#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>
#include <unistd.h> // lockf

int main (int argc, char **argv)
{
   int fd;
   
   if ( (fd = open(argv[1], O_RDWR)) <0)
   {
   	printf("opening file %s failed\n",argv[1]);
	exit(1);
   } //if
   
   printf("locking file %s\n",argv[1]);
   
   lseek(fd, 0L, 0); // rewined to start of file
   if (lockf(fd, F_LOCK, 0L))
   {
      printf("locking of file %s failed\n",argv[1]);
      exit(1);
   }
   else
      printf("file locked successfully\n");
   
   getchar();
   
   if (lockf(fd, F_ULOCK, 0L) == -1 ) 
   {
      printf("cannot unlock file %s\n",argv[1]);
      exit(1);
   }
   else
      printf("file %s successfully unlocked\n",argv[1]); 
   
   
   return (0);
} // main()