The initial code contains a function waitfg:

    1   /* 
    2    * waitfg - Block until process pid is no longer the foreground process
    3    * If pid is no longer in the job list, just return.
    4    * Otherwise, wait while the state of pid is FG. 
    5    * Wait 1 sec and check the state again. Keep waiting for 1 sec and 
    6    * checking the state until the state is not FG. 
    7    */
    8   void waitfg(pid_t pid)
    9   {
   10       return;
   11   }

It is a bad idea to have this function and the SIGCHLD handler both call waitpid.

So this function should not!