(More or less) OS-independent headers and code. options/ is divided into subdirectories that can be enabled or disabled by ports.
sysdeps/
OS-specific headers and code. 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. 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:
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.
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.
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.
