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<h4 class="subsection">3.10.4 The Location Counter</h4>
<p><a name="index-_002e-468"></a><a name="index-dot-469"></a><a name="index-location-counter-470"></a><a name="index-current-output-location-471"></a>The special linker variable <dfn>dot</dfn> <span class="samp">.</span> 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-472"></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 <span class="samp">.text</span> section from <span class="file">file1</span> is
located at the beginning of the output section <span class="samp">output</span>. It is
followed by a 1000 byte gap. Then the <span class="samp">.text</span> section from
<span class="file">file2</span> appears, also with a 1000 byte gap following before the
<span class="samp">.text</span> section from <span class="file">file3</span>. The notation <span class="samp">= 0x12345678</span>
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-473"></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 <span class="samp">.text</span> 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 <span class="samp">.text</span> 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 <span class="samp">.data</span> section will start at 0x500
and it will have an extra 0x600 bytes worth of space after the end of
the values from the <span class="samp">.data</span> input sections and before the end of
the <span class="samp">.data</span> output section itself.
<p><a name="index-dot-outside-sections-474"></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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