sys_handler
struct sys_handler *sysh;
sysh->sys_handler = func;
struct sys_handler *sysh;
if (sysh->sys_handler(esr, frame))
LIST_ENTRY(sys_handler) sys_link;
undef_sys_handler_t sys_handler;
LIST_HEAD(, sys_handler) sys_handlers = LIST_HEAD_INITIALIZER(sys_handler);