Semaphore Files

Let's write a program that will either update or print out a file's contents:

         $FILE = '/data/hitcounter';

         print "Content-type: text/html\n\n";

         if (something) {
           open F, $FILE or die ...;
           print while <F>;             # Display file contents
         } else {
           open F, "> $FILE" or die ...;
           my $data = qx{some command}; # Calculate new contents
           print F $data;               # Overwrite file
         }

         close F;

Oops, it has a race condition
Better slap a little file locking on it