timers/real_timer.c

+/* real_timer.c
+
+   A demonstration of the use of (real-time) timers created using setitimer().
+
+   Usage: real_timer [secs [usecs [int-secs [int-usecs]]]]
+             Defaults: 2      0        0         0
+
+   The command-line arguments are the second and microsecond settings for
+   the timer's initial value and interval.
+
+   The version of this code shown in the first print of the book contained
+   an error in the main() function whereby 'maxSigs' was initialized before
+   'itv', even though the initialization of the former variable depends on the
+   initialization of the latter variable. See the erratum for page 983 to 984.
+*/
 #include <signal.h>
 #include <sys/time.h>
 #include <time.h>
 #include "tlpi_hdr.h"
 
 static volatile sig_atomic_t gotAlarm = 0;
                         /* Set nonzero on receipt of SIGALRM */
 
 /* Retrieve and display the real time, and (if 'includeTimer' is
    TRUE) the current value and interval for the ITIMER_REAL timer */
 
 static void
 displayTimes(const char *msg, Boolean includeTimer)
 {
     struct itimerval itv;
     static struct timeval start;
     struct timeval curr;
     static int callNum = 0;             /* Number of calls to this function */
 
     if (callNum == 0)                   /* Initialize elapsed time meter */
         if (gettimeofday(&start, NULL) == -1)
             errExit("gettimeofday");
 
     if (callNum % 20 == 0)              /* Print header every 20 lines */
         printf("       Elapsed   Value Interval\n");
 
     if (gettimeofday(&curr, NULL) == -1)
         errExit("gettimeofday");
     printf("%-7s %6.2f", msg, curr.tv_sec - start.tv_sec +
                         (curr.tv_usec - start.tv_usec) / 1000000.0);
 
     if (includeTimer) {
         if (getitimer(ITIMER_REAL, &itv) == -1)
             errExit("getitimer");
         printf("  %6.2f  %6.2f",
                 itv.it_value.tv_sec + itv.it_value.tv_usec / 1000000.0,
                 itv.it_interval.tv_sec + itv.it_interval.tv_usec / 1000000.0);
     }
 
     printf("\n");
     callNum++;
 }
 
 static void
 sigalrmHandler(int sig)
 {
     gotAlarm = 1;
 }
 
 int
 main(int argc, char *argv[])
 {
     struct itimerval itv;
     clock_t prevClock;
     int maxSigs;                /* Number of signals to catch before exiting */
     int sigCnt;                 /* Number of signals so far caught */
     struct sigaction sa;
 
     if (argc > 1 && strcmp(argv[1], "--help") == 0)
         usageErr("%s [secs [usecs [int-secs [int-usecs]]]]\n", argv[0]);
 
     sigemptyset(&sa.sa_mask);
     sa.sa_flags = 0;
     sa.sa_handler = sigalrmHandler;
     if (sigaction(SIGALRM, &sa, NULL) == -1)
         errExit("sigaction");
 
     /* Set timer from the command-line arguments */
 
     itv.it_value.tv_sec = (argc > 1) ? getLong(argv[1], 0, "secs") : 2;
     itv.it_value.tv_usec = (argc > 2) ? getLong(argv[2], 0, "usecs") : 0;
     itv.it_interval.tv_sec = (argc > 3) ? getLong(argv[3], 0, "int-secs") : 0;
     itv.it_interval.tv_usec = (argc > 4) ? getLong(argv[4], 0, "int-usecs") : 0;
 
     /* Exit after 3 signals, or on first signal if interval is 0 */
 
     maxSigs = (itv.it_interval.tv_sec == 0 &&
                 itv.it_interval.tv_usec == 0) ? 1 : 3;
 
     displayTimes("START:", FALSE);
     if (setitimer(ITIMER_REAL, &itv, NULL) == -1)
         errExit("setitimer");
 
     prevClock = clock();
     sigCnt = 0;
 
     for (;;) {
 
         /* Inner loop consumes at least 0.5 seconds CPU time */
 
         while (((clock() - prevClock) * 10 / CLOCKS_PER_SEC) < 5) {
             if (gotAlarm) {                     /* Did we get a signal? */
                 gotAlarm = 0;
                 displayTimes("ALARM:", TRUE);
 
                 sigCnt++;
                 if (sigCnt >= maxSigs) {
                     printf("That's all folks\n");
                     exit(EXIT_SUCCESS);
                 }
             }
         }
 
         prevClock = clock();
         displayTimes("Main: ", TRUE);
     }
 }