An Alarm Clock Program

The following program is a simple "alarm clock" program. You give it a time of day, and an optional message. At the given time, it prints the message on the standard output. In addition, you can give it the number of times to repeat the message, and also a delay between repetitions.

This program uses the gettimeofday function from section Managing the Time of Day.

All the work is done in the BEGIN rule. The first part is argument checking and setting of defaults; the delay, the count, and the message to print. If the user supplied a message, but it does not contain the ASCII BEL character (known as the "alert" character, `\a'), then it is added to the message. (On many systems, printing the ASCII BEL generates some sort of audible alert. Thus, when the alarm goes off, the system calls attention to itself, in case the user is not looking at their computer or terminal.)

# alarm --- set an alarm
# Arnold Robbins, arnold@gnu.ai.mit.edu, Public Domain
# May 1993

# usage: alarm time [ "message" [ count [ delay ] ] ]

BEGIN    \
{
    # Initial argument sanity checking
    usage1 = "usage: alarm time ['message' [count [delay]]]"
    usage2 = sprintf("\t(%s) time ::= hh:mm", ARGV[1])

    if (ARGC < 2) {
        print usage > "/dev/stderr"
        exit 1
    } else if (ARGC == 5) {
        delay = ARGV[4] + 0
        count = ARGV[3] + 0
        message = ARGV[2]
    } else if (ARGC == 4) {
        count = ARGV[3] + 0
        message = ARGV[2]
    } else if (ARGC == 3) {
        message = ARGV[2]
    } else if (ARGV[1] !~ /[0-9]?[0-9]:[0-9][0-9]/) {
        print usage1 > "/dev/stderr"
        print usage2 > "/dev/stderr"
        exit 1
    }

    # set defaults for once we reach the desired time
    if (delay == 0)
        delay = 180    # 3 minutes
    if (count == 0)
        count = 5
    if (message == "")
        message = sprintf("\aIt is now %s!\a", ARGV[1])
    else if (index(message, "\a") == 0)
        message = "\a" message "\a"

The next section of code turns the alarm time into hours and minutes, and converts it if necessary to a 24-hour clock. Then it turns that time into a count of the seconds since midnight. Next it turns the current time into a count of seconds since midnight. The difference between the two is how long to wait before setting off the alarm.

    # split up dest time
    split(ARGV[1], atime, ":")
    hour = atime[1] + 0    # force numeric
    minute = atime[2] + 0  # force numeric

    # get current broken down time
    gettimeofday(now)

    # if time given is 12-hour hours and it's after that
    # hour, e.g., `alarm 5:30' at 9 a.m. means 5:30 p.m.,
    # then add 12 to real hour
    if (hour < 12 && now["hour"] > hour)
        hour += 12

    # set target time in seconds since midnight
    target = (hour * 60 * 60) + (minute * 60)

    # get current time in seconds since midnight
    current = (now["hour"] * 60 * 60) + \
               (now["minute"] * 60) + now["second"]

    # how long to sleep for
    naptime = target - current
    if (naptime <= 0) {
        print "time is in the past!" > "/dev/stderr"
        exit 1
    }

Finally, the program uses the system function (see section Built-in Functions for Input/Output) to call the sleep utility. The sleep utility simply pauses for the given number of seconds. If the exit status is not zero, the program assumes that sleep was interrupted, and exits. If sleep exited with an OK status (zero), then the program prints the message in a loop, again using sleep to delay for however many seconds are necessary.

    # zzzzzz..... go away if interrupted
    if (system(sprintf("sleep %d", naptime)) != 0)
        exit 1

    # time to notify!
    command = sprintf("sleep %d", delay)
    for (i = 1; i <= count; i++) {
        print message
        # if sleep command interrupted, go away
        if (system(command) != 0)
            break
    }

    exit 0
}

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