Use cosmocc to cross‐compile a CMake project

We know how to compile an Autotools project using cosmocc but how do you compile a CMake project with cosmocc?

Here's an example project:

main.c

#include <stdio.h>

int main() {
    puts("Hello world!");
    return 0;
}

CMakeLists.txt

cmake_minimum_required(VERSION 3.29)
project(hello-world LANGUAGES C)
add_executable(hello-world main.c)

You can almost just do CC=cosmocc cmake ... and call it day. Almost. But not quite! 😉 There are some other CMake settings you need to set to get things to work well with cosmocc.

First, you'll need to add a CMAKE_USER_MAKE_RULES_OVERRIDE file to define the object suffix as .o instead of .obj. cosmocc currently doesn't work if the object extension is not .o. 🤷‍♀️

cosmocc-override.cmake

set(CMAKE_ASM_OUTPUT_EXTENSION .o)
set(CMAKE_C_OUTPUT_EXTENSION .o)
set(CMAKE_CXX_OUTPUT_EXTENSION .o)

Why can't we just specify -DCMAKE_C_OUTPUT_EXTENSION=".o" on the command line? It needs to be defined after the Generic configuration is loaded so it doesn't get reset to the default .obj. CMAKE_USER_MAKE_RULES_OVERRIDE is the recommended way to do it. https://gitlab.kitware.com/cmake/cmake/-/issues/18713

ASM="cosmocc" \
CC="cosmocc" \
CXX="cosmoc++" \
cmake \
  -DCMAKE_SYSTEM_NAME="Generic" \
  -UCMAKE_SYSTEM_PROCESSOR \
  -DCMAKE_USER_MAKE_RULES_OVERRIDE="$PWD/cosmocc-override.cmake" \
  -DCMAKE_AR="$(command -v cosmoar)" \
  -UCMAKE_RANLIB \
  -B build

-- The C compiler identification is GNU 12.3.0
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /somewhere/cosmocc/bin/cosmocc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Configuring done (2.9s)
-- Generating done (0.0s)
-- Build files have been written to: /somewhere/hello-world/build

Do I have to specify CMAKE_SYSTEM_NAME and CMAKE_SYSTEM_PROCESSOR? Yes. They configure some CMake magic internally and configure what your CMakeLists.txt thinks it's compilation target is.

Why not CMAKE_SYSTEM_NAME="Linux"? So that if(LINUX) and other Linux-specific stuff in CMakeLists.txt doesn't activate for unknown-unknown-cosmo.

Why not CMAKE_SYSTEM_PROCESSOR="unknown"? unset is what means unknown-unknown-cosmo to CMake. CMake internals use if(CMAKE_SYSTEM_PROCESSOR), not if(CMAKE_SYSTEM_PROCESSOR STREQUAL unknown).

Why does CMAKE_AR need to be an absolute path? Unfixed issue. https://gitlab.kitware.com/cmake/cmake/-/issues/18087

Why does CMAKE_RANLIB need to be unset? CMake gates using ranlib behind if(CMAKE_RANLIB) so it must be unset not just an empty string. cosmocc doesn't bundle a cosmoar.

Why can't we do AR="cosmoar" like CC and CXX? Unfixed issue. https://gitlab.kitware.com/cmake/cmake/-/issues/18712

Now that the project is configured with all the compiler settings and other magic✨ we can run the build step:

cmake --build build

[ 50%] Building C object CMakeFiles/hello-world.dir/main.c.o
[100%] Linking C executable hello-world
[100%] Built target hello-world

You just made a magic✨ binary! 🥳

^{On Windows you need to rename it or symlink it to have an .exe or .com suffix to execute it.}

./build/hello-world     

Hello world!