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

[Alan's Home for Wayward Notes File.]

In article <1990Nov7.015529.15623 at wrl.dec.com>, mogul at wrl.dec.com (Jeffrey Mogul) writes:
> 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.

	Actually it depends on what version of ULTRIX you're
	running.  In V3.1 and earlier.  It's the really hairy
	stuff that Jeff mentioned.  In V4.0 there is a data
	structure called "swapu" that has how much is being
	used for various things.  The amount free is just the
	total - the amount used.  You can get total from adding
	amount indicated in the "swdevt" table.
> 
> 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.
> 

	In general such things look something like:

/*
 *	Correctly dealing with errors, counting up the amount of
 *	page/swap space and getting the units right is left as an 
 *	exercise to the reader.
 */
#include <sys/types.h>
#include <sys/file.h>
#include <sys/dmap.h>

#include <nlist.h>

void	exit(), nlist() ;
off_t	lseek() ;

struct nlist nl[] = {
	{ "_swapu" },
#define	NM_SWAPU	0
	{ 0 },
};

main()
{
	register rc, fd ;
	struct swapu_t data ;

	nlist("/vmunix", nl) ;

	if((fd = open("/dev/kmem", O_RDONLY)) == -1 )
		exit(-1) ;

	if( lseek(fd, (off_t)nl[NM_SWAPU].n_value, L_SET) == -1 )
		exit(-1) ;

	if( read(fd, (char *)&data, sizeof(data)) == -1 )
		exit(-1) ;

	printf("%d\n", data.total_used) ;
	printf("%d\n", data.wasted) ;

	if( close(fd) == -1 )
		exit(-1) ;

	return 0 ;
}

	You may season to taste.

> -Jeff


-- 
Alan Rollow				alan at nabeth.enet.dec.com