# Garbage Collection: Marking RData ptr **URL:** \ **Category:** Ruby\ **Created:** [September 16, 2006, 2:13am UTC](https://www.ruby-forum.com/t/garbage-collection-marking-rdata-ptr/68869 "2006-09-16T02:13:09Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![vtphysguy](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/77aa72/32.png) [@vtphysguy](https://www.ruby-forum.com/u/vtphysguy)\ **Post date:** [September 16, 2006, 2:13am UTC](https://www.ruby-forum.com/t/garbage-collection-marking-rdata-ptr/68869/1 "2006-09-16T02:13:09Z")
I’m working on creating a binding for an object-based library[1] and am trying to find a way to make it work well with the garbage collector. Constructor functions from the library return a pointer to their data structure, making it easy to wrap in a Ruby object using Data\_Wrap\_Struct. The problem I’m having is when one of the C objects takes responsibility for it’s associated object(s). For example, a Statement[2] has three member Nodes[3]. When the statement’s free function is called, it will also free the associated nodes. There are functions to get and set each of these nodes. The problem I’m having is marking the associated nodes so the garbage collector won’t prematurely free them. There may be zero or more Ruby objects wrapping each one of the nodes, if any one of these is destroyed the statement will enter an undefined state. I’d like to define a mark function for the statement’s that will mark an RData object pointing to a specific pointer. As I understand it, rb\_gc\_mark will only mark a VALUE, is there a good way to do this? An alternative I’m considering would be to create my own struct that points to the librdf\_statement as well as VALUEs for each of the nodes. I’d have to write helper functions to make sure the VALUE is updated if the statement is changed externally, but I think it should work. The safe (and naive) approach is to copy anything going into or out of the statement, but I’d rather not do that. Thanks, Justin [1] [http://librdf.org/](http://librdf.org/) [2] [RDF Triple (librdf\_statement)](http://librdf.org/docs/api/redland-statement.html) [3] [RDF term (librdf\_node)](http://librdf.org/docs/api/redland-node.html) ---
**Author:** ![vtphysguy](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/77aa72/32.png) [@vtphysguy](https://www.ruby-forum.com/u/vtphysguy)\ **Post date:** [September 16, 2006, 4:06pm UTC](https://www.ruby-forum.com/t/garbage-collection-marking-rdata-ptr/68869/2 "2006-09-16T16:06:34Z")
> > > > > “J” == Justin B. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes: J\> The safe (and naive) approach is to copy anything going into or out of J\> the statement, but I’d rather not do that. Well, what do you do is someone re-use a node after it was set in the statement ? You have The object passed in becomes owned by the statement object and must not ^^^^^^^^ be used by the caller after this call. ^^^^^^^ and This method returns a SHARED pointer to the object which must be copied by ^^^^^^^^^^^^^^ the caller if needed. – Guy Decoux ---
**Author:** ![vtphysguy](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/v/77aa72/32.png) [@vtphysguy](https://www.ruby-forum.com/u/vtphysguy)\ **Post date:** [September 16, 2006, 7:39pm UTC](https://www.ruby-forum.com/t/garbage-collection-marking-rdata-ptr/68869/3 "2006-09-16T19:39:57Z")
Redland nodes are immutable so it’s not really a problem. I just have to make sure that any object that takes possession of or depends on a node makes a copy for itself. In practice this is just statements, for which I have to do this anyway. I was looking for a more efficient way to provide access to the the nodes in a statement than copying the underlying datastructure and rewrapping it on every access. I think I’ll leave it as is for now and if it becomes a performance problem I can cache the nodes in the object. Unless of course there’s away to mark all RData objects wrapping a particular pointer 🙂 -Justin