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 <div class="node">
 <a name="Expression-Section"></a>
 <p>
 Next:&nbsp;<a rel="next" accesskey="n" href="Builtin-Functions.html#Builtin-Functions">Builtin Functions</a>,
 Previous:&nbsp;<a rel="previous" accesskey="p" href="Evaluation.html#Evaluation">Evaluation</a>,
 Up:&nbsp;<a rel="up" accesskey="u" href="Expressions.html#Expressions">Expressions</a>
 <hr>
 </div>

 <h4 class="subsection">3.10.8 The Section of an Expression</h4>

 <p><a name="index-expression-sections-527"></a><a name="index-absolute-expressions-528"></a><a name="index-relative-expressions-529"></a><a name="index-absolute-and-relocatable-symbols-530"></a><a name="index-relocatable-and-absolute-symbols-531"></a><a name="index-symbols_002c-relocatable-and-absolute-532"></a>Addresses and symbols may be section relative, or absolute.  A section
 relative symbol is relocatable.  If you request relocatable output
 using the &lsquo;<samp><span class="samp">-r</span></samp>&rsquo; 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 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>Unary operations on an absolute address, and binary operations on one
 or more absolute addresses or on two relative addresses not in the
 same section, 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, &lsquo;<samp><span class="samp">&amp;&amp;</span></samp>&rsquo; and &lsquo;<samp><span class="samp">||</span></samp>&rsquo; 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 addresess
 (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 &lsquo;<samp><span class="samp">.data</span></samp>&rsquo;:
 <pre class="smallexample">     SECTIONS
        {
          .data : { *(.data) _edata = ABSOLUTE(.); }
        }
 </pre>
    <p class="noindent">If &lsquo;<samp><span class="samp">ABSOLUTE</span></samp>&rsquo; were not used, &lsquo;<samp><span class="samp">_edata</span></samp>&rsquo; would be relative to the
 &lsquo;<samp><span class="samp">.data</span></samp>&rsquo; section.

    <p>Using <code>LOADADDR</code> also forces an expression absolute, since this
 particular builtin function returns an absolute address.

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	<p>
	Next: <a rel="next" accesskey="n" href="Builtin-Functions.html#Builtin-Functions">Builtin Functions</a>,
	Previous: <a rel="previous" accesskey="p" href="Evaluation.html#Evaluation">Evaluation</a>,
	Up: <a rel="up" accesskey="u" href="Expressions.html#Expressions">Expressions</a>
	<hr>
	</div>

	<h4 class="subsection">3.10.8 The Section of an Expression</h4>

	<p><a name="index-expression-sections-527"></a><a name="index-absolute-expressions-528"></a><a name="index-relative-expressions-529"></a><a name="index-absolute-and-relocatable-symbols-530"></a><a name="index-relocatable-and-absolute-symbols-531"></a><a name="index-symbols_002c-relocatable-and-absolute-532"></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 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>Unary operations on an absolute address, and binary operations on one
	or more absolute addresses or on two relative addresses not in the
	same section, 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 addresess
	(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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