writing a clock app

peets · 2008-11-23 18:46:48

I'm finally getting around to setting me up a dzen. I want to display the time in minutes (hh:mm). I'm not sure how best to go about this.

Ideally, I'd ask the kernel to tell me whenever the minute changes on the system clock. Reading time(7) and the man pages in its "SEE ALSO" section has led me to believe that there's no way to do this. Did I miss something?

Otherwise, I guess I have to poll for the time periodically using ctime(3) or some such function. Boo hoo.

thayer · 2008-11-23 18:54:49

I might be misunderstanding your dilemma, but what about this?

$ date +%H:%M

Procyon · 2008-11-23 18:59:26

and wrap it around while true; do date....; sleep 5; done

peets · 2008-11-23 20:05:47

Yeah. I guess I'm just a perfectionist: I don't want to call "date" every second when I only need it once per minute. But hey, it's not that bad. Thanks.

moljac024 · 2008-11-23 20:23:40

I don't see what's wrong with conky....

string · 2008-11-23 20:24:51

peets wrote:

Yeah. I guess I'm just a perfectionist: I don't want to call "date" every second when I only need it once per minute. But hey, it's not that bad. Thanks.

"sleep 60" then?

haxit · 2008-11-23 20:26:09

Improvement of script:

while true; do date && sleep 1 && clear; done

Xyne · 2008-11-23 20:29:04

#!/bin/bash
while [ 1 ]
do
    echo -e -n "\r$(date +"%H:%M")"
    SEC=$(date +"%S")
    sleep $((60-$SEC))
done

This will display the hour and minute once every full minute then sleep the remaining seconds until the next minute. It resynchronizes itself every loop too so there's no need to worry about its interval lapsing.

Last edited by Xyne (2008-11-23 20:48:27)

peets · 2008-11-24 06:36:38

Xyne, that's pretty smart.
In the meantime, I went ahead and wrote this:

#include <sys/time.h>
#include <time.h>
#include <stdio.h>
#include <signal.h>

void print_time(int signum)
{
    char buf[21];
    struct timeval now;
    gettimeofday(&now, NULL);
    strftime(buf, sizeof buf, "%a %Y-%m-%d %H:%M", localtime(&now.tv_sec));
    printf("%s\n", buf);
}

int main()
{
    print_time(0);

    /* set up SIGALRM-handling callback */
    struct sigaction alarm;
    alarm.sa_handler = print_time;
    sigemptyset(&alarm.sa_mask);
    alarm.sa_flags = 0;
    sigaction(SIGALRM, &alarm, NULL);

    /* set up timer. Sends a SIGALRM when countdown reaches 0,
       then starts counting down again from 60 seconds. */
    struct timeval interval = {60, 0};
    struct timeval timeleft;
    struct timeval now;
    gettimeofday(&now, NULL);
    struct tm *now_tm = localtime(&now.tv_sec);
    if(now.tv_usec == 0) {
        timeleft.tv_sec = 60 - now_tm->tm_sec;
    } else {
        timeleft.tv_sec = 59 - now_tm->tm_sec;
        timeleft.tv_usec = 1000000 - now.tv_usec;
    }
    struct itimerval timer = {interval, timeleft};
    setitimer(ITIMER_REAL, &timer, NULL);

    /* do nothing until a signal is received, then do nothing again */
    while(pause()) {
    }

    /* this should never be reached */
    return 0;
}

I learned about signals (I need an excuse for my hours-long solution vs your minutes-long one). It gives the time about 15-25 microseconds late on my system --I guess that's the cool feature.

Arch Linux

#1 2008-11-23 18:46:48

writing a clock app

#2 2008-11-23 18:54:49

Re: writing a clock app

#3 2008-11-23 18:59:26

Re: writing a clock app

#4 2008-11-23 20:05:47

Re: writing a clock app

#5 2008-11-23 20:23:40

Re: writing a clock app

#6 2008-11-23 20:24:51

Re: writing a clock app

#7 2008-11-23 20:26:09

Re: writing a clock app

#8 2008-11-23 20:29:04

Re: writing a clock app

#9 2008-11-24 06:36:38

Re: writing a clock app

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