Question about ustat(2) and free disk space

[John Rogers]

Hello.  I was reading "The Programmer's Workbench" (an article in the 1978
special UNIX issue of the Bell Systems Technical Journal), and I noticed the
following sentence:

	"The ustat system call was added to allow programs to discover the
	amount of free space remaining in a file system."

Since I haven't seen ustat in the V7, Sys III, or 4.1BSD documentation, I'd
like to know:

	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?

Thanks in advance...
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				JR (John Rogers)
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