Functionally it's the same. The functions called could live in the kernel (a system call), in the process itself or in a shared lib loaded into the process addr

Functionally it's the same. The functions called could live in the kernel (a system call), in the process itself or in a shared lib loaded into the process address space. Interpreting bytes is message passing and this communication happens through the handle/descriptor to the file/object/whatever. As above, this handle could have come from the kernel, another process or the executing process itself (the process opened some file or whatever). I think the message passing version happening through a handle can be more versatile as you could send this handle over to another process. Could copy it and distribute it to other processes in a easier way than injecting/changing a (presumably C ABI) into a process address space.