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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: |
| ‘<samp><span class="samp">SECTIONS</span></samp>’. You use the ‘<samp><span class="samp">SECTIONS</span></samp>’ command to describe the |
| memory layout of the output file. |
| |
| <p>The ‘<samp><span class="samp">SECTIONS</span></samp>’ 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 |
| ‘<samp><span class="samp">.text</span></samp>’, ‘<samp><span class="samp">.data</span></samp>’, and ‘<samp><span class="samp">.bss</span></samp>’ 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 ‘<samp><span class="samp">SECTIONS</span></samp>’ command as the keyword ‘<samp><span class="samp">SECTIONS</span></samp>’, |
| followed by a series of symbol assignments and output section |
| descriptions enclosed in curly braces. |
| |
| <p>The first line inside the ‘<samp><span class="samp">SECTIONS</span></samp>’ command of the above example |
| sets the value of the special symbol ‘<samp><span class="samp">.</span></samp>’, 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 |
| ‘<samp><span class="samp">SECTIONS</span></samp>’ command, the location counter has the value ‘<samp><span class="samp">0</span></samp>’. |
| |
| <p>The second line defines an output section, ‘<samp><span class="samp">.text</span></samp>’. 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 ‘<samp><span class="samp">*</span></samp>’ is a |
| wildcard which matches any file name. The expression ‘<samp><span class="samp">*(.text)</span></samp>’ |
| means all ‘<samp><span class="samp">.text</span></samp>’ input sections in all input files. |
| |
| <p>Since the location counter is ‘<samp><span class="samp">0x10000</span></samp>’ when the output section |
| ‘<samp><span class="samp">.text</span></samp>’ is defined, the linker will set the address of the |
| ‘<samp><span class="samp">.text</span></samp>’ section in the output file to be ‘<samp><span class="samp">0x10000</span></samp>’. |
| |
| <p>The remaining lines define the ‘<samp><span class="samp">.data</span></samp>’ and ‘<samp><span class="samp">.bss</span></samp>’ sections in |
| the output file. The linker will place the ‘<samp><span class="samp">.data</span></samp>’ output section |
| at address ‘<samp><span class="samp">0x8000000</span></samp>’. After the linker places the ‘<samp><span class="samp">.data</span></samp>’ |
| output section, the value of the location counter will be |
| ‘<samp><span class="samp">0x8000000</span></samp>’ plus the size of the ‘<samp><span class="samp">.data</span></samp>’ output section. The |
| effect is that the linker will place the ‘<samp><span class="samp">.bss</span></samp>’ output section |
| immediately after the ‘<samp><span class="samp">.data</span></samp>’ 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 ‘<samp><span class="samp">.text</span></samp>’ and ‘<samp><span class="samp">.data</span></samp>’ |
| sections will probably satisfy any alignment constraints, but the linker |
| may have to create a small gap between the ‘<samp><span class="samp">.data</span></samp>’ and ‘<samp><span class="samp">.bss</span></samp>’ |
| sections. |
| |
| <p>That's it! That's a simple and complete linker script. |
| |
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