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:    // https://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 timer_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 absolutetimer based on that clock is armed, then the expiration of the timerwill be appropriately adjusted. Adjustments to the CLOCK_REALTIMEclock 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));}