Re: kern.msgbuf_show_timestamp=1

On Fri, 19-Dec-2025 at 20:37:41 +0000, Jamie Landeg-Jones wrote:
> void <void@f-m.fm> wrote:
> 
> >...
> > and so dmsg has things like
> >
> > [1353919] swp_pager_getswapspace(32): failed
> >
> > the [1353919] I guess being the timestamp.
> >
> > ...
> >
> Yes! You can get the value from sysctl kern.boottime.
> 
> Here is a small program I wrote a while back, just call it
> as a dmesg substitute.
> ...
> Cheers, Jamie
> 
> #!/bin/sh -efu
> set -efu
> 
> boottime="$(/sbin/sysctl -n kern.boottime | /usr/local/bin/gawk '{printf "%d", gensub ("^.* sec = ([1-9][0-9]*), .*$", "\\1", 1)}')"
> 
> [ -z "$(printf '%s' "$boottime" | /usr/bin/egrep '^0$|^[1-9][0-9]*$')" ] && { printf 'Invalid boottime retrieved.\n' >& 2; exit 1; }
> 
> /sbin/dmesg "$@" | /usr/local/bin/gawk -v boottime="$boottime" '
> 
>     {
>       uptime = gensub ("^\\[([1-9][0-9]*)\\] .*$", "\\1", 1)
>       if (uptime == $0) realtime = "??? ?? ??:??;??"
>        else realtime = strftime ("%b %d %T", uptime + boottime)
> 
>       print realtime " " $0
>     }'

FWIW, this is a patch for dmesg(1) I use. You can enable timestamp
conversion using -t and optionally specify a different output format
using -f "fmt".

If interested for inclusion in base, I could try to re-format it
and come up with a manpage diff...

--- sbin/dmesg/dmesg.c	2023-12-27 17:49:57.000000000 +0100
+++ sbin/dmesg/dmesg.c	2026-01-22 09:20:28.987020000 +0100
@@ -60,6 +60,7 @@
 #include <unistd.h>
 #include <vis.h>
 #include <sys/syslog.h>
+#include <time.h>
 
 static struct nlist nl[] = {
 #define	X_MSGBUF	0
@@ -76,18 +77,23 @@
 main(int argc, char *argv[])
 {
 	struct msgbuf *bufp, cur;
+	struct timeval boottime;
+	char timebuf[64];
 	char *bp, *ep, *memf, *nextp, *nlistf, *p, *q, *visbp;
+	const char *timefmt = "%d %b %T";
 	kvm_t *kd;
+	time_t time;
 	size_t buflen, bufpos;
 	long pri;
-	int ch, clear;
+	int ch, clear, timeconv;
 	bool all;
 
 	all = false;
 	clear = false;
+	timeconv = false;
 	(void) setlocale(LC_CTYPE, "");
 	memf = nlistf = NULL;
-	while ((ch = getopt(argc, argv, "acM:N:")) != -1)
+	while ((ch = getopt(argc, argv, "actf:M:N:")) != -1)
 		switch(ch) {
 		case 'a':
 			all = true;
@@ -95,6 +101,12 @@
 		case 'c':
 			clear = true;
 			break;
+		case 't':
+			timeconv = true;
+			break;
+		case 'f':
+			timefmt = optarg;
+			break;
 		case 'M':
 			memf = optarg;
 			break;
@@ -109,6 +121,13 @@
 	if (argc != 0)
 		usage();
 
+	if( timeconv ) {
+		int mib[6] = { CTL_KERN, KERN_BOOTTIME };
+		size_t l = sizeof boottime;
+		if (sysctl(mib, 2, &boottime, &l, 0, 0) < 0)
+			err(1, "sysctl kern.boottime");
+        }
+
 	if (memf == NULL) {
 		/*
 		 * Running kernel.  Use sysctl.  This gives an unwrapped buffer
@@ -200,7 +219,22 @@
 		}
 
 		(void)strvisx(visbp, p, nextp - p, 0);
-		(void)printf("%s", visbp);
+		bufpos = strspn( visbp+1, "0123456789" ) + 1;
+		if( timeconv != true
+		 || bufpos < 2
+		 || visbp[0] != '['
+		 || visbp[bufpos] != ']'
+		 || visbp[bufpos+1] != ' ' ) {
+			(void)printf("%s", visbp);
+			continue;
+		}
+
+		visbp[bufpos] = '\0';
+		time = boottime.tv_sec + strtoull( visbp+1, NULL, 10 );
+		if( strftime( timebuf, sizeof timebuf, timefmt, localtime( &time ) ) != 0 )
+			(void)printf( "[%s]%s", timebuf, visbp + bufpos + 1 );
+		else
+			(void)printf("%s", visbp);
 	}
 	exit(0);
 }
@@ -208,6 +242,6 @@
 void
 usage(void)
 {
-	fprintf(stderr, "usage: dmesg [-ac] [-M core [-N system]]\n");
+	fprintf(stderr, "usage: dmesg [-ac] [-t [-f output_fmt]] [-M core [-N system]]\n");
 	exit(1);
 }


	-Andre