#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/file.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <errno.h>

void usage(char *progname)
{
   fprintf(stderr, "usage: %s [-s] [filename]\n", progname);
   exit(EXIT_FAILURE);
}

typedef enum { false, true, FALSE = 0, TRUE = 1 } bool;

int main(int argc, char *argv[])
{
   int  fd;
   char *filename = "shared.txt";
   bool done;     
   short lmode = F_WRLCK;
   struct stat st;
   char  line[128];
   int fsize;
   int offset;
   int length;
   struct flock fl;

   // get filename and locking options
   if ( argc == 2 ) {
      if ( strcmp("-s", argv[1]) == 0 ) {
         lmode = F_RDLCK;
      } else {
         filename = argv[1];
      }
   } else if ( argc == 3 ) {
      if ( strcmp("-s", argv[1]) == 0 ) {
         lmode = F_RDLCK;
         filename = argv[2];
      } else if ( strcmp("-s", argv[2]) == 0 ) {
         lmode = F_RDLCK;
         filename = argv[1];
      } else {
         usage(argv[0]);
      }
   } else if ( argc > 3 ) {
      usage(argv[0]);
   }

   // open file
   fd = open(filename, O_RDWR);

   if ( fd < 0 ) {
      fprintf(stderr, "Could not open file %s\n", filename);
      exit(EXIT_FAILURE);
   }

   // get stats
   fstat(fd, &st);
   fsize = (int) st.st_size;

   // ask user where to start the range locking
   done = false;
   while ( ! done ) {
      fprintf(stdout, "Enter start offset for range locking: ");
      fflush(stdout);
      fgets(line, sizeof(line), stdin);
      offset = atoi(line);
      if ( offset < 0 ) {
         fprintf(stdout, "Enter a valid integer larger than or equal to 0.\n");
         fflush(stdout);
      } else {
         done = TRUE;
      }
   }

   done = false;
   while ( ! done ) {
      fprintf(stdout, "Enter start length: ");
      fflush(stdout);
      fgets(line, sizeof(line), stdin);
      length = atoi(line);
      if ( length < 1 ) {
         fprintf(stdout, "Enter a valid integer larger than 0.\n");
         fflush(stdout);
      } else {
         done = true;
      }
   }

   fl.l_type = lmode;
   fl.l_start = offset;
   fl.l_whence = SEEK_SET;
   fl.l_len = length;

   errno = 0;
   fprintf(stdout, "trying to lock %s (%s)\n", filename, lmode == F_WRLCK ? "write" : "read");
   fcntl(fd, F_SETLKW, &fl);
   fprintf(stdout, "successfully locked %s (%d, %d)\n", filename, offset, offset+length);

   while ( 1 ) {
      sleep(1);
   }
}