emacs 源码分析（三）

<2023-11-06 Mon>

emacs源码分析（三）

这次来分析TAG_PTR宏定义（将指针变成Lisp_Object），首先看LISP_INITIALLY在gdb中展开的样子：

(gdb) macro exp LISP_INITIALLY(w)
expands to: (w)

从上面输出可以看出LISP_INITIALLY的相关定义走的是这个分支：

/* A Lisp_Object is a tagged pointer or integer.  Ordinarily it is a
   Lisp_Word.  However, if CHECK_LISP_OBJECT_TYPE, it is a wrapper
   around Lisp_Word, to help catch thinkos like 'Lisp_Object x = 0;'.

   LISP_INITIALLY (W) initializes a Lisp object with a tagged value
   that is a Lisp_Word W.  It can be used in a static initializer.  */

typedef Lisp_Word Lisp_Object;
# define LISP_INITIALLY(w) (w)
enum CHECK_LISP_OBJECT_TYPE {
    CHECK_LISP_OBJECT_TYPE = false };

再看关于LISP_WORD_TAG(tag)的gdb相关输出：

(gdb) p pI
$23 = "l"

(gdb) p (int)EMACS_INT_WIDTH
$24 = 64

(gdb) p (int)VALBITS
$25 = 61

(gdb) p EMACS_INT_MAX
$26 = 9223372036854775807

(gdb) p VAL_MAX
$27 = 2305843009213693951

(gdb) macro exp LISP_WORD_TAG(tag)
expands to: ((Lisp_Word_tag) (tag) << (((0x7fffffffffffffffL >> (3 - 1)) / 2 < (0x7fffffffffffffffL)) ? 0 : VALBITS))

对应代码：

enum Lisp_Bits
  {