" type="text/css" href="../cs.css"> |
| </head> |
| <body> |
| <div class="node"> |
| <a name="Alignment"></a> |
| <p> |
| Next: <a rel="next" accesskey="n" href="Inline.html#Inline">Inline</a>, |
| Previous: <a rel="previous" accesskey="p" href="Type-Attributes.html#Type-Attributes">Type Attributes</a>, |
| Up: <a rel="up" accesskey="u" href="C-Extensions.html#C-Extensions">C Extensions</a> |
| <hr> |
| </div> |
| |
| <h3 class="section">6.35 Inquiring on Alignment of Types or Variables</h3> |
| |
| <p><a name="index-alignment-2481"></a><a name="index-type-alignment-2482"></a><a name="index-variable-alignment-2483"></a> |
| The keyword <code>__alignof__</code> allows you to inquire about how an object |
| is aligned, or the minimum alignment usually required by a type. Its |
| syntax is just like <code>sizeof</code>. |
| |
| <p>For example, if the target machine requires a <code>double</code> value to be |
| aligned on an 8-byte boundary, then <code>__alignof__ (double)</code> is 8. |
| This is true on many RISC machines. On more traditional machine |
| designs, <code>__alignof__ (double)</code> is 4 or even 2. |
| |
| <p>Some machines never actually require alignment; they allow reference to any |
| data type even at an odd address. For these machines, <code>__alignof__</code> |
| reports the smallest alignment that GCC will give the data type, usually as |
| mandated by the target ABI. |
| |
| <p>If the operand of <code>__alignof__</code> is an lvalue rather than a type, |
| its value is the required alignment for its type, taking into account |
| any minimum alignment specified with GCC's <code>__attribute__</code> |
| extension (see <a href="Variable-Attributes.html#Variable-Attributes">Variable Attributes</a>). For example, after this |
| declaration: |
| |
| <pre class="smallexample"> struct foo { int x; char y; } foo1; |
| </pre> |
| <p class="noindent">the value of <code>__alignof__ (foo1.y)</code> is 1, even though its actual |
| alignment is probably 2 or 4, the same as <code>__alignof__ (int)</code>. |
| |
| <p>It is an error to ask for the alignment of an incomplete type. |
| |
| </body></html> |
| |
