Unix-jun72: Restoration of 1st Edition UNIX kernel sources

Seems like they're trying to accommodate the default "fallthrough" behavior of the switch without using break, either because break hadn't been invented or because they explicitly wanted to bypass the code below the switch block.

EDIT: On further inspection, the code below the switch block is meant to behave like a "default" block. I bet this is before they invented "default".

As it is now, you push all your function arguments to the stack before you push your locals. If you call printf(a1, a2, ... , an), it pushes n arguments, then the instruction pointer, then your locals. No clobbering necessary.

Maybe 1972-era C pushed arguments to the stack according to the signature defined in the function definition rather than the function call? (i.e. printf would have 10 pushes no matter how many arguments you actually passed to it). If it pushed according to the function signature, you'd waste memory and have a hard limit to how many arguments you could pass. And you'd waste push instructions. So I don't know why it's written the way it is, but corrupting your locals doesn't seem like a likely explanation. Maybe they're trying to mollify the compiler?

I've used a C compiler[1] that implements old K&R semantics without "...". In fact, I used it to write a printf just last week[2]. All you need to do is make a pointer to one of the arguments and increment the pointer to take in variable arguments--which is exactly what this code does. The difference is, the compiler I used didn't require any parameters except fmt to be explicitly declared.

[1] http://linux.die.net/man/1/bcc

[2] http://philwelch.net/io.c

(Why do I have experience with strange, non-compliant 16 bit C compilers? Because I have an eccentric professor for my operating systems class, apparently.)