how do you write a program to determine free swap (like pstat -s)

[Jeffrey Mogul]

In article <1990Nov6.033516.18772 at NCoast.ORG> simpsong at NCoast.ORG (Gregory R. Simpson @ The North Coast) writes:
>I'd like to write a simple vm_check program that returns
>nothing except the amount of swap remaining... (which is
>all I'm interested in...

As you might have guessed from the output of /etc/pstat -s,
there isn't simple value stored in the kernel that says "this
much swap space is in use"; rather, pstat grovels through
kernel data structures counting things up, and computes
a total through a fairly hairy computation.  I counted over
160 lines in pstat just to do what you're asking for.

Since the Ultrix version of pstat.c carries the usual Digital/AT&T
copyright, I can't post that on the net.  If you get a 4.2BSD
source tape, I suspect that the code to do this calculation hasn't
changed all that much, so perhaps starting with that version of
pstat.c, you can create a program to do what you want.

-Jeff