#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <signal.h>
#include <limits.h>

void catch(int c)        {       }

int main(int argc, char *argv[])
{
        unsigned long start=time(NULL), len;

        if(argc != 2)
        {
                fprintf(stderr, "usage: %s 60\n", argv[0]);
                return(1);
        }

        len=atol(argv[1]);

        if((len == LONG_MIN) || (len == LONG_MAX) || (len < 0))
        {
                fprintf(stderr, "Bad input value, must be positive integer\n");
                return(1);
        }

        signal(SIGINT, catch);

        while( (time(NULL) < (start+len)))
        if(sleep((start+len) - time(NULL)))
        {
                int off=INT_MAX;
                signal(SIGINT, SIG_DFL);
                while(off == INT_MAX)
                {
                        printf("\r%d seconds remain, offset:",
                                (start+len)-time(NULL));

                        if(scanf("%d", &off) != 1)
                        {
                                fflush(stdin);
                                off=-1;
                        }
                }

                printf("%d%+d seconds, %d remain\n", len, off,
                        (start+len+off)-time(NULL));
                len += off;

                signal(SIGINT, catch);
        }

        return(0);
}