HOMEWORK 4 (COMMENTS AND COMMON MISTAKES)

o Many students gave no explanation for whether their output was correct or
  not.

o "$!" is the PID of the latest backgrounded process; i.e., the value
  returned from Fork() to the parent when evaluating a command to be
  run in the background.

o There is no requirement that you have to use a function map to implement 
  recognition of builtin commands.  Some approaches are:

  Approach 1 (Used in HW 4 solution):

    - Bool is_builtin(char *cmnd):  Returns YES if cmnd is builtin
    		Uses array of builtin command names "char *builtin[]".
    - void eval_builtin(int nwords, char *xword[]):  Evaluates builtin commands

    Criticism of Approach:

    - is_builtin() and eval_builtin() are NOT integrated:
    
    	+ is_builtin() uses builtin[] array but eval_builtin() doesn't

  Approach 2 (Improvement to Approach 1):

    - Might be better to have a lookup function that mapped each builtin
      commands to a unique integer which could then be used by
      eval_builtin() in a switch statement.

    - Something like:

	enum	BuiltinOp	{  ECHO,   QUIT,   WAIT,   SET };
	char    *builtin[]  =	{ "echo", "quit", "wait", "set", 0};

	static inline int
	get_builtin(char *cmnd) {
	  for (int i=0; builtin[i] != NULL; i++) {
	    if (strcmp(builtin[i], cmnd) == 0)  return i;
	  }
	  return -1;
	}

	void
	eval_builtin(int op, int nwords, char *xword[]) {
	  switch (op) {
	    case ECHO:	...
	    case QUIT:	...
	    case WAIT:	...
	    case SET:	...
	    default:	... shouldn't happen ...
	  }
	}

  Approach 3 (Function Map):

    - See test28f.C.

  	typedef void (*func_t)(char *[]);
	map<const char *, func_t, ltstr> eval;          // functions
	...
	(*(eval[xword[0]]))(xword);			// call eval builtin