Comment by dfox

On linux the stable API and ABI that you are supposed to use is also just libc. But due to various reasons people think that the libc ABI is less stable than how stable it really is.

Edit: NT with its split OS API dll and separate libc is its own can of worms. You can have multiple instances of libc in the same address space, which among other things implies that you cannot safely malloc() in one module and free() in another, and don't even think about passing FILE* around.