174 lines
7.7 KiB
HTML
174 lines
7.7 KiB
HTML
<html lang="en">
|
|
<head>
|
|
<title>Location Counter - 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="Orphan-Sections.html#Orphan-Sections" title="Orphan Sections">
|
|
<link rel="next" href="Operators.html#Operators" title="Operators">
|
|
<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="Location-Counter"></a>
|
|
<p>
|
|
Next: <a rel="next" accesskey="n" href="Operators.html#Operators">Operators</a>,
|
|
Previous: <a rel="previous" accesskey="p" href="Orphan-Sections.html#Orphan-Sections">Orphan Sections</a>,
|
|
Up: <a rel="up" accesskey="u" href="Expressions.html#Expressions">Expressions</a>
|
|
<hr>
|
|
</div>
|
|
|
|
<h4 class="subsection">3.10.5 The Location Counter</h4>
|
|
|
|
<p><a name="index-g_t_002e-567"></a><a name="index-dot-568"></a><a name="index-location-counter-569"></a><a name="index-current-output-location-570"></a>The special linker variable <dfn>dot</dfn> ‘<samp><span class="samp">.</span></samp>’ always contains the
|
|
current output location counter. Since the <code>.</code> always refers to a
|
|
location in an output section, it may only appear in an expression
|
|
within a <code>SECTIONS</code> command. The <code>.</code> symbol may appear
|
|
anywhere that an ordinary symbol is allowed in an expression.
|
|
|
|
<p><a name="index-holes-571"></a>Assigning a value to <code>.</code> will cause the location counter to be
|
|
moved. This may be used to create holes in the output section. The
|
|
location counter may not be moved backwards inside an output section,
|
|
and may not be moved backwards outside of an output section if so
|
|
doing creates areas with overlapping LMAs.
|
|
|
|
<pre class="smallexample"> SECTIONS
|
|
{
|
|
output :
|
|
{
|
|
file1(.text)
|
|
. = . + 1000;
|
|
file2(.text)
|
|
. += 1000;
|
|
file3(.text)
|
|
} = 0x12345678;
|
|
}
|
|
</pre>
|
|
<p class="noindent">In the previous example, the ‘<samp><span class="samp">.text</span></samp>’ section from <samp><span class="file">file1</span></samp> is
|
|
located at the beginning of the output section ‘<samp><span class="samp">output</span></samp>’. It is
|
|
followed by a 1000 byte gap. Then the ‘<samp><span class="samp">.text</span></samp>’ section from
|
|
<samp><span class="file">file2</span></samp> appears, also with a 1000 byte gap following before the
|
|
‘<samp><span class="samp">.text</span></samp>’ section from <samp><span class="file">file3</span></samp>. The notation ‘<samp><span class="samp">= 0x12345678</span></samp>’
|
|
specifies what data to write in the gaps (see <a href="Output-Section-Fill.html#Output-Section-Fill">Output Section Fill</a>).
|
|
|
|
<p><a name="index-dot-inside-sections-572"></a>Note: <code>.</code> actually refers to the byte offset from the start of the
|
|
current containing object. Normally this is the <code>SECTIONS</code>
|
|
statement, whose start address is 0, hence <code>.</code> can be used as an
|
|
absolute address. If <code>.</code> is used inside a section description
|
|
however, it refers to the byte offset from the start of that section,
|
|
not an absolute address. Thus in a script like this:
|
|
|
|
<pre class="smallexample"> SECTIONS
|
|
{
|
|
. = 0x100
|
|
.text: {
|
|
*(.text)
|
|
. = 0x200
|
|
}
|
|
. = 0x500
|
|
.data: {
|
|
*(.data)
|
|
. += 0x600
|
|
}
|
|
}
|
|
</pre>
|
|
<p>The ‘<samp><span class="samp">.text</span></samp>’ section will be assigned a starting address of 0x100
|
|
and a size of exactly 0x200 bytes, even if there is not enough data in
|
|
the ‘<samp><span class="samp">.text</span></samp>’ input sections to fill this area. (If there is too
|
|
much data, an error will be produced because this would be an attempt to
|
|
move <code>.</code> backwards). The ‘<samp><span class="samp">.data</span></samp>’ section will start at 0x500
|
|
and it will have an extra 0x600 bytes worth of space after the end of
|
|
the values from the ‘<samp><span class="samp">.data</span></samp>’ input sections and before the end of
|
|
the ‘<samp><span class="samp">.data</span></samp>’ output section itself.
|
|
|
|
<p><a name="index-dot-outside-sections-573"></a>Setting symbols to the value of the location counter outside of an
|
|
output section statement can result in unexpected values if the linker
|
|
needs to place orphan sections. For example, given the following:
|
|
|
|
<pre class="smallexample"> SECTIONS
|
|
{
|
|
start_of_text = . ;
|
|
.text: { *(.text) }
|
|
end_of_text = . ;
|
|
|
|
start_of_data = . ;
|
|
.data: { *(.data) }
|
|
end_of_data = . ;
|
|
}
|
|
</pre>
|
|
<p>If the linker needs to place some input section, e.g. <code>.rodata</code>,
|
|
not mentioned in the script, it might choose to place that section
|
|
between <code>.text</code> and <code>.data</code>. You might think the linker
|
|
should place <code>.rodata</code> on the blank line in the above script, but
|
|
blank lines are of no particular significance to the linker. As well,
|
|
the linker doesn't associate the above symbol names with their
|
|
sections. Instead, it assumes that all assignments or other
|
|
statements belong to the previous output section, except for the
|
|
special case of an assignment to <code>.</code>. I.e., the linker will
|
|
place the orphan <code>.rodata</code> section as if the script was written
|
|
as follows:
|
|
|
|
<pre class="smallexample"> SECTIONS
|
|
{
|
|
start_of_text = . ;
|
|
.text: { *(.text) }
|
|
end_of_text = . ;
|
|
|
|
start_of_data = . ;
|
|
.rodata: { *(.rodata) }
|
|
.data: { *(.data) }
|
|
end_of_data = . ;
|
|
}
|
|
</pre>
|
|
<p>This may or may not be the script author's intention for the value of
|
|
<code>start_of_data</code>. One way to influence the orphan section
|
|
placement is to assign the location counter to itself, as the linker
|
|
assumes that an assignment to <code>.</code> is setting the start address of
|
|
a following output section and thus should be grouped with that
|
|
section. So you could write:
|
|
|
|
<pre class="smallexample"> SECTIONS
|
|
{
|
|
start_of_text = . ;
|
|
.text: { *(.text) }
|
|
end_of_text = . ;
|
|
|
|
. = . ;
|
|
start_of_data = . ;
|
|
.data: { *(.data) }
|
|
end_of_data = . ;
|
|
}
|
|
</pre>
|
|
<p>Now, the orphan <code>.rodata</code> section will be placed between
|
|
<code>end_of_text</code> and <code>start_of_data</code>.
|
|
|
|
</body></html>
|
|
|