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| |
| <h4 class="subsection">3.10.8 The Section of an Expression</h4> |
| |
| <p><a name="index-expression-sections-579"></a><a name="index-absolute-expressions-580"></a><a name="index-relative-expressions-581"></a><a name="index-absolute-and-relocatable-symbols-582"></a><a name="index-relocatable-and-absolute-symbols-583"></a><a name="index-symbols_002c-relocatable-and-absolute-584"></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 when |
| <code>LD_FEATURE ("SANE_EXPR")</code> or inside an output section definition |
| but an absolute address otherwise. |
| <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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| |
