| <html lang="en"> |
| <head> |
| <title>Evaluation - Untitled</title> |
| <meta http-equiv="Content-Type" content="text/html"> |
| <meta name="description" content="Untitled"> |
| <meta name="generator" content="makeinfo 4.13"> |
| <link title="Top" rel="start" href="index.html#Top"> |
| <link rel="up" href="Expressions.html#Expressions" title="Expressions"> |
| <link rel="prev" href="Operators.html#Operators" title="Operators"> |
| <link rel="next" href="Expression-Section.html#Expression-Section" title="Expression Section"> |
| <link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage"> |
| <!-- |
| This file documents the GNU linker LD |
| (Sourcery G++ Lite 2011.03-41) |
| version 2.20.51. |
| |
| Copyright (C) 1991, 1992, 1993, 1994, 1995, 1996, 1997, 1998, 1999, 2000, |
| 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009 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 no Invariant Sections, with no Front-Cover Texts, and with no |
| Back-Cover Texts. A copy of the license is included in the |
| section entitled ``GNU Free Documentation License''.--> |
| <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="Evaluation"></a> |
| <p> |
| Next: <a rel="next" accesskey="n" href="Expression-Section.html#Expression-Section">Expression Section</a>, |
| Previous: <a rel="previous" accesskey="p" href="Operators.html#Operators">Operators</a>, |
| Up: <a rel="up" accesskey="u" href="Expressions.html#Expressions">Expressions</a> |
| <hr> |
| </div> |
| |
| <h4 class="subsection">3.10.7 Evaluation</h4> |
| |
| <p><a name="index-lazy-evaluation-515"></a><a name="index-expression-evaluation-order-516"></a>The linker evaluates expressions lazily. It only computes the value of |
| an expression when absolutely necessary. |
| |
| <p>The linker needs some information, such as the value of the start |
| address of the first section, and the origins and lengths of memory |
| regions, in order to do any linking at all. These values are computed |
| as soon as possible when the linker reads in the linker script. |
| |
| <p>However, other values (such as symbol values) are not known or needed |
| until after storage allocation. Such values are evaluated later, when |
| other information (such as the sizes of output sections) is available |
| for use in the symbol assignment expression. |
| |
| <p>The sizes of sections cannot be known until after allocation, so |
| assignments dependent upon these are not performed until after |
| allocation. |
| |
| <p>Some expressions, such as those depending upon the location counter |
| ‘<samp><span class="samp">.</span></samp>’, must be evaluated during section allocation. |
| |
| <p>If the result of an expression is required, but the value is not |
| available, then an error results. For example, a script like the |
| following |
| <pre class="smallexample"> SECTIONS |
| { |
| .text 9+this_isnt_constant : |
| { *(.text) } |
| } |
| </pre> |
| <p class="noindent">will cause the error message ‘<samp><span class="samp">non constant expression for initial |
| address</span></samp>’. |
| |
| </body></html> |
| |
