sleep.c

/**
   @file      sleep.c
   @author    Mitch Richling <http://www.mitchr.me/>
   @Copyright Copyright 1996 by Mitch Richling.  All rights reserved.
   @brief     How to use UNIX sleep()@EOL
   @Keywords  UNIX signal sleep
   @Std       ISOC POSIX

              This C program is intended to illustrate the correct way
              to make a program sleep for at least n seconds.  This is
              more complex that simply calling the sleep() system
              call.  The sleep system call 1) stores the current
              signal handler for alarm, 2) sets an alarm for n
              seconds, 3) waits until a signal is received, 4) sets
              the signal handler back to the correct thing.  Thus the
              sleep system call can be prematurely interrupted by a
              signal.  To get it to wait the correct amount of time,
              we need to add some code.  This program illustrates how
              to do do this.  This program sets up a signal handler
              for SIGUSR1 to help you understand the process.

   @Tested    
              - Solaris 2.8
              - MacOS X.2
              - Linux (RH 7.3)
*/

#include <stdlib.h>             /* Standard Lib    ISOC  */
#include <unistd.h>             /* UNIX std stf    POSIX */
#include <stdio.h>              /* I/O lib         ISOC  */
#include <time.h>               /* time            ISOC  */
#include <signal.h>             /* UNIX signals    POSIX */

int main(int argc, char *argv[]);

int main(int argc, char *argv[]) {
  int secToSleep = 600;
  long startTime;
  long endTime;

  /* First set the SIGUSR1 signal to be ignored so that the user
     can send this program SIGUSR1 signals to test it. */
  if(SIG_ERR == signal(SIGUSR1, SIG_IGN)) {
    printf("ERROR: Couldn't set signal handler for SIGUSR1\n");
    return 1;
  } /* end if */
  printf("My PID(try SIGUSR1): %ld\n", (long)getpid());

  startTime = time(NULL);
  endTime = startTime + secToSleep;

  /* Loop until we have waited secToSleep seconds. */
  printf("Current Time=%ld\n", (long)time(NULL));
  while(time(NULL) <= endTime) {
    sleep(endTime - time(NULL));
    printf("Current Time=%ld\n", (long)time(NULL));
  } /* end while */
  printf("Time gone by: %ld\n", time(NULL) - startTime);
  
  return 0;

} /* end func main */