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<h4 class="subsection">Evaluation</h4>



   <p>The linker evaluates expressions lazily.  It only computes the value of

an expression when absolutely necessary.



   <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>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>The sizes of sections cannot be known until after allocation, so

assignments dependent upon these are not performed until after

allocation.



   <p>Some expressions, such as those depending upon the location counter

<code>.</code>, must be evaluated during section allocation.



   <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

<pre class="smallexample">     SECTIONS

       {

         .text 9+this_isnt_constant :

           { *(.text) }

       }

     </pre>



<p>will cause the error message <code>non constant expression for initial

address</code>.



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