This directory must be run in Linux IA-32.
Linker scripts determine how exactly ld is going to transform object files into the executable.
In particular, how the sections are going to me mashed up into segments.
The default linker script operation is determined by the default linker scripts.
Those are hardcoded into the ld executable.
make install also generates an informational copy of the scripts at:
x86_64-unknown-linux-gnu/lib/ldscripts
which some distributions provide. But those are only informational. The usual location is:
/usr/lib/ldscripts
The actual script being used is determined by the ld options, and can be viewed with:
ld --verbose
which spits out the entire script, but not it's name. You can then match this output to the scripts under /usr/lib/ldscripts.
Minimal example: http://stackoverflow.com/questions/7182409/how-to-correctly-use-a-simple-linker-script TODO make into a repo.
The main case where linker scripts are needed is when building more "exotic" executables, typically boot sectors or a multiboot file. E.g.: http://wiki.osdev.org/Bare_Bones
Replace the linker script with a custom one:
ld -T script a.o
You will often use an existing script as the basis for this.
Change the address of some predefined segments:
-Ttext-segment=0x80000
The default linker script reads:
. = SEGMENT_START("text-segment", 0x400000)
which chooses takes the value passed, or 4M by default.
Most important directive: contains the section o segment mappings.
You can select sections by object file.
For example, for a multiboot elf file, we must put the multiboot header before everything else.
So if we had our header in the .text section of the boot.o file (bad practice, it would be better to have a specially named section), we could write:
.text 0x100000 :
{
boot.o(.text)
*(.text)
}
This way, boot.o(.text) is guaranteed to put boot.o first, and *(.text) then matches the others.
Used to insert raw bytes directly into the output.
Application: insert boot sector magic bytes: https://github.com/cirosantilli/x86-bare-metal-examples/blob/2b79ac21df801fbf4619d009411be6b9cd10e6e0/a.ld#L14
Like QUAD but sign extends.
https://sourceware.org/binutils/docs/ld/PROVIDE.html#index-PROVIDE-408
Make provided symbols weak: if they are defined by the object files, that definition wins.
Without PROVIDE, a multiple definition error occurs.
E.g.:
__symbol_weak = PROVIDE(.);
__symbol = .;
Note that:
__symbol = .;
sets the address of __symbol, relative to the current segment. . in this context is also relative to the current segment.
To set the value of the symbol, you'd need:
__symbol = .;
DWORD
Used on the default userland script for section position symbols like etext and edata: http://stackoverflow.com/questions/1765969/where-are-the-symbols-etext-edata-and-end-defined
Those symbols are placed on the output right where they are defined, relative to other sections and symbols.
TODO
Returns the address of the next multiple.
Applications:
Extend the output to be a multiple of 512 bytes:
. = ALIGN(512)
This can be useful to generate boot sectors with multiple stages.
Some segment names are magic: .text and .data, and will have fixed permissions no matter what.
If you use a non-magic name lie .blurb, the permission is the sum of the permissions of all sections that make up the segment.
It is also possible to set the segment permissions with the FLAGS option of the PHDRS linker script command.