\datethis
@* Extended Hello World program.
This is a medium-short demonstration of \.{CWEB}.
@c
@<include files@>@;
@<global variables@>@;
@<subroutines@>@;
main()
{
 @<local variables@>;
 @<print a greeting@>;
 @<print the date and time@>;
 @<print an interesting date and time from the past@>;
}
@ First we brush up our Shakespeare by quoting
from The Merchant of Venice (Act~I, Scene~I,
Line~77). This makes the program literate.
@<print a greeting@>=
printf("Greetings ... to\n"); /* Hello, */
printf(" `the stage, "); /* world */
printf("where every man must play a part'.\n");
@ Since we're using the &#124;printf&#124; routine, we had
better include the standard input/output header
file.
@<include files@>=
#include <stdio.h>
@ Now we brush up our knowledge of \Cee\ runtime
routines, by recalling that the function &#124;time(0)&#124;
returns the current time in seconds. 
@<print the date and time@>=
current_time=time(0);
printf("Today is ");
print_date(current_time);
printf(",\n and the time is ");
print_time(current_time);
printf(".\n");
@ The value returned by &#124;time(0)&#124; is, more
precisely, a value of type &#124;time_t&#124;, representing
seconds elapsed since 00:00:00 Greenwich Mean Time
on January 1, 1970.
At present, &#124;time_t&#124; is equivalent to &#124;long&#124;.
But a 32-bit integer will become too small to
hold the result of &#124;time(0)&#124; on
January 18, 2038, at 19:14:08, Pacific
Standard Time. We will try to write a program
that will still work on January 19, 2038
(although it will have to be recompiled), by
declaring &#124;current_time&#124; to have type &#124;time_t&#124;
instead of type &#124;long&#124;.
@<local variables@>=
time_t current_time; /* seconds after the epoch */
@ Ten million minutes is 600,000,000 seconds.
@<print an interesting date and time from the past@>=
printf("Ten million minutes ago it was\n ");
print_date(current_time-600000000);
printf(", at ");
print_time(current_time-600000000);
printf(".\n");
@* Date and time. The remaining task is to
write subroutines to print dates and times.
\UNIX/'s &#124;localtime&#124; function does most of the
work for us, but we need to include another
system header file before we can use it.
@<include files@>=
#include <time.h>
@ First, let's work on the date. We want to
produce an American-style date such as
``Monday, January 18, 2038''.
The result of &#124;localtime&#124; is a pointer to a &#124;tm&#124;
structure, which has 11 fields, as explained in
the man page for &#124;ctime(3V)&#124;. For example,
one of the fields is &#124;tm_year&#124;, the year minus
1900.
Note that the parameter to &#124;localtime&#124;
must be a pointer to the time in seconds,
not the time itself.
@<subroutines@>=
print_date(clk)
 time_t clk; /* seconds since the epoch */
{
 struct tm *t; /* data deduced from &#124;clk&#124; */
 t=localtime(&clk);
 printf("%s, %s %d, %d",@&#124;
 day_name[t->tm_wday],
 month_name[t->tm_mon],@&#124;
 t->tm_mday,
 t->tm_year+1900);
}
@ @<global variables@>=
char *day_name[]={"Sunday","Monday","Tuesday",
 "Wednesday","Thursday","Friday","Saturday"};
char *month_name[]={"January","February","March",
 "April","May","June","July","August","September",
 "October","November","December"};
@ The subroutine for time is similar to the
routine for date. We could make use of the fact
that &#124;print_time&#124; is always called after
&#124;print_date&#124;, with the same parameter; that
would save a call on &#124;localtime&#124;. But let's
make the subroutine more general, so that we
can use it later in another program.
@<subroutines@>=
print_time(clk)
 time_t clk; /* seconds since the epoch */
{
 struct tm *t; /* data deduced from &#124;clk&#124; */
 t=localtime(&clk);
 @<print the hours and minutes@>;
 @<print ``am'' or ``pm'' as appropriate@>;
 printf(", %s",t->tm_zone);
}
@ The tricky thing here is to make 0 hours come
out as 12, yet 13 is changed to~1. If the
number of minutes is less than 10, we want a
leading zero to be printed.
@<print the hours and minutes@>=
printf("%d:%02d",
 ((t->tm_hour+11)%12)+1,
 t->tm_min);
@ Instead of trying to figure out whether noon
and midnight are ``am'' or ``pm,'' we treat them
as special cases.
@<print ``am''...@>=
if (t->tm_min==0 && (t->tm_hour % 12)==0)
 printf("%s",t->tm_hour==0? "midnight": "noon");
else printf("%s",t->tm_hour<12? "am": "pm");
@* Index. \.{CWEB} prepares an index that shows
where each identifier is used and/or declared.
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