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| <h4 class="subsection">3.10.8 The Section of an Expression</h4> |
| |
| <p><a name="index-expression-sections-517"></a><a name="index-absolute-expressions-518"></a><a name="index-relative-expressions-519"></a><a name="index-absolute-and-relocatable-symbols-520"></a><a name="index-relocatable-and-absolute-symbols-521"></a><a name="index-symbols_002c-relocatable-and-absolute-522"></a>When the linker evaluates an expression, the result is either absolute |
| or relative to some section. A relative expression is expressed as a |
| fixed offset from the base of a section. |
| |
| <p>The position of the expression within the linker script determines |
| whether it is absolute or relative. An expression which appears within |
| an output section definition is relative to the base of the output |
| section. An expression which appears elsewhere will be absolute. |
| |
| <p>A symbol set to a relative expression will be relocatable if you request |
| relocatable output using the ‘<samp><span class="samp">-r</span></samp>’ option. That means that a |
| further link operation may change the value of the symbol. The symbol's |
| section will be the section of the relative expression. |
| |
| <p>A symbol set to an absolute expression will retain the same value |
| through any further link operation. The symbol will be absolute, and |
| will not have any particular associated section. |
| |
| <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. |
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