Recursive Data Type

Submitted by Gerardo Pardo on Fri, 11/06/2015 - 14:12

Concept

Some data-types are recursive in the sense that they contain members elements of the same data-type as the containing structure. A classic example is a Tree.

A Tree is traditionally defined as a type that contains a Node (data) and a list of children who themselves can be Tree structures. A simple example in C would be:

/* C definition of a Tree structure */ 
struct Node { 
    long data; 
};

struct Tree {
    Node node;
    struct Tree *children; // NULL terminated list of children 
};

Representing these kinds of recursive data-types in IDL such that they can be sent using DDS presents challenges.

While recursive data-types are not officially supported by RTI Connext DDS it is possible to use them in RTI Connext DDS 5.2, as long as you program in least in C or C++. In future releases the support will be expanded to Java and C#.

Using recursive data-types requires taking advantage of some advanced features of rtiddsgen. This example illustrates the approach.

Example Description

The simplest mapping of the above Tree to IDL would be to define a Tree as:

// IDL definition of a Tree structure. This will not compile... 
struct Node { 
    long data;
}; 

struct Tree { 
    Node node; 
    sequence<Node> children;
};

If you try this and run rtiddsgen you will encounter several problems:

  1. rtiddsgen may fail as it is unable to resolve the recursive dependency on the type getting into an infinite recursion that overflows the stack.
  2. rtiddsgen may fail as it is unable to compute the "TypeCode" for the data-type.
  3. rtiddsgen may fail as it is unable to compute the "max serialized size" for the data-type.
  4. The generated header files may give compile-time errors due to them referencing types before they are fully defined

To overcome these problems use the following techniques:

  1. Annotate the recursive type member with the annotation //@resolve-name false. This prevents rtiddsgen from trying to resolve the type and getting into the infinite recursion
  2. Run rtiddsgen with the option -notypecode. This option prevents it use the recursive type inside an unbounded sequence and run rtiddsgen with the option trying to compute the typecode.
  3. Run rtiddsgen with the option  -unboundedSupport this disables the computation of the max serialized size as being unlimited there would be no maximum size. Note that this option is currently only available in C and C++.
  4. Declare the recursive type with the annotation //@Optional this causes rtiddsgen to generate a reference/pointer avoiding the self-reference compile-time issue.

Applying these techniques the resulting IDL is:

// IDL file: recursiveType.idl
struct Node { 
    long data; 
}; 

struct Tree {
    Node node;
    sequence<Node> children; //@Optional 
                       //@resolve-name false
};

And the rtiddsgen command called to generate the data-types and marshaling code is:

rtiddsgen RecursiveType.idl -ppDisable -language C++ -update typefiles -notypecode -unboundedSupport

As mentioned in Connext DDS 5.2 this techniques are only usable in C and C++.

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