Overview

Now that I had picked mlibc as the implementation to port, and release-5.0 as the version, it was time to start working. I forked the GitHub repository to the PincerOS organization (pincerOS/mlibc), and cloned it to my local machine.

Starting the Port

1. Following the instructions in the README, I create a new sysdeps subdirectory, pinceros. I add an empty meson.build file, and then add a new entry to the top-level meson.build file to register the new sysdep, following the pattern of the other ports.

   

2. Then to verify that the new sysdep was being recognized, I try to meson build and just see what would happen. I get errors about linux_kernel_headers not being set, and scrolling up in the terminal, it looks like it was compiling for x86-64, rather than Aarch64. I had forgotten to configure meson to build for our system!

   

3. To fix this, I create a cross compile configuration file, and tell meson about it by running meson setup build --cross-file aarch64-pinceros.txt.
   • I eventually end up moving the file to the scripts directory, and renaming it to differentiate between compilers, but more on that later.

   

4. Now, there is a new error, but at least it lists the right host machine information. The error reports a directory not existing, which I resolve by simply creating an empty directory at that location. So now meson setup succeeds.

   

   • As a side tangent, I noticed a list titled “Subprojects” which listed cshim, cxxshim, and frigg. Out of curiosity, I looked into the Managarm GitHub and found the repos for each of them.
     • cshim is “a collection of freestanding C headers, for use with GCC or Clang.” It contains header files such as stdint.h, stddef.h, stdbool.h, and stdarg.h, which define types and macros, but not functions.
     • cxxshim is similar, but for C++. It contains header files such as algorithm, cstddef, cstdint, iterator, memory, new, type_traits, and utility.
     • frigg provides “lightweight C++ utilities and algorithms for system programming” and contains many header files, including mutex.hpp, optional.hpp, and unique.hpp.

5. Now that meson setup works, I try compiling with meson compile -C build. There are lots of build errors because I have not yet created the ABI header files.

   

Getting mlibc to Build

1. Following the instructions in the README, I know I have to create all of the ABI header files in the abis directory and create a symlink to each of them in sysdeps. There are a lot of them (45), so it would be kind of painful to do manually. There is a script in the scripts directory called abi-link.sh, but all it does is create the symlinks, not the actual header files themselves. At this point in time while I’m setting up mlibc, we don’t have a very fleshed out syscall ABI in our OS, so we do not have anything to put in the header files. But I know that there were specific named constants that have to be defined in each of them for mlibc to work, so I decide to just write a new script to copy the Linux headers and generate the symlinks, at scripts/gen-abi-bits.py.

   

2. Now, I get compiler errors relating to invalid instruction mnemonics, so despite providing the cross compile file, there is still something wrong with it. I update the cross file to not only set the cpu, but also pass the triple target argument --target=aarch64-none-elf into clang using c_args and cpp_args.

   

3. But now we’re back to failing meson setup…

   

4. I notice in the logs that it is failing to find ld.lld, so I try installing it. And get errors from ld.lld being unable to find libclang_rt.builtins, lc, and lm. I do have compiler-rt on my Linux system for x86-64, but not for Aarch64. I search for it in the Arch Linux package repository and in the AUR, but the only results I see are for armv6 and armv7, not for armv8/aarch64.

   

5. At this point I fear I will have to build llvm’s compiler-rt from source (which I would prefer to not have to do), so I try pivoting to gcc first and see if I can make it further than with clang. So I install aarch64-linux-gnu-gcc and create a new cross compile file for it, aarch64-pinceros-gcc.txt.

   

6. Now compiling yields a ton of undefined reference linker errors, which is actually a good sign because it means that it was at least able to compile everything, and then just fails to link because I have not provided any implementation for the necessary functions yet.