| <html lang="en"> |
| <head> |
| <title>Obstacks Data Alignment - The GNU C Library</title> |
| <meta http-equiv="Content-Type" content="text/html"> |
| <meta name="description" content="The GNU C Library"> |
| <meta name="generator" content="makeinfo 4.13"> |
| <link title="Top" rel="start" href="index.html#Top"> |
| <link rel="up" href="Obstacks.html#Obstacks" title="Obstacks"> |
| <link rel="prev" href="Status-of-an-Obstack.html#Status-of-an-Obstack" title="Status of an Obstack"> |
| <link rel="next" href="Obstack-Chunks.html#Obstack-Chunks" title="Obstack Chunks"> |
| <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> |
| <!-- |
| This file documents the GNU C library. |
| |
| This is Edition 0.12, last updated 2007-10-27, |
| of `The GNU C Library Reference Manual', for version |
| 2.8 (Sourcery G++ Lite 2011.03-41). |
| |
| Copyright (C) 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2001, 2002, |
| 2003, 2007, 2008, 2010 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 1.3 or |
| any later version published by the Free Software Foundation; with the |
| Invariant Sections being ``Free Software Needs Free Documentation'' |
| and ``GNU Lesser General Public License'', the Front-Cover texts being |
| ``A GNU Manual'', and with the Back-Cover Texts as in (a) below. A |
| copy of the license is included in the section entitled "GNU Free |
| Documentation License". |
| |
| (a) The FSF's Back-Cover Text is: ``You have the freedom to |
| copy and modify this GNU manual. Buying copies from the FSF |
| supports it in developing GNU and promoting software freedom.''--> |
| <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="Obstacks-Data-Alignment"></a> |
| <p> |
| Next: <a rel="next" accesskey="n" href="Obstack-Chunks.html#Obstack-Chunks">Obstack Chunks</a>, |
| Previous: <a rel="previous" accesskey="p" href="Status-of-an-Obstack.html#Status-of-an-Obstack">Status of an Obstack</a>, |
| Up: <a rel="up" accesskey="u" href="Obstacks.html#Obstacks">Obstacks</a> |
| <hr> |
| </div> |
| |
| <h5 class="subsubsection">3.2.4.9 Alignment of Data in Obstacks</h5> |
| |
| <p><a name="index-alignment-_0028in-obstacks_0029-336"></a> |
| Each obstack has an <dfn>alignment boundary</dfn>; each object allocated in |
| the obstack automatically starts on an address that is a multiple of the |
| specified boundary. By default, this boundary is aligned so that |
| the object can hold any type of data. |
| |
| <p>To access an obstack's alignment boundary, use the macro |
| <code>obstack_alignment_mask</code>, whose function prototype looks like |
| this: |
| |
| <!-- obstack.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Macro: int <b>obstack_alignment_mask</b> (<var>struct obstack *obstack-ptr</var>)<var><a name="index-obstack_005falignment_005fmask-337"></a></var><br> |
| <blockquote><p>The value is a bit mask; a bit that is 1 indicates that the corresponding |
| bit in the address of an object should be 0. The mask value should be one |
| less than a power of 2; the effect is that all object addresses are |
| multiples of that power of 2. The default value of the mask is a value |
| that allows aligned objects to hold any type of data: for example, if |
| its value is 3, any type of data can be stored at locations whose |
| addresses are multiples of 4. A mask value of 0 means an object can start |
| on any multiple of 1 (that is, no alignment is required). |
| |
| <p>The expansion of the macro <code>obstack_alignment_mask</code> is an lvalue, |
| so you can alter the mask by assignment. For example, this statement: |
| |
| <pre class="smallexample"> obstack_alignment_mask (obstack_ptr) = 0; |
| </pre> |
| <p class="noindent">has the effect of turning off alignment processing in the specified obstack. |
| </p></blockquote></div> |
| |
| <p>Note that a change in alignment mask does not take effect until |
| <em>after</em> the next time an object is allocated or finished in the |
| obstack. If you are not growing an object, you can make the new |
| alignment mask take effect immediately by calling <code>obstack_finish</code>. |
| This will finish a zero-length object and then do proper alignment for |
| the next object. |
| |
| </body></html> |
| |
