E.6 Object Internals

GNU Emacs Lisp manipulates many different types of data. The actual data are stored in a heap and the only access that programs have to it is through pointers. Pointers are thirty-two bits wide in most implementations. Depending on the operating system and type of machine for which you compile Emacs, twenty-eight bits are used to address the object, and the remaining four bits are used for a GC mark bit and the tag that identifies the object's type.

Because Lisp objects are represented as tagged pointers, it is always possible to determine the Lisp data type of any object. The C data type Lisp_Object can hold any Lisp object of any data type. Ordinary variables have type Lisp_Object, which means they can hold any type of Lisp value; you can determine the actual data type only at run time. The same is true for function arguments; if you want a function to accept only a certain type of argument, you must check the type explicitly using a suitable predicate (see section 2.6 Type Predicates).

E.6.1 Buffer Internals		Components of a buffer structure.
E.6.2 Window Internals		Components of a window structure.
E.6.3 Process Internals		Components of a process structure.