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<a name="Simple-Linker-Script-Example"></a>
<h3 class="section">3.3 Simple Linker Script Example</h3>
<a name="index-linker-script-example"></a>
<a name="index-example-of-linker-script"></a>
<p>Many linker scripts are fairly simple.
</p>
<p>The simplest possible linker script has just one command:
&lsquo;<samp>SECTIONS</samp>&rsquo;.  You use the &lsquo;<samp>SECTIONS</samp>&rsquo; command to describe the
memory layout of the output file.
</p>
<p>The &lsquo;<samp>SECTIONS</samp>&rsquo; command is a powerful command.  Here we will
describe a simple use of it.  Let&rsquo;s assume your program consists only of
code, initialized data, and uninitialized data.  These will be in the
&lsquo;<samp>.text</samp>&rsquo;, &lsquo;<samp>.data</samp>&rsquo;, and &lsquo;<samp>.bss</samp>&rsquo; sections, respectively.
Let&rsquo;s assume further that these are the only sections which appear in
your input files.
</p>
<p>For this example, let&rsquo;s say that the code should be loaded at address
0x10000, and that the data should start at address 0x8000000.  Here is a
linker script which will do that:
</p><div class="smallexample">
<pre class="smallexample">SECTIONS
{
  . = 0x10000;
  .text : { *(.text) }
  . = 0x8000000;
  .data : { *(.data) }
  .bss : { *(.bss) }
}
</pre></div>

<p>You write the &lsquo;<samp>SECTIONS</samp>&rsquo; command as the keyword &lsquo;<samp>SECTIONS</samp>&rsquo;,
followed by a series of symbol assignments and output section
descriptions enclosed in curly braces.
</p>
<p>The first line inside the &lsquo;<samp>SECTIONS</samp>&rsquo; command of the above example
sets the value of the special symbol &lsquo;<samp>.</samp>&rsquo;, which is the location
counter.  If you do not specify the address of an output section in some
other way (other ways are described later), the address is set from the
current value of the location counter.  The location counter is then
incremented by the size of the output section.  At the start of the
&lsquo;<samp>SECTIONS</samp>&rsquo; command, the location counter has the value &lsquo;<samp>0</samp>&rsquo;.
</p>
<p>The second line defines an output section, &lsquo;<samp>.text</samp>&rsquo;.  The colon is
required syntax which may be ignored for now.  Within the curly braces
after the output section name, you list the names of the input sections
which should be placed into this output section.  The &lsquo;<samp>*</samp>&rsquo; is a
wildcard which matches any file name.  The expression &lsquo;<samp>*(.text)</samp>&rsquo;
means all &lsquo;<samp>.text</samp>&rsquo; input sections in all input files.
</p>
<p>Since the location counter is &lsquo;<samp>0x10000</samp>&rsquo; when the output section
&lsquo;<samp>.text</samp>&rsquo; is defined, the linker will set the address of the
&lsquo;<samp>.text</samp>&rsquo; section in the output file to be &lsquo;<samp>0x10000</samp>&rsquo;.
</p>
<p>The remaining lines define the &lsquo;<samp>.data</samp>&rsquo; and &lsquo;<samp>.bss</samp>&rsquo; sections in
the output file.  The linker will place the &lsquo;<samp>.data</samp>&rsquo; output section
at address &lsquo;<samp>0x8000000</samp>&rsquo;.  After the linker places the &lsquo;<samp>.data</samp>&rsquo;
output section, the value of the location counter will be
&lsquo;<samp>0x8000000</samp>&rsquo; plus the size of the &lsquo;<samp>.data</samp>&rsquo; output section.  The
effect is that the linker will place the &lsquo;<samp>.bss</samp>&rsquo; output section
immediately after the &lsquo;<samp>.data</samp>&rsquo; output section in memory.
</p>
<p>The linker will ensure that each output section has the required
alignment, by increasing the location counter if necessary.  In this
example, the specified addresses for the &lsquo;<samp>.text</samp>&rsquo; and &lsquo;<samp>.data</samp>&rsquo;
sections will probably satisfy any alignment constraints, but the linker
may have to create a small gap between the &lsquo;<samp>.data</samp>&rsquo; and &lsquo;<samp>.bss</samp>&rsquo;
sections.
</p>
<p>That&rsquo;s it!  That&rsquo;s a simple and complete linker script.
</p>
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