Question about ustat(2) and free dis - (nf)

[jab at uokvax.UUCP]

#R:forcm5:-12800:uokvax:6100021:000:2703
uokvax!jab    Feb 21 18:49:00 1984

/***** uokvax:net.unix / forcm5!jr /  3:47 am  Feb 18, 1984 */
	1) What happened to ustat?
	2) What are/were it's arguments?  I assume it returned a long, for
	   the number of bytes (where -1 might mean an error of some sort).
	3) How can someone do the same thing without using the ustat system
	   call?
	4) How does df(1) work?
-- 
				JR (John Rogers)
				UUCP: forcm5!jr, fortune!jr, proper!jr
				CompuServe: 70140,213
/* ---------- */

I can answer a little of that. If I had the "PWB/Unix" manual, I could answer
a whole lot more!

i)	"ustat" was a system call that you gave a special device name and a
	structure to be filled in. It looked up that device in the in-core
	mount table and copied specific items out. As I remember, the old
	RJE used it a lot. (I seem to remember our local wizard, one of only
	of few I've ever met, bumping into this  when he put RJE on our top
	of a V6 system.)
ii)	I just looked through the System III kernel, and it's there. It's 
	under the name "utssys", although there is a C library interface:
		#include <sys/types.h>
		#include <ustat.h>

		int ustat(dev, buf)
			int dev;	/* major/minor of special device */
			struct ustat *buf;
	The "struct ustat" contains the volume name and pack name and the
	number of free inodes/blocks.
iii)	It's on system V, also --- I stole the above C statements from a
	system V manual.
iv)	I assume that you could do the same thing FOR AN UNMOUNTED FILESYSTEM
	by opening that device and copying out the information from the 
	superblock --- it's all there. For a mounted filesystem, you need to
	be able to get to the in-core superblock, and that's more trouble than
	it's worth. (Well, I suppose you could just "sync()" before getting it
	off the special device, but...)
v)	Different versions of "df" do different things. The Berkeley 2.8 "df",
	if I remember, went out to the disk and counted the number of free
	blocks! (Most of the older "df" commands do this.) On a lot of systems
	that don't have "ustat", you just grab the info you want out of the
	superblock. Peter Staubach here at OU hacked the "ustat" system call
	into his 2.8 pdp-11 system, and "df" does a "ustat" first, and if that
	fails, THEN it gets the info off the superblock. To put it mildly, it
	runs pretty quickly.

I think I messed up the numbering scheme. Oh, well. The "ustat" system call,
along with several other PWB notions, was never bought back by the people that
gave us Version 7 and Unix/32V. (Version 7 begat 2.8bsd, and Unix/32V begat
3bsd and 4.Xbsd.) This explains why it wasn't in any of those versions --- the
system III and system V are direct decendents of PWB, so those notions are in
system {III,V}.

	Jeff Bowles
	Lisle, IL