Portability Primitives

This is the portability layer where all primitives needed from the OS are defined.

• include/mimalloc/prim.h: primitive portability API definition.
• prim.c: Selects one of unix/prim.c, wasi/prim.c, or windows/prim.c depending on the host platform (and on macOS, osx/prim.c defers to unix/prim.c).

Note: still work in progress, there may still be places in the sources that still depend on OS ifdef's.