time/calendar_time.c

+/* calendar_time.c
+
+   Demonstrate the use of functions for working with calendar time.
+
+   This program retrieves the current time and displays it in various forms.
+*/
 #include <locale.h>
 #include <time.h>
 #include <sys/time.h>
 #include "tlpi_hdr.h"
 
 #define SECONDS_IN_TROPICAL_YEAR (365.24219 * 24 * 60 * 60)
 
 int
 main(int argc, char *argv[])
 {
     time_t t;
     struct tm *gmp, *locp;
     struct tm gm, loc;
     struct timeval tv;
 
+    /* Retrieve time, convert and display it in various forms */
+
     t = time(NULL);
     printf("Seconds since the Epoch (1 Jan 1970): %ld", (long) t);
     printf(" (about %6.3f years)\n", t / SECONDS_IN_TROPICAL_YEAR);
 
     if (gettimeofday(&tv, NULL) == -1)
         errExit("gettimeofday");
     printf("  gettimeofday() returned %ld secs, %ld microsecs\n",
             (long) tv.tv_sec, (long) tv.tv_usec);
 
     gmp = gmtime(&t);
     if (gmp == NULL)
         errExit("gmtime");
 
     gm = *gmp;          /* Save local copy, since *gmp may be modified
                            by asctime() or gmtime() */
     printf("Broken down by gmtime():\n");
     printf("  year=%d mon=%d mday=%d hour=%d min=%d sec=%d ", gm.tm_year,
             gm.tm_mon, gm.tm_mday, gm.tm_hour, gm.tm_min, gm.tm_sec);
     printf("wday=%d yday=%d isdst=%d\n", gm.tm_wday, gm.tm_yday, gm.tm_isdst);
 
+    /* The TZ environment variable will affect localtime().
+       Try, for example:
+
+                TZ=Pacific/Auckland calendar_time
+    */
+
     locp = localtime(&t);
     if (locp == NULL)
         errExit("localtime");
 
     loc = *locp;        /* Save local copy */
 
     printf("Broken down by localtime():\n");
     printf("  year=%d mon=%d mday=%d hour=%d min=%d sec=%d ",
             loc.tm_year, loc.tm_mon, loc.tm_mday,
             loc.tm_hour, loc.tm_min, loc.tm_sec);
     printf("wday=%d yday=%d isdst=%d\n\n",
             loc.tm_wday, loc.tm_yday, loc.tm_isdst);
 
     printf("asctime() formats the gmtime() value as: %s", asctime(&gm));
     printf("ctime() formats the time() value as:     %s", ctime(&t));
 
     printf("mktime() of gmtime() value:    %ld secs\n", (long) mktime(&gm));
     printf("mktime() of localtime() value: %ld secs\n", (long) mktime(&loc));
 
     exit(EXIT_SUCCESS);
 }