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| Next: <a href="Expression-Section.html#Expression-Section" accesskey="n" rel="next">Expression Section</a>, Previous: <a href="Operators.html#Operators" accesskey="p" rel="prev">Operators</a>, Up: <a href="Expressions.html#Expressions" accesskey="u" rel="up">Expressions</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="LD-Index.html#LD-Index" title="Index" rel="index">Index</a>]</p> |
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| <a name="Evaluation-1"></a> |
| <h4 class="subsection">3.10.7 Evaluation</h4> |
| <a name="index-lazy-evaluation"></a> |
| <a name="index-expression-evaluation-order"></a> |
| <p>The linker evaluates expressions lazily. It only computes the value of |
| an expression when absolutely necessary. |
| </p> |
| <p>The linker needs some information, such as the value of the start |
| address of the first section, and the origins and lengths of memory |
| regions, in order to do any linking at all. These values are computed |
| as soon as possible when the linker reads in the linker script. |
| </p> |
| <p>However, other values (such as symbol values) are not known or needed |
| until after storage allocation. Such values are evaluated later, when |
| other information (such as the sizes of output sections) is available |
| for use in the symbol assignment expression. |
| </p> |
| <p>The sizes of sections cannot be known until after allocation, so |
| assignments dependent upon these are not performed until after |
| allocation. |
| </p> |
| <p>Some expressions, such as those depending upon the location counter |
| ‘<samp>.</samp>’, must be evaluated during section allocation. |
| </p> |
| <p>If the result of an expression is required, but the value is not |
| available, then an error results. For example, a script like the |
| following |
| </p><div class="smallexample"> |
| <pre class="smallexample">SECTIONS |
| { |
| .text 9+this_isnt_constant : |
| { *(.text) } |
| } |
| </pre></div> |
| <p>will cause the error message ‘<samp>non constant expression for initial |
| address</samp>’. |
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