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<h4 class="subsection">3.10.8 The Section of an Expression</h4>
<p><a name="index-expression-sections-551"></a><a name="index-absolute-expressions-552"></a><a name="index-relative-expressions-553"></a><a name="index-absolute-and-relocatable-symbols-554"></a><a name="index-relocatable-and-absolute-symbols-555"></a><a name="index-symbols_002c-relocatable-and-absolute-556"></a>Addresses and symbols may be section relative, or absolute. A section
relative symbol is relocatable. If you request relocatable output
using the ‘<samp><span class="samp">-r</span></samp>’ option, a further link operation may change the
value of a section relative symbol. On the other hand, an absolute
symbol will retain the same value throughout any further link
operations.
<p>Some terms in linker expressions are addresses. This is true of
section relative symbols and for builtin functions that return an
address, such as <code>ADDR</code>, <code>LOADADDR</code>, <code>ORIGIN</code> and
<code>SEGMENT_START</code>. Other terms are simply numbers, or are builtin
functions that return a non-address value, such as <code>LENGTH</code>.
One complication is that unless you set <code>LD_FEATURE ("SANE_EXPR")</code>
(see <a href="Miscellaneous-Commands.html#Miscellaneous-Commands">Miscellaneous Commands</a>), numbers and absolute symbols are treated
differently depending on their location, for compatibility with older
versions of <code>ld</code>. Expressions appearing outside an output
section definition treat all numbers as absolute addresses.
Expressions appearing inside an output section definition treat
absolute symbols as numbers. If <code>LD_FEATURE ("SANE_EXPR")</code> is
given, then absolute symbols and numbers are simply treated as numbers
everywhere.
<p>In the following simple example,
<pre class="smallexample"> SECTIONS
{
. = 0x100;
__executable_start = 0x100;
.data :
{
. = 0x10;
__data_start = 0x10;
*(.data)
}
...
}
</pre>
<p>both <code>.</code> and <code>__executable_start</code> are set to the absolute
address 0x100 in the first two assignments, then both <code>.</code> and
<code>__data_start</code> are set to 0x10 relative to the <code>.data</code>
section in the second two assignments.
<p>For expressions involving numbers, relative addresses and absolute
addresses, ld follows these rules to evaluate terms:
<ul>
<li>Unary operations on an absolute address or number, and binary
operations on two absolute addresses or two numbers, or between one
absolute address and a number, apply the operator to the value(s).
<li>Unary operations on a relative address, and binary operations on two
relative addresses in the same section or between one relative address
and a number, apply the operator to the offset part of the address(es).
<li>Other binary operations, that is, between two relative addresses not
in the same section, or between a relative address and an absolute
address, first convert any non-absolute term to an absolute address
before applying the operator.
</ul>
<p>The result section of each sub-expression is as follows:
<ul>
<li>An operation involving only numbers results in a number.
<li>The result of comparisons, ‘<samp><span class="samp">&&</span></samp>’ and ‘<samp><span class="samp">||</span></samp>’ is also a number.
<li>The result of other binary arithmetic and logical operations on two
relative addresses in the same section or two absolute addresses
(after above conversions) is also a number.
<li>The result of other operations on relative addresses or one
relative address and a number, is a relative address in the same
section as the relative operand(s).
<li>The result of other operations on absolute addresses (after above
conversions) is an absolute address.
</ul>
<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.
<p>Using <code>LOADADDR</code> also forces an expression absolute, since this
particular builtin function returns an absolute address.
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