visus
/
toolchain-gcc-arm-embedded


			
							<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<!-- This file documents the GNU linker LD
(GNU Arm Embedded Toolchain 10-2020-q4-major)
version 2.35.1.

Copyright (C) 1991-2020 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". -->
<!-- Created by GNU Texinfo 6.5, http://www.gnu.org/software/texinfo/ -->
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Expression Section (LD)</title>

<meta name="description" content="Expression Section (LD)">
<meta name="keywords" content="Expression Section (LD)">
<meta name="resource-type" content="document">
<meta name="distribution" content="global">
<meta name="Generator" content="makeinfo">
<link href="index.html#Top" rel="start" title="Top">
<link href="LD-Index.html#LD-Index" rel="index" title="LD Index">
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
<link href="Expressions.html#Expressions" rel="up" title="Expressions">
<link href="Builtin-Functions.html#Builtin-Functions" rel="next" title="Builtin Functions">
<link href="Evaluation.html#Evaluation" rel="prev" title="Evaluation">
<style type="text/css">
<!--
a.summary-letter {text-decoration: none}
blockquote.indentedblock {margin-right: 0em}
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
blockquote.smallquotation {font-size: smaller}
div.display {margin-left: 3.2em}
div.example {margin-left: 3.2em}
div.lisp {margin-left: 3.2em}
div.smalldisplay {margin-left: 3.2em}
div.smallexample {margin-left: 3.2em}
div.smalllisp {margin-left: 3.2em}
kbd {font-style: oblique}
pre.display {font-family: inherit}
pre.format {font-family: inherit}
pre.menu-comment {font-family: serif}
pre.menu-preformatted {font-family: serif}
pre.smalldisplay {font-family: inherit; font-size: smaller}
pre.smallexample {font-size: smaller}
pre.smallformat {font-family: inherit; font-size: smaller}
pre.smalllisp {font-size: smaller}
span.nolinebreak {white-space: nowrap}
span.roman {font-family: initial; font-weight: normal}
span.sansserif {font-family: sans-serif; font-weight: normal}
ul.no-bullet {list-style: none}
-->
</style>


</head>

<body lang="en">
<a name="Expression-Section"></a>
<div class="header">
<p>
Next: <a href="Builtin-Functions.html#Builtin-Functions" accesskey="n" rel="next">Builtin Functions</a>, Previous: <a href="Evaluation.html#Evaluation" accesskey="p" rel="prev">Evaluation</a>, Up: <a href="Expressions.html#Expressions" accesskey="u" rel="up">Expressions</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="LD-Index.html#LD-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="The-Section-of-an-Expression"></a>
<h4 class="subsection">3.10.8 The Section of an Expression</h4>
<a name="index-expression-sections"></a>
<a name="index-absolute-expressions"></a>
<a name="index-relative-expressions"></a>
<a name="index-absolute-and-relocatable-symbols"></a>
<a name="index-relocatable-and-absolute-symbols"></a>
<a name="index-symbols_002c-relocatable-and-absolute"></a>
<p>Addresses and symbols may be section relative, or absolute.  A section
relative symbol is relocatable.  If you request relocatable output
using the &lsquo;<samp>-r</samp>&rsquo; option, a further link operation may change the
value of a section relative symbol.  On the other hand, an absolute
symbol will retain the same value throughout any further link
operations.
</p>
<p>Some terms in linker expressions are addresses.  This is true of
section relative symbols and for builtin functions that return an
address, such as <code>ADDR</code>, <code>LOADADDR</code>, <code>ORIGIN</code> and
<code>SEGMENT_START</code>.  Other terms are simply numbers, or are builtin
functions that return a non-address value, such as <code>LENGTH</code>.
One complication is that unless you set <code>LD_FEATURE (&quot;SANE_EXPR&quot;)</code>
(see <a href="Miscellaneous-Commands.html#Miscellaneous-Commands">Miscellaneous Commands</a>), numbers and absolute symbols are treated
differently depending on their location, for compatibility with older
versions of <code>ld</code>.  Expressions appearing outside an output
section definition treat all numbers as absolute addresses.
Expressions appearing inside an output section definition treat
absolute symbols as numbers.  If <code>LD_FEATURE (&quot;SANE_EXPR&quot;)</code> is
given, then absolute symbols and numbers are simply treated as numbers
everywhere.
</p>
<p>In the following simple example,
</p>
<div class="smallexample">
<pre class="smallexample">SECTIONS
  {
    . = 0x100;
    __executable_start = 0x100;
    .data :
    {
      . = 0x10;
      __data_start = 0x10;
      *(.data)
    }
    &hellip;
  }
</pre></div>

<p>both <code>.</code> and <code>__executable_start</code> are set to the absolute
address 0x100 in the first two assignments, then both <code>.</code> and
<code>__data_start</code> are set to 0x10 relative to the <code>.data</code>
section in the second two assignments.
</p>
<p>For expressions involving numbers, relative addresses and absolute
addresses, ld follows these rules to evaluate terms:
</p>
<ul>
<li> Unary operations on an absolute address or number, and binary
operations on two absolute addresses or two numbers, or between one
absolute address and a number, apply the operator to the value(s).
</li><li> Unary operations on a relative address, and binary operations on two
relative addresses in the same section or between one relative address
and a number, apply the operator to the offset part of the address(es).
</li><li> Other binary operations, that is, between two relative addresses not
in the same section, or between a relative address and an absolute
address, first convert any non-absolute term to an absolute address
before applying the operator.
</li></ul>

<p>The result section of each sub-expression is as follows:
</p>
<ul>
<li> An operation involving only numbers results in a number.
</li><li> The result of comparisons, &lsquo;<samp>&amp;&amp;</samp>&rsquo; and &lsquo;<samp>||</samp>&rsquo; is also a number.
</li><li> The result of other binary arithmetic and logical operations on two
relative addresses in the same section or two absolute addresses
(after above conversions) is also a number when
<code>LD_FEATURE (&quot;SANE_EXPR&quot;)</code> or inside an output section definition
but an absolute address otherwise.
</li><li> The result of other operations on relative addresses or one
relative address and a number, is a relative address in the same
section as the relative operand(s).
</li><li> The result of other operations on absolute addresses (after above
conversions) is an absolute address.
</li></ul>

<p>You can use the builtin function <code>ABSOLUTE</code> to force an expression
to be absolute when it would otherwise be relative.  For example, to
create an absolute symbol set to the address of the end of the output
section &lsquo;<samp>.data</samp>&rsquo;:
</p><div class="smallexample">
<pre class="smallexample">SECTIONS
  {
    .data : { *(.data) _edata = ABSOLUTE(.); }
  }
</pre></div>
<p>If &lsquo;<samp>ABSOLUTE</samp>&rsquo; were not used, &lsquo;<samp>_edata</samp>&rsquo; would be relative to the
&lsquo;<samp>.data</samp>&rsquo; section.
</p>
<p>Using <code>LOADADDR</code> also forces an expression absolute, since this
particular builtin function returns an absolute address.
</p>
<hr>
<div class="header">
<p>
Next: <a href="Builtin-Functions.html#Builtin-Functions" accesskey="n" rel="next">Builtin Functions</a>, Previous: <a href="Evaluation.html#Evaluation" accesskey="p" rel="prev">Evaluation</a>, Up: <a href="Expressions.html#Expressions" accesskey="u" rel="up">Expressions</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="LD-Index.html#LD-Index" title="Index" rel="index">Index</a>]</p>
</div>


</body>
</html>