iostat(8): implement periodic display with setitimer(2)

[Scott Cheloha]

As with e.g. kstat(1), implementing a periodic display with
nanosleep(2) causes the display interval to drift.  Doing it with
setitimer(2) prevents drift.

ok?

Index: iostat.c
===================================================================
RCS file: /cvs/src/usr.sbin/iostat/iostat.c,v
retrieving revision 1.45
diff -u -p -r1.45 iostat.c
--- iostat.c    4 Dec 2022 23:50:51 -0000       1.45
+++ iostat.c    10 Dec 2022 14:00:50 -0000
@@ -68,10 +68,12 @@
 
 #include <err.h>
 #include <ctype.h>
+#include <limits.h>
 #include <signal.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <time.h>
 #include <unistd.h>
 #include <kvm.h>
 
@@ -85,7 +87,8 @@ extern int    dk_ndrive;
 kvm_t *kd;
 char   *nlistf, *memf;
 
-int            hz, reps, interval;
+int            hz, reps;
+time_t         interval;
 static int     todo = 0;
 
 volatile sig_atomic_t wantheader;
@@ -100,6 +103,7 @@ volatile sig_atomic_t wantheader;
 static void cpustats(void);
 static void disk_stats(double);
 static void disk_stats2(double);
+static void sigalarm(int);
 static void sigheader(int);
 static void header(void);
 static void usage(void);
@@ -113,9 +117,10 @@ int dkinit(int);
 int
 main(int argc, char *argv[])
 {
+       struct itimerval itv;
        const char *errstr;
+       sigset_t empty;
        int ch, hdrcnt;
-       struct timespec ts;
 
        while ((ch = getopt(argc, argv, "Cc:dDIM:N:Tw:")) != -1)
                switch(ch) {
@@ -146,7 +151,7 @@ main(int argc, char *argv[])
                        todo |= SHOW_TTY;
                        break;
                case 'w':
-                       interval = strtonum(optarg, 1, 100000000, &errstr);
+                       interval = strtonum(optarg, 1, UINT_MAX, &errstr);
                        if (errstr)
                                errx(1, "wait is %s", errstr);
                        break;
@@ -169,12 +174,20 @@ main(int argc, char *argv[])
        dkreadstats();
        selectdrives(argv);
 
-       ts.tv_sec = interval;
-       ts.tv_nsec = 0;
-
        /* print a new header on sigcont */
        signal(SIGCONT, sigheader);
 
+       if (interval != 0) {
+               if (signal(SIGALRM, sigalarm) == SIG_ERR)
+                       err(1, "signal");
+               sigemptyset(&empty);
+               itv.it_value.tv_sec = interval;
+               itv.it_value.tv_usec = 0;
+               itv.it_interval = itv.it_value;
+               if (setitimer(ITIMER_REAL, &itv, NULL) == -1)
+                       err(1, "setitimer");
+       }
+
        for (hdrcnt = 1;;) {
                if (!--hdrcnt || wantheader) {
                        header();
@@ -188,7 +201,7 @@ main(int argc, char *argv[])
 
                if (reps >= 0 && --reps <= 0)
                        break;
-               nanosleep(&ts, NULL);
+               sigsuspend(&empty);
                dkreadstats();
                if (last.dk_ndrive != cur.dk_ndrive)
                        wantheader = 1;
@@ -196,6 +209,11 @@ main(int argc, char *argv[])
        exit(0);
 }
 
+static void
+sigalarm(int signo)
+{
+}
+
 /*ARGSUSED*/
 static void
 sigheader(int signo)
@@ -421,7 +439,7 @@ selectdrives(char *argv[])
                        errx(1, "invalid interval or drive name: %s", *argv);
        }
        if (*argv) {
-               interval = strtonum(*argv, 1, 100000000, &errstr);
+               interval = strtonum(*argv, 1, UINT_MAX, &errstr);
                if (errstr)
                        errx(1, "interval is %s", errstr);
                if (*++argv) {