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

#define SEQFILE "seqno"      /*filename*/
#define MAXBUF   100
#define MAXDELAY 3 
#define MAXREPEAT 10

#define bool short
#define TRUE 1
#define FALSE 0

//#define LINUX

#ifndef LINUX
#define LOCK_SH 1
#define LOCK_EX 2
#define LOCK_NB 4
#define LOCK_UN 8
#endif


//#define debug


/* Locking routines for System V */

bool tryLock(int fd) 
{
   if (flock(fd, (LOCK_NB|LOCK_EX)) == -1)
   {
      #ifdef debug
		printf("process %d cannot lock\n",getpid());
      #endif
		return FALSE;
   }
   else
      return TRUE;	
} //tryLock()

void my_unlock(int fd)
{
   if(flock(fd, (LOCK_NB|LOCK_UN)) == -1)
      printf("process %d cannot unlock file\n",getpid() );
}//my_unlock()
      
void increment2(unsigned int ignoredLockingFailures, unsigned int WaitSecondsBeforeLockingAgain, int delay)
{
   int  fd,
	 	  n, 
        seqno,
		  failedLocks=0;
   char buffer[MAXBUF + 1];

   if ( (fd = open(SEQFILE,O_RDWR) ) < 0 ) 
   {
      perror("SEQFILE");
      exit(1);
   } //if
	//file sucessfully opened

	#ifdef DEBUG
	printf("file sucessully openend\n");
	#endif
	
	while ( (tryLock(fd)== FALSE) && (failedLocks<ignoredLockingFailures) )
	{
		//try again after a delay
		failedLocks++;
		#ifdef debug
		printf("%d unsuccessfull locks applied so far\n",failedLocks);
		#endif
		sleep(WaitSecondsBeforeLockingAgain);
	} //while
	if (failedLocks == ignoredLockingFailures)
	{
		#ifdef debug
		printf("process %d is exiting after %d unsuccessfull locking approaches\n",getpid(),failedLocks);
		#endif
		exit(1);	
	}
	//file sucessfully locked

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

   if ( (n = sscanf(buffer, "%d\n", &seqno)) != 1) 
	{
   	fprintf(stderr, "sscanf error\n");
      exit(1);
   }//endif
   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");
      exit(1);
   }
   
   my_unlock(fd);
   
   close(fd);
}//increment2()   
   
int main(int argc, char **argv) 
{
   register unsigned int i;
   unsigned int startTime;
   
   if (argc != 4)
   {
      printf("SYNTAX: %s #ignoredLockingFailures waitSecondsBeforeLockingAgain incrementRepreat\n",argv[0]); 
      exit(1);
   }//if
   
	srandom(time(0));
   startTime = time(0);
    
   #ifdef debug
	printf("incrementing by %d, ignoring %d locking failures and waiting %d seconds before trying to lock again\n",atoi(argv[3]),atoi(argv[1]), atoi(argv[2]));
	#endif
	
	for (i=0; i<atoi(argv[3]); i++)
   {
   	increment2(atoi(argv[1]), atoi(argv[2]), random() );
   }
   printf("total calculation time of process %d: %d seconds\n",getpid(),time(0)-startTime);

   return(0);
} //main()