struct stackinfo {
    AV *                  si_stack;               /* stack for current runlevel */
    PERL_CONTEXT *         si_cxstack;          /* context stack for runlevel */
    struct stackinfo *         si_prev;
    struct stackinfo *         si_next;
    I32                                      si_cxix; /* current context index */
    I32                                      si_cxmax;            /* maximum allocated index */
    I32                                      si_cxsubix;         /* topmost sub/eval/format */
    I32                                      si_type; /* type of runlevel */
    I32                                      si_markoff;         /* offset where markstack begins for us.
                                                                                * currently used only with DEBUGGING,
                                                                                * but not #ifdef-ed for bincompat */
#if defined DEBUGGING && !defined DEBUGGING_RE_ONLY
/* high water mark: for checking if the stack was correctly extended /
* tested for extension by each pp function */
    SSize_t             si_stack_hwm;
#endif

};