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### 22.3 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), 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.

#### 22.3.1 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), 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:

1. Type `TP` and all the argument types `T` must be marked with `GTY`.
2. Type `TP` can only have type names in its argument list.
3. The pointer walker functions are different for `TP<T>` and `TP<T *>`. In the case of `TP<T>`, references to `T` must be handled by calling `gt_pch_nx` (which will, in turn, walk all the pointers inside fields of `T`). In the case of `TP<T *>`, references to `T *` must be handled by calling the `op` function on the address of the pointer (see the code snippets above).

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