/* ----------- incr.c -------------*/

# include <stdio.h>
# include <fcntl.h>
# include <unistd.h>
# include <sys/wait.h>
# include <stdlib.h>

#define SEQFILE "seqno"      /*filename*/
#define MAXBUF   100
#define MAXDELAY 5 
#define MAXREPEAT 10


/* Locking routines for System V */

void my_lock(int fd) 
{
	/* rewind before lockf: */
	lseek(fd, 0L, 0);
	if (lockf(fd, F_LOCK, 0L) == -1 )
		/* 0L -> lock entire file */
		perror("can't lock");
	return;	
} //my_lock()

void my_unlock(int fd) {
	lseek(fd, 0L, 0);
	if ( lockf(fd, F_ULOCK, 0L) == -1 )
		perror("can't unlock");
	return;
}

int incr(int delay) {
   int fd, n, seqno;
   char buffer[MAXBUF + 1];

   if ( (fd = open(SEQFILE,O_RDWR) ) < 0 ) {
      perror("SEQFILE");
      return -1;
   }
   
   my_lock(fd);

   /* rewind before read: */
   lseek(fd, 0L, 0); 
   
   if ( (n = read(fd, buffer, MAXBUF) ) <= 0) {
      perror("read");
      return -1;
   }
   buffer[n] = '\0';
	

   if ( (n = sscanf(buffer, "%d\n", &seqno)) != 1) {
         fprintf(stderr, "sscanf error\n");
         return -1;
   }
   seqno++;
     
   sprintf(buffer, "%03d\n", seqno);
   n = strlen(buffer);
   
   /* wait delay sec */
   sleep(delay % MAXDELAY);
   
   /* rewind before write: */
   lseek(fd,0L,0);
   if (write(fd, buffer, n) != n) {
      perror("write");
      return -1;
   }
   my_unlock(fd);
   close(fd);
   return 0;
}


char increment(int start, int end, int maxduration)
{
   int delay,
       current=start;
   char totalCalculationTime=0;

   srandom(time(0));
   delay = random();

   while ( (current <= end) && incr(delay) == 0 ) 
   {
      sleep(delay = random() % MAXDELAY);
      totalCalculationTime+=delay;
      fprintf(stderr, "long running increment within process %d took %d seconds\n", getpid(), delay);
      current++;
   } //while
   return (totalCalculationTime);
}// increment
   
int main() {
   int pid1, pid2, i, status, delay;
   switch(pid1 = fork()){
     case -1: 
             perror("fork");
             exit(1);
     case  0:
	     printf("process %d will wait one moment...\n",getpid() );
	     srandom(time(0));
	     sleep(1+ random() % MAXDELAY );
	     
	     //fprintf(stderr, "waiting %d seconds before starting action within process %d\n", delay=(1+ random() % MAXDELAY), getpid() );
	     
             fprintf(stderr, "process %d needed %d seconds for calculation\n",getpid(), increment(1,MAXREPEAT,MAXDELAY) ); 
	     exit(0);
     default:
          break;
    }
   fprintf(stderr,"forking again...\n");
   switch(pid2 = fork()){
     case -1: 
             perror("fork");
             exit(1);
     case  0:
	     printf("second process (%d) is running!\n",getpid());
	
	     
	     fprintf(stderr, "process %d needed %d seconds for calculation\n",getpid(), increment(1,MAXREPEAT,MAXDELAY) ); 
	      
             exit(0);
     default:
         break;
    }
    printf("Child %d terminated\n", wait(&status));
    printf("Child %d terminated\n", wait(&status));
    exit(0);
}