请问下边这段代码中DEFUN宏这样定义的作用？

请问下边这段代码中DEFUN宏这样定义的作用？
#define DEFUN(funcname, cmdname, cmdstr, helpstr) \
  int funcname (struct cmd_element *, struct vty *, int, char **); \
  struct cmd_element cmdname = \
  { \
    cmdstr, \
    funcname, \
    helpstr \
  }; \
  int funcname \
  (struct cmd_element *self, struct vty *vty, int argc, char **argv)
谢谢！