| `options/` | (More or less) OS-independent headers and code.<br>`options/` is divided into subdirectories that can be enabled or disabled by ports.|
| `sysdeps/` | OS-specific headers and code.<br>`sysdeps/` is divded into per-port subdirectories. Exactly one of those subdirectories is enabled in each build.|
| `abis/` | OS-specific interface headers ("ABI headers"). Those contain the constants and structs of the OS interface. For example, the numerical values of `SEEK_SET` or `O_CREAT` live here, as well as structs like `struct stat`. ABI headers are _only_ allowed to contain constants, structs and unions but _no_ function declarations or logic.<br>`abis/` is divided into per-OS subdirectories but this division is for organizational purposes only. Ports can still mix headers from different `abis/` subdirectories.|
**Porting mlibc to a new OS**: Ports to new OSes are welcome. To port mlibc to another OS, the following changes need to be made:
1. Add new `sysdeps/` subdirectory `sysdeps/some-new-os/` and a `meson.build` to compile it. Integreate `sysdeps/some-new-os/meson.build` into the toplevel `meson.build`.
2. Create ABI headers in `abis/some-new-os/`. Add symlinks in `sysdeps/some-new-os/include/abi-bits` to your ABI headers. Look at existing ports to figure out the ABI headers required for the options enabled by `sysdeps/some-new-os/meson.build`.
3. In `sysdeps/some-new-os/`, add code to implement (a subset of) the functions from `options/internal/include/mlibc/sysdeps.hpp`. Which subset you need depends on the options that `sysdeps/some-new-os/meson.build` enables.
