is the string passed by expat handler available ...

hello everyone, who can tell me whether a string passed by an handler available after
the handler return. for code like below:
const char *reserve_for_use;
static void XMLCALL start_element(void *data, const char *el, const char **attr)
{
   reserve_for_use = el;   // or attr
}
after the start_element return, is reserve_for_use available for future use ?
I have tested this and it works (before XML_ParserFree), I wonder is this a valid usage in expat.