bison question (memory deallocation)

I'm implementing a parser in bison which has a pointer to the
following structure specified as one option in the "%union" statement
for bison:


struct unittype {
      char *numerator[MAXSUBUNITS];
      char *denominator[MAXSUBUNITS];
      double factor;
};


In the process of scanning and reducing, various instances of this
structure get allocated and entries of the numerator[] and
denominator[] arrays also get allocated. A well behaved program
should free this stuff at some point. When and how do I free it?


Is it ok for my actions to destroy their arguments? So like an action
that does { $$ = foo($1, $2); kill($1);kill($2);} after which time $1
and $2 (which are pointers to struct unittype) no longer point
anywhere valid?


What about when parse errors occur? Is there some way I can clean up
the parser stack and free anything on it that needs to be freed? Or
do I have to do my own memory management?
[In principle you could put in error rules and catch everything as yacc
unwinds the stack, in practice that's too fragile to do. I use a malloc
wrapper that chains together everything it allocates, then after the
parse, I zip down the chain and free everything. -John]

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