#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>

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

int main(int argc, char *argv[])
{
   int fd;
   char *filename = "shared.txt";
   int  lockmode = LOCK_EX;

   if ( argc == 2 ) {
      if ( strcmp("-s", argv[1]) == 0 ) {
         lockmode = LOCK_SH;
      } else {
         filename = argv[1];
      }
   } else if ( argc == 3 ) {
      if ( strcmp("-s", argv[1]) == 0 ) {
         lockmode = LOCK_SH;
         filename = argv[2];
      } else if ( strcmp("-s", argv[2]) == 0 ) {
         lockmode = LOCK_SH;
         filename = argv[1];
      } else {
         usage(argv[0]);
      }
   } else if ( argc > 3 ) {
      usage(argv[0]);
   }

   fd = open(filename, O_RDWR);

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

   fprintf(stdout, "trying to lock %s (%s)\n", filename, lockmode == LOCK_EX ? "exclusive" : "shared");
   flock(fd, lockmode);
   fprintf(stdout, "successfully locked %s\n", filename);

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