Dan Lenski Dan Lenski - 6 months ago 24
Linux Question

Real-time aware sleep() call?

I've written a utility to talk to TomTom GPS watches over Bluetooth, and am trying to make it run nicely as a run-and-forget background daemon.

The program periodically communicates with the GPS device and then

s for a while until it's needed again.

I've noticed that
sleep()
interacts strangely with system suspend: when I enter system suspend (laptop running Linux 3.16.0 kernel) and then wake the computer back up,
sleep
doesn't appear to notice the suspend time. For example, take the following
sleep.c
:

#include <time.h>
#include <stdio.h>
#include <unistd.h>

int main(int argc, char**argv)
{
time_t t=time(NULL);
printf("sleep at: %s", ctime(&t));

sleep(atoi(argv[1]));

t=time(NULL);
printf("wake at: %s", ctime(&t));
}
dlenski@dlenski-ultra:~$


Compile, run, and sleep halfway through;

$ gcc sleep.c -o sleep
$ ./sleep 30
sleep at: Fri Aug 21 21:05:36 2015
<suspend computer for 17 seconds>
wake at: Fri Aug 21 21:06:23 2015


Is there a correct way to suspend program execution in a way that's clock-time-aware rather than system-uptime-aware?

(I've also tried
usleep
,
nanosleep
, and
alarm
and found that they behave similarly.)

UPDATE:
setitimer
, as suggested by @HuStmpHrrr, sounded quite promising... but appears to have the same problem.

This version pauses for more than the requested 30s when I suspend in the middle of it...

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

void sighandler() {}

int main(int argc, char**argv)
{
time_t t=time(NULL);
printf("sleep at: %s", ctime(&t));

signal(SIGALRM, sighandler);
struct itimerval timer = {.it_interval={0,0},
.it_value={atoi(argv[1]),0}};
setitimer(ITIMER_REAL, &timer, NULL);
pause();

t=time(NULL);
printf("wake at: %s", ctime(&t));
}

Answer

A few solutions and potential solutions:

Sleep in a loop and check the real elapsed time

There is almost certainly a better way to do it—or there should be!—but here's an acceptable solution for my application for the time being:

  • When needing to sleep for a long interval (>30s), I do sleep(30) repeatedly, checking that the real elapsed time (time(NULL)-start_time) is not longer than the total desired pause.

  • This way, if the system is suspended for, say, 3 hours, while the desired program pause is 1 hour, the extra delay caused by the suspend will not exceed 30 seconds.

Code to do it:

void nullhandler(int signal) {}

int isleep(int seconds, int verbose)
{
    if (verbose) {
        fprintf(stderr, "Sleeping for %d seconds...", seconds);
        fflush(stderr);
    }
    signal(SIGALRM, nullhandler);

    // weird workaround for non-realtime-awareness of sleep:
    // http://stackoverflow.com/questions/32152276/real-time-aware-sleep-call
    int res=0, elapsed=0;
    for (time_t t=time(NULL); (elapsed<seconds) && (res<=0); elapsed=time(NULL)-t)
        res = sleep((seconds-elapsed > 30) ? 30 : seconds-elapsed);

    signal(SIGALRM, SIG_IGN);
    if (res && verbose)
        fprintf(stderr, "%s\n\n", res ? " woken by signal!" : "");
    return (res>0);
}

Wakeup callback

@Speed8ump suggested registering a callback for system-wakeup notifications. This thread over at AskUbuntu shows how to do it. A good solution, but I'd like to avoid tying my relatively-low-level code too strongly to a particular desktop environment for now.

timer_settime

This is the most elegant and correct solution, in my opinion, and was suggested by @NominalAnimal. It uses time_settime() and friends from librt.

Note that in order to make this work correctly with system suspend, you must use CLOCK_REALTIME with TIMER_ABSTIME, per the timer_settime documentation:

If the value of the CLOCK_REALTIME clock is adjusted while an absolute timer based on that clock is armed, then the expiration of the timer will be appropriately adjusted. Adjustments to the CLOCK_REALTIME clock have no effect on relative timers based on that clock.

Here's a demo (link with librt, e.g. gcc -lrt -o sleep sleep.c):

#include <time.h>
#include <stdio.h>
#include <unistd.h>
#include <signal.h>
#include <sys/time.h>
#include <string.h>
#include <stdlib.h>

void sighandler(int signal) {}

void isleep(int seconds)
{
    timer_t timerid;
    struct sigevent sev = { .sigev_notify=SIGEV_SIGNAL,
                            .sigev_signo=SIGALRM,
                            .sigev_value=(union sigval){ .sival_ptr = &timerid } };
    signal(SIGALRM, sighandler);
    timer_create(CLOCK_REALTIME, &sev, &timerid);

    struct itimerspec its = {.it_interval={0,0}};
    clock_gettime(CLOCK_REALTIME, &its.it_value);
    its.it_value.tv_sec += seconds;
    timer_settime(timerid, TIMER_ABSTIME, &its, NULL);
    pause();
    signal(SIGALRM, SIG_IGN);
}

int main(int argc, char**argv)
{
    time_t t=time(NULL);
    printf("sleep at: %s", ctime(&t));

    isleep(atoi(argv[1]));

    t=time(NULL);
    printf("wake at: %s", ctime(&t));
}