ld.md

ld

https://sourceware.org/binutils/docs-2.25/ld/index.html

Does the linking process, which converts object files into executables.

Basic usage:

nasm -o hello_world.o hello_world.asm
ld -o hello_world.out hello_world.o
./hello_world

Order of libraries passed

The order matters for static libraries:

• http://stackoverflow.com/questions/45135/why-does-the-order-in-which-libraries-are-linked-sometimes-cause-errors-in-gcc

References undefined references of later libraries can only be resolved by the earlier ones.

start-group

end-group

Those weird options act on the -l libraries that appear in between them, e.g.:

ld --start-group lib1 lib2 lib3 --end-group

With them, ld loops between the given libraries until all references have been resolved.

This has a performance impact.

Standard library

If you program is compiled with gcc, it expects a few things from the linker which ld does not do by default: http://stackoverflow.com/questions/3577922/how-to-link-a-gas-assembly-file-as-a-c-program-with-ld-without-using-gcc

ld -dynamic-linker /lib64/ld-linux-x86-64.so.2 \
  /usr/lib/x86_64-linux-gnu/crt1.o \
  /usr/lib/x86_64-linux-gnu/crti.o \
  -lc hello_world.o \
  /usr/lib/x86_64-linux-gnu/crtn.o

m

Cross compile

http://stackoverflow.com/questions/16004206/force-gnu-linker-to-generate-32-bit-elf-executables

-m <arch>, where <arch> can be found with ld -V, sample output:

elf_x86_64
elf32_x86_64
elf_i386
i386linux
elf_l1om
elf_k1om
i386pep
i386pe

TODO meaning of each type?

verbose

Print ld information, including the linker script used:

ld --verbose

e

Sets the ELF entry point to a given symbol or address.

This determines e_entry of the ELF header.

You can set it to any address, but keep in mind that sections have alignment constraints.

So for example if you something like:

. = 0x4FFFFF;
.text :
{
    *(*)
}

and the page size is 2M, then your executable will be about 2M in size:

• .text segment will go to 0x4FFFFF
• all bytes up to 0x4FFFFE will be filled with zeroes
• e_entry is set to 0x4FFFFF

Library and weak TODOs

• http://stackoverflow.com/questions/24390267/why-redefinition-of-a-function-which-is-already-present-in-dynamic-or-static-lib
• http://stackoverflow.com/questions/13089166/how-to-make-gcc-link-strong-symbol-in-static-library-to-overwittren-weak-symbol