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| <h4 class="subsection">3.10.5 The Location Counter</h4> |
| |
| <p><a name="index-g_t_002e-505"></a><a name="index-dot-506"></a><a name="index-location-counter-507"></a><a name="index-current-output-location-508"></a>The special linker variable <dfn>dot</dfn> ‘<samp><span class="samp">.</span></samp>’ always contains the |
| current output location counter. Since the <code>.</code> always refers to a |
| location in an output section, it may only appear in an expression |
| within a <code>SECTIONS</code> command. The <code>.</code> symbol may appear |
| anywhere that an ordinary symbol is allowed in an expression. |
| |
| <p><a name="index-holes-509"></a>Assigning a value to <code>.</code> will cause the location counter to be |
| moved. This may be used to create holes in the output section. The |
| location counter may not be moved backwards inside an output section, |
| and may not be moved backwards outside of an output section if so |
| doing creates areas with overlapping LMAs. |
| |
| <pre class="smallexample"> SECTIONS |
| { |
| output : |
| { |
| file1(.text) |
| . = . + 1000; |
| file2(.text) |
| . += 1000; |
| file3(.text) |
| } = 0x12345678; |
| } |
| </pre> |
| <p class="noindent">In the previous example, the ‘<samp><span class="samp">.text</span></samp>’ section from <samp><span class="file">file1</span></samp> is |
| located at the beginning of the output section ‘<samp><span class="samp">output</span></samp>’. It is |
| followed by a 1000 byte gap. Then the ‘<samp><span class="samp">.text</span></samp>’ section from |
| <samp><span class="file">file2</span></samp> appears, also with a 1000 byte gap following before the |
| ‘<samp><span class="samp">.text</span></samp>’ section from <samp><span class="file">file3</span></samp>. The notation ‘<samp><span class="samp">= 0x12345678</span></samp>’ |
| specifies what data to write in the gaps (see <a href="Output-Section-Fill.html#Output-Section-Fill">Output Section Fill</a>). |
| |
| <p><a name="index-dot-inside-sections-510"></a>Note: <code>.</code> actually refers to the byte offset from the start of the |
| current containing object. Normally this is the <code>SECTIONS</code> |
| statement, whose start address is 0, hence <code>.</code> can be used as an |
| absolute address. If <code>.</code> is used inside a section description |
| however, it refers to the byte offset from the start of that section, |
| not an absolute address. Thus in a script like this: |
| |
| <pre class="smallexample"> SECTIONS |
| { |
| . = 0x100 |
| .text: { |
| *(.text) |
| . = 0x200 |
| } |
| . = 0x500 |
| .data: { |
| *(.data) |
| . += 0x600 |
| } |
| } |
| </pre> |
| <p>The ‘<samp><span class="samp">.text</span></samp>’ section will be assigned a starting address of 0x100 |
| and a size of exactly 0x200 bytes, even if there is not enough data in |
| the ‘<samp><span class="samp">.text</span></samp>’ input sections to fill this area. (If there is too |
| much data, an error will be produced because this would be an attempt to |
| move <code>.</code> backwards). The ‘<samp><span class="samp">.data</span></samp>’ section will start at 0x500 |
| and it will have an extra 0x600 bytes worth of space after the end of |
| the values from the ‘<samp><span class="samp">.data</span></samp>’ input sections and before the end of |
| the ‘<samp><span class="samp">.data</span></samp>’ output section itself. |
| |
| <p><a name="index-dot-outside-sections-511"></a>Setting symbols to the value of the location counter outside of an |
| output section statement can result in unexpected values if the linker |
| needs to place orphan sections. For example, given the following: |
| |
| <pre class="smallexample"> SECTIONS |
| { |
| start_of_text = . ; |
| .text: { *(.text) } |
| end_of_text = . ; |
| |
| start_of_data = . ; |
| .data: { *(.data) } |
| end_of_data = . ; |
| } |
| </pre> |
| <p>If the linker needs to place some input section, e.g. <code>.rodata</code>, |
| not mentioned in the script, it might choose to place that section |
| between <code>.text</code> and <code>.data</code>. You might think the linker |
| should place <code>.rodata</code> on the blank line in the above script, but |
| blank lines are of no particular significance to the linker. As well, |
| the linker doesn't associate the above symbol names with their |
| sections. Instead, it assumes that all assignments or other |
| statements belong to the previous output section, except for the |
| special case of an assignment to <code>.</code>. I.e., the linker will |
| place the orphan <code>.rodata</code> section as if the script was written |
| as follows: |
| |
| <pre class="smallexample"> SECTIONS |
| { |
| start_of_text = . ; |
| .text: { *(.text) } |
| end_of_text = . ; |
| |
| start_of_data = . ; |
| .rodata: { *(.rodata) } |
| .data: { *(.data) } |
| end_of_data = . ; |
| } |
| </pre> |
| <p>This may or may not be the script author's intention for the value of |
| <code>start_of_data</code>. One way to influence the orphan section |
| placement is to assign the location counter to itself, as the linker |
| assumes that an assignment to <code>.</code> is setting the start address of |
| a following output section and thus should be grouped with that |
| section. So you could write: |
| |
| <pre class="smallexample"> SECTIONS |
| { |
| start_of_text = . ; |
| .text: { *(.text) } |
| end_of_text = . ; |
| |
| . = . ; |
| start_of_data = . ; |
| .data: { *(.data) } |
| end_of_data = . ; |
| } |
| </pre> |
| <p>Now, the orphan <code>.rodata</code> section will be placed between |
| <code>end_of_text</code> and <code>start_of_data</code>. |
| |
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