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<h3 class="section">3.3 Simple Linker Script Example</h3>

<p><a name="index-linker-script-example-339"></a><a name="index-example-of-linker-script-340"></a>Many linker scripts are fairly simple.

   <p>The simplest possible linker script has just one command:
&lsquo;<samp><span class="samp">SECTIONS</span></samp>&rsquo;.  You use the &lsquo;<samp><span class="samp">SECTIONS</span></samp>&rsquo; command to describe the
memory layout of the output file.

   <p>The &lsquo;<samp><span class="samp">SECTIONS</span></samp>&rsquo; command is a powerful command.  Here we will
describe a simple use of it.  Let's assume your program consists only of
code, initialized data, and uninitialized data.  These will be in the
&lsquo;<samp><span class="samp">.text</span></samp>&rsquo;, &lsquo;<samp><span class="samp">.data</span></samp>&rsquo;, and &lsquo;<samp><span class="samp">.bss</span></samp>&rsquo; sections, respectively. 
Let's assume further that these are the only sections which appear in
your input files.

   <p>For this example, let'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:
<pre class="smallexample">     SECTIONS
     {
       . = 0x10000;
       .text : { *(.text) }
       . = 0x8000000;
       .data : { *(.data) }
       .bss : { *(.bss) }
     }
</pre>
   <p>You write the &lsquo;<samp><span class="samp">SECTIONS</span></samp>&rsquo; command as the keyword &lsquo;<samp><span class="samp">SECTIONS</span></samp>&rsquo;,
followed by a series of symbol assignments and output section
descriptions enclosed in curly braces.

   <p>The first line inside the &lsquo;<samp><span class="samp">SECTIONS</span></samp>&rsquo; command of the above example
sets the value of the special symbol &lsquo;<samp><span class="samp">.</span></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><span class="samp">SECTIONS</span></samp>&rsquo; command, the location counter has the value &lsquo;<samp><span class="samp">0</span></samp>&rsquo;.

   <p>The second line defines an output section, &lsquo;<samp><span class="samp">.text</span></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><span class="samp">*</span></samp>&rsquo; is a
wildcard which matches any file name.  The expression &lsquo;<samp><span class="samp">*(.text)</span></samp>&rsquo;
means all &lsquo;<samp><span class="samp">.text</span></samp>&rsquo; input sections in all input files.

   <p>Since the location counter is &lsquo;<samp><span class="samp">0x10000</span></samp>&rsquo; when the output section
&lsquo;<samp><span class="samp">.text</span></samp>&rsquo; is defined, the linker will set the address of the
&lsquo;<samp><span class="samp">.text</span></samp>&rsquo; section in the output file to be &lsquo;<samp><span class="samp">0x10000</span></samp>&rsquo;.

   <p>The remaining lines define the &lsquo;<samp><span class="samp">.data</span></samp>&rsquo; and &lsquo;<samp><span class="samp">.bss</span></samp>&rsquo; sections in
the output file.  The linker will place the &lsquo;<samp><span class="samp">.data</span></samp>&rsquo; output section
at address &lsquo;<samp><span class="samp">0x8000000</span></samp>&rsquo;.  After the linker places the &lsquo;<samp><span class="samp">.data</span></samp>&rsquo;
output section, the value of the location counter will be
&lsquo;<samp><span class="samp">0x8000000</span></samp>&rsquo; plus the size of the &lsquo;<samp><span class="samp">.data</span></samp>&rsquo; output section.  The
effect is that the linker will place the &lsquo;<samp><span class="samp">.bss</span></samp>&rsquo; output section
immediately after the &lsquo;<samp><span class="samp">.data</span></samp>&rsquo; output section in memory.

   <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><span class="samp">.text</span></samp>&rsquo; and &lsquo;<samp><span class="samp">.data</span></samp>&rsquo;
sections will probably satisfy any alignment constraints, but the linker
may have to create a small gap between the &lsquo;<samp><span class="samp">.data</span></samp>&rsquo; and &lsquo;<samp><span class="samp">.bss</span></samp>&rsquo;
sections.

   <p>That's it!  That's a simple and complete linker script.

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