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From: nerd@percival.rain.com (Michael Galassi)
Newsgroups: comp.sys.next
Subject: Re: Trimmer for /usr/adm/wtmp
Message-ID: <1991Jun19.162436.13902@percy.rain.com>
Date: 19 Jun 91 16:24:36 GMT
References: <1991Jun19.022344.26696@agate.berkeley.edu>
Sender: news@percy.rain.com (news maintainer)
Organization: Percy's UNIX, Portland, OR.
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izumi@pinoko.berkeley.edu (Izumi Ohzawa) writes:
>Here's a little more reasonable file trimmer for /usr/adm/wtmp file.
>With the default monthly script (/usr/adm/monthly), the wtmp file
>is wiped clean on the first of each month.  It also wipes out
>/usr/adm/lastlog, which shouldn't be necessary (lastlog doesn't grow).

You got it right there, whoever wrote monthly was asleep.  I use a
slightly more paranoid program which is somewhat  faster.  It only
does one read of the wtmp file and stores the part to keep in a
malloced buffer.  This works nicely as the wtmp structure is only
36 bytes so even keeping 5000 entries only consumes 176 K.

cheers,
-m

--- cut here for wtrim, watch out for .signature at the end ---
/*
** by Michael Galassi -- nerd@percival.rain.com
** this is public domain, you can even say you wrote it,
** I don't care compile with:
**	 cc -ansi -O wtrim.c -o wtrim -object -s
*/

#include <stdio.h>
#include <stdlib.h>
#include <utmp.h>
#include <sys/file.h>
#include <sys/stat.h>

#define WTMP "/usr/adm/wtmp"
#define OWTMP "/usr/adm/wtmp.old"
#define DEFTRIM 1000

void main(int argc, char *argv[])
{
	int size;
	int fd;
	struct stat st;
	struct utmp *ptr;

	if (argc == 2)
		size = atoi(argv[1]) * sizeof(struct utmp);
	else if (argc == 1)
		size = DEFTRIM * sizeof(struct utmp);
	else {
		fprintf(stderr, "usage: %s [count]\n", argv[0]);
		exit(1);
	}
	if (stat(WTMP, &st) < 0) {
		fprintf(stderr, "%s: can't stat " WTMP "\n", argv[0]);
		exit(1);
	}
	if (st.st_size <= size)		/* already OK */
		exit(0);
	if ((fd = open(WTMP, O_CREAT|O_RDWR, 0644)) < 0) {
		fprintf(stderr, "%s: can't open " WTMP "\n", argv[0]);
		exit(1);
	}
	if (lseek(fd, st.st_size - size, L_SET) != st.st_size - size) {
		fprintf(stderr, "%s: can't lseek in " WTMP "\n", argv[0]);
		close(fd);
		exit(1);
	}
	if ((ptr = (struct utmp *) malloc(size)) == (void *)NULL) {
		fprintf(stderr, "%s: can't malloc %d bytes\n", argv[0], size);
		close(fd);
		exit(1);
	}
	if (read(fd, ptr, size) != size) {
		fprintf(stderr, "%s: can't malloc %d bytes\n", argv[0], size);
		free(ptr);
		close(fd);
		exit(1);
	}
	close(fd);
	unlink(OWTMP);
	if (rename(WTMP, OWTMP) < 0) {
		fprintf(stderr, "%s: can't rename " WTMP " to " OWTMP "\n", argv[0]);
		free(ptr);
		exit(1);
	}
	if ((fd = open(WTMP, O_CREAT|O_WRONLY, 0644)) < 0) {
		fprintf(stderr, "%s: can't open new " WTMP "\n", argv[0]);
		free(ptr);
		rename(OWTMP, WTMP);
		exit(1);
	}
	if (write(fd, ptr, size) != size) {
		fprintf(stderr, "%s: can't write new " WTMP "\n", argv[0]);
		free(ptr);
		close(fd);
		unlink(WTMP);
		rename(OWTMP, WTMP);
		exit(1);
	}
	close(fd);
	free(ptr);
	exit(0);
}
-- 
Michael Galassi				| nerd@percival.rain.com
MS-DOS:  The ultimate PC virus.		| ...!tektronix!percy!nerd