Node: Builtin Functions, Previous: Expression Section, Up: Expressions
The linker script language includes a number of builtin functions for use in linker script expressions.
ABSOLUTE(
exp)
ADDR(
section)
symbol_1
and symbol_2
are assigned identical values:
SECTIONS { ... .output1 : { start_of_output_1 = ABSOLUTE(.); ... } .output : { symbol_1 = ADDR(.output1); symbol_2 = start_of_output_1; } ... }
ALIGN(
align)
ALIGN(
exp,
align)
.
) or arbitrary expression aligned to the next align boundary. The single operand ALIGN
doesn't change the value of the location counter--it just does arithmetic on it. The two operand ALIGN
allows an arbitrary expression to be aligned upwards (ALIGN(
align)
is equivalent to ALIGN(.,
align)
).
Here is an example which aligns the output .data
section to the next 0x2000
byte boundary after the preceding section and sets a variable within the section to the next 0x8000
boundary after the input sections:
SECTIONS { ... .data ALIGN(0x2000): { *(.data) variable = ALIGN(0x8000); } ... }
The first use of ALIGN
in this example specifies the location of a section because it is used as the optional address attribute of a section definition (see Output Section Address). The second use of ALIGN
is used to defines the value of a symbol.
The builtin function NEXT
is closely related to ALIGN
.
BLOCK(
exp)
ALIGN
, for compatibility with older linker scripts. It is most often seen when setting the address of an output section.DATA_SEGMENT_ALIGN(
maxpagesize,
commonpagesize)
(ALIGN(maxpagesize) + (. & (maxpagesize - 1)))or
(ALIGN(maxpagesize) + (. & (maxpagesize - commonpagesize)))
depending on whether the latter uses fewer commonpagesize sized pages for the data segment (area between the result of this expression and DATA_SEGMENT_END
) than the former or not. If the latter form is used, it means commonpagesize bytes of runtime memory will be saved at the expense of up to commonpagesize wasted bytes in the on-disk file.
This expression can only be used directly in SECTIONS
commands, not in any output section descriptions and only once in the linker script. commonpagesize should be less or equal to maxpagesize and should be the system page size the object wants to be optimized for (while still working on system page sizes up to maxpagesize).
Example:
. = DATA_SEGMENT_ALIGN(0x10000, 0x2000);
DATA_SEGMENT_END(
exp)
DATA_SEGMENT_ALIGN
evaluation purposes.
. = DATA_SEGMENT_END(.);
DEFINED(
symbol)
begin
to the first location in the .text
section--but if a symbol called begin
already existed, its value is preserved:
SECTIONS { ... .text : { begin = DEFINED(begin) ? begin : . ; ... } ... }
LOADADDR(
section)
ADDR
, but it may be different if the AT
attribute is used in the output section definition (see Output Section LMA).MAX(
exp1,
exp2)
MIN(
exp1,
exp2)
NEXT(
exp)
ALIGN(
exp)
; unless you use the MEMORY
command to define discontinuous memory for the output file, the two functions are equivalent.SIZEOF(
section)
symbol_1
and symbol_2
are assigned identical values:
SECTIONS{ ... .output { .start = . ; ... .end = . ; } symbol_1 = .end - .start ; symbol_2 = SIZEOF(.output); ... }
SIZEOF_HEADERS
sizeof_headers
When producing an ELF output file, if the linker script uses the SIZEOF_HEADERS
builtin function, the linker must compute the number of program headers before it has determined all the section addresses and sizes. If the linker later discovers that it needs additional program headers, it will report an error not enough room for program headers
. To avoid this error, you must avoid using the SIZEOF_HEADERS
function, or you must rework your linker script to avoid forcing the linker to use additional program headers, or you must define the program headers yourself using the PHDRS
command (see PHDRS).