Today, I want to show you how to link an object file produces with
any compiled language into an executable using the LLVM linker
I will limit myself to a unix operating system and linking using
musl C standard library.
Steps for other systems will be similar.
Introduction to Linking
Many compilers such as GCC or Clang for C or rustc for Rust, parse the code and convert it to machine instructions, storing the result in object files. These files contain the function and structure definitions.
After this step is done, the compiler invokes the linker.
The job of the linker is to take these object files,
other object files coming from the libraries your code is using
libc) and “links” the functions together,
for instance ensuring that when your code calls a library function,
the correct code is called.
The output of this operation is the final executable file. What I just described is plain static linking. I won’t discuss anything else here.
The linking step is usually performed by the compiler calling the linker, so it’s typically transparent to the user. For this reason, there isn’t so much information online on how to do it manually. But if you are developing your own compiled language, you might have to do this step manually, or at least understand how it’s done.
The linking step is also crucial to add small pieces of assembly that ensure the program can be started correctly and returns the right result to the invoking process.
Context & Prerequisites
In particular, I had this problem when writing my own compiled
language using LLVM. I used
llc to compile LLVM IR code
into object files and I wanted to link this with
to be able to create my own standalone executables.
I will assume you already have some object files
*.o) which contain a
(the entrypoint to our program) and a copy of
If you want to get your own copy of the libc, just
download the tarball from the project website and build it.
The compilation will generate certain files. The minimum we need is:
libc.a- The file containing all functionality such as
crtn.o- These files contain some special assembly code that is called before the
mainfunction to initialize the C standard library and to pass input and output correctly between our program and the invoking process. You can find more information in the crt man page.
All these files are of course platform specific.
To link your program run:
ld.lld <paths/to/objects_files> <path/to/library_files> -o <path/to/executable>
(of course replacing the placeholders within
That is actually all you need to do!
It took me a while to figure this out, by checking how clang does
it and trying to reduce the number of options to
ld.lld to a