Support for user-provided GC marking routines#
The garbage collector supports types for which no automatic marking code is generated. For these types, the user is required to provide three functions: one to act as a marker for garbage collection, and two functions to act as marker and pointer walker for pre-compiled headers.
Given a structure struct GTY((user)) my_struct
, the following functions
should be defined to mark my_struct
:
void gt_ggc_mx (my_struct *p)
{
/* This marks field 'fld'. */
gt_ggc_mx (p->fld);
}
void gt_pch_nx (my_struct *p)
{
/* This marks field 'fld'. */
gt_pch_nx (tp->fld);
}
void gt_pch_nx (my_struct *p, gt_pointer_operator op, void *cookie)
{
/* For every field 'fld', call the given pointer operator. */
op (&(tp->fld), NULL, cookie);
}
In general, each marker M
should call M
for every
pointer field in the structure. Fields that are not allocated in GC
or are not pointers must be ignored.
For embedded lists (e.g., structures with a next
or prev
pointer), the marker must follow the chain and mark every element in
it.
Note that the rules for the pointer walker gt_pch_nx (my_struct
*, gt_pointer_operator, void *)
are slightly different. In this
case, the operation op
must be applied to the address of
every pointer field.
User-provided marking routines for template types#
When a template type TP
is marked with GTY
, all
instances of that type are considered user-provided types. This means
that the individual instances of TP
do not need to be marked
with GTY
. The user needs to provide template functions to mark
all the fields of the type.
The following code snippets represent all the functions that need to
be provided. Note that type TP
may reference to more than one
type. In these snippets, there is only one type T
, but there
could be more.
template<typename T>
void gt_ggc_mx (TP<T> *tp)
{
extern void gt_ggc_mx (T&);
/* This marks field 'fld' of type 'T'. */
gt_ggc_mx (tp->fld);
}
template<typename T>
void gt_pch_nx (TP<T> *tp)
{
extern void gt_pch_nx (T&);
/* This marks field 'fld' of type 'T'. */
gt_pch_nx (tp->fld);
}
template<typename T>
void gt_pch_nx (TP<T *> *tp, gt_pointer_operator op, void *cookie)
{
/* For every field 'fld' of 'tp' with type 'T *', call the given
pointer operator. */
op (&(tp->fld), NULL, cookie);
}
template<typename T>
void gt_pch_nx (TP<T> *tp, gt_pointer_operator, void *cookie)
{
extern void gt_pch_nx (T *, gt_pointer_operator, void *);
/* For every field 'fld' of 'tp' with type 'T', call the pointer
walker for all the fields of T. */
gt_pch_nx (&(tp->fld), op, cookie);
}
Support for user-defined types is currently limited. The following restrictions apply:
Type
TP
and all the argument typesT
must be marked withGTY
.Type
TP
can only have type names in its argument list.The pointer walker functions are different for
TP<T>
andTP<T *>
. In the case ofTP<T>
, references toT
must be handled by callinggt_pch_nx
(which will, in turn, walk all the pointers inside fields ofT
). In the case ofTP<T *>
, references toT *
must be handled by calling theop
function on the address of the pointer (see the code snippets above).