How do "df" from within a C program?

[Richard H. Gumpertz]

I have a C program that would like to take special action when disk
space is getting tight for the group of files that it writes (to avoid
running out completely).
	1) How can a C program get the space available on a file system
	   (i.e., do the equivalent of a "df" command)?
	2) How can a C program determine which file system a particular
	   directory is on (so that it can find the appropriate argument
	   for doing the equivalent of "df")?
	3) I am using an AT&T Unix PC, OS release 3.51 (roughly Vr2); a
	   system-specific answer will do IF NECESSARY.
	4) What are the portability issues of the answers to the above?
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