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The special linker variable dot ‘.’ always contains the
current output location counter. Since the .
always refers to a
location in an output section, it may only appear in an expression
within a SECTIONS
command. The .
symbol may appear
anywhere that an ordinary symbol is allowed in an expression.
Assigning a value to .
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.
SECTIONS { output : { file1(.text) . = . + 1000; file2(.text) . += 1000; file3(.text) } = 0x12345678; }
In the previous example, the ‘.text’ section from file1 is located at the beginning of the output section ‘output’. It is followed by a 1000 byte gap. Then the ‘.text’ section from file2 appears, also with a 1000 byte gap following before the ‘.text’ section from file3. The notation ‘= 0x12345678’ specifies what data to write in the gaps (see Output Section Fill).
Note: .
actually refers to the byte offset from the start of the
current containing object. Normally this is the SECTIONS
statement, whose start address is 0, hence .
can be used as an
absolute address. If .
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:
SECTIONS { . = 0x100 .text: { *(.text) . = 0x200 } . = 0x500 .data: { *(.data) . += 0x600 } }
The ‘.text’ 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 ‘.text’ 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 .
backwards). The ‘.data’ section will start at 0x500
and it will have an extra 0x600 bytes worth of space after the end of
the values from the ‘.data’ input sections and before the end of
the ‘.data’ output section itself.
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:
SECTIONS { start_of_text = . ; .text: { *(.text) } end_of_text = . ; start_of_data = . ; .data: { *(.data) } end_of_data = . ; }
If the linker needs to place some input section, e.g. .rodata
,
not mentioned in the script, it might choose to place that section
between .text
and .data
. You might think the linker
should place .rodata
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 .
. I.e., the linker will
place the orphan .rodata
section as if the script was written
as follows:
SECTIONS { start_of_text = . ; .text: { *(.text) } end_of_text = . ; start_of_data = . ; .rodata: { *(.rodata) } .data: { *(.data) } end_of_data = . ; }
This may or may not be the script author’s intention for the value of
start_of_data
. One way to influence the orphan section
placement is to assign the location counter to itself, as the linker
assumes that an assignment to .
is setting the start address of
a following output section and thus should be grouped with that
section. So you could write:
SECTIONS { start_of_text = . ; .text: { *(.text) } end_of_text = . ; . = . ; start_of_data = . ; .data: { *(.data) } end_of_data = . ; }
Now, the orphan .rodata
section will be placed between
end_of_text
and start_of_data
.
Next: Operators, Previous: Orphan Sections, Up: Expressions in Linker Scripts [Contents][Index]