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| <h3 class="section">4.2 Linker Sections</h3> |
| |
| <p><code>ld</code> deals with just four kinds of sections, summarized below. |
| |
| |
| <a name="index-named-sections-192"></a> |
| <a name="index-sections_002c-named-193"></a> |
| <dl><dt><strong>named sections</strong><dd><a name="index-text-section-194"></a><a name="index-data-section-195"></a><dt><strong>text section</strong><dt><strong>data section</strong><dd>These sections hold your program. <samp><span class="command">as</span></samp> and <code>ld</code> treat them as |
| separate but equal sections. Anything you can say of one section is |
| true of another. |
| <!-- @ifset aout-bout --> |
| When the program is running, however, it is |
| customary for the text section to be unalterable. The |
| text section is often shared among processes: it contains |
| instructions, constants and the like. The data section of a running |
| program is usually alterable: for example, C variables would be stored |
| in the data section. |
| <!-- @end ifset --> |
| |
| <p><a name="index-bss-section-196"></a><br><dt><strong>bss section</strong><dd>This section contains zeroed bytes when your program begins running. It |
| is used to hold uninitialized variables or common storage. The length of |
| each partial program's bss section is important, but because it starts |
| out containing zeroed bytes there is no need to store explicit zero |
| bytes in the object file. The bss section was invented to eliminate |
| those explicit zeros from object files. |
| |
| <p><a name="index-absolute-section-197"></a><br><dt><strong>absolute section</strong><dd>Address 0 of this section is always “relocated” to runtime address 0. |
| This is useful if you want to refer to an address that <code>ld</code> must |
| not change when relocating. In this sense we speak of absolute |
| addresses being “unrelocatable”: they do not change during relocation. |
| |
| <p><a name="index-undefined-section-198"></a><br><dt><strong>undefined section</strong><dd>This “section” is a catch-all for address references to objects not in |
| the preceding sections. |
| <!-- FIXME: ref to some other doc on obj-file formats could go here. --> |
| </dl> |
| |
| <p><a name="index-relocation-example-199"></a>An idealized example of three relocatable sections follows. |
| The example uses the traditional section names ‘<samp><span class="samp">.text</span></samp>’ and ‘<samp><span class="samp">.data</span></samp>’. |
| Memory addresses are on the horizontal axis. |
| |
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| <pre class="smallexample"> +-----+----+--+ |
| partial program # 1: |ttttt|dddd|00| |
| +-----+----+--+ |
| |
| text data bss |
| seg. seg. seg. |
| |
| +---+---+---+ |
| partial program # 2: |TTT|DDD|000| |
| +---+---+---+ |
| |
| +--+---+-----+--+----+---+-----+~~ |
| linked program: | |TTT|ttttt| |dddd|DDD|00000| |
| +--+---+-----+--+----+---+-----+~~ |
| |
| addresses: 0 ... |
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