142 lines
6.8 KiB
HTML
142 lines
6.8 KiB
HTML
<html lang="en">
|
|
<head>
|
|
<title>Expression Section - 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="Evaluation.html#Evaluation" title="Evaluation">
|
|
<link rel="next" href="Builtin-Functions.html#Builtin-Functions" title="Builtin Functions">
|
|
<link href="http://www.gnu.org/software/texinfo/" rel="generator-home" title="Texinfo Homepage">
|
|
<!--
|
|
This file documents the GNU linker LD
|
|
(GNU Toolchain for the A-profile Architecture 9.2-2019.12 (arm-9.10))
|
|
version 2.33.1.
|
|
|
|
Copyright (C) 1991-2019 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>
|
|
</head>
|
|
<body>
|
|
<div class="node">
|
|
<a name="Expression-Section"></a>
|
|
<p>
|
|
Next: <a rel="next" accesskey="n" href="Builtin-Functions.html#Builtin-Functions">Builtin Functions</a>,
|
|
Previous: <a rel="previous" accesskey="p" href="Evaluation.html#Evaluation">Evaluation</a>,
|
|
Up: <a rel="up" accesskey="u" href="Expressions.html#Expressions">Expressions</a>
|
|
<hr>
|
|
</div>
|
|
|
|
<h4 class="subsection">3.10.8 The Section of an Expression</h4>
|
|
|
|
<p><a name="index-expression-sections-579"></a><a name="index-absolute-expressions-580"></a><a name="index-relative-expressions-581"></a><a name="index-absolute-and-relocatable-symbols-582"></a><a name="index-relocatable-and-absolute-symbols-583"></a><a name="index-symbols_002c-relocatable-and-absolute-584"></a>Addresses and symbols may be section relative, or absolute. A section
|
|
relative symbol is relocatable. If you request relocatable output
|
|
using the ‘<samp><span class="samp">-r</span></samp>’ 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>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 ("SANE_EXPR")</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 ("SANE_EXPR")</code> is
|
|
given, then absolute symbols and numbers are simply treated as numbers
|
|
everywhere.
|
|
|
|
<p>In the following simple example,
|
|
|
|
<pre class="smallexample"> SECTIONS
|
|
{
|
|
. = 0x100;
|
|
__executable_start = 0x100;
|
|
.data :
|
|
{
|
|
. = 0x10;
|
|
__data_start = 0x10;
|
|
*(.data)
|
|
}
|
|
...
|
|
}
|
|
</pre>
|
|
<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>For expressions involving numbers, relative addresses and absolute
|
|
addresses, ld follows these rules to evaluate terms:
|
|
|
|
<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>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>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.
|
|
</ul>
|
|
|
|
<p>The result section of each sub-expression is as follows:
|
|
|
|
<ul>
|
|
<li>An operation involving only numbers results in a number.
|
|
<li>The result of comparisons, ‘<samp><span class="samp">&&</span></samp>’ and ‘<samp><span class="samp">||</span></samp>’ is also a number.
|
|
<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 ("SANE_EXPR")</code> or inside an output section definition
|
|
but an absolute address otherwise.
|
|
<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>The result of other operations on absolute addresses (after above
|
|
conversions) is an absolute address.
|
|
</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 ‘<samp><span class="samp">.data</span></samp>’:
|
|
<pre class="smallexample"> SECTIONS
|
|
{
|
|
.data : { *(.data) _edata = ABSOLUTE(.); }
|
|
}
|
|
</pre>
|
|
<p class="noindent">If ‘<samp><span class="samp">ABSOLUTE</span></samp>’ were not used, ‘<samp><span class="samp">_edata</span></samp>’ would be relative to the
|
|
‘<samp><span class="samp">.data</span></samp>’ section.
|
|
|
|
<p>Using <code>LOADADDR</code> also forces an expression absolute, since this
|
|
particular builtin function returns an absolute address.
|
|
|
|
</body></html>
|
|
|