Interface Compatibility

Many types of change can be made to an object. In their simplest terms, these changes can be categorized into one of two groups:

• Compatible updates. These updates are additive, in that all previously available interfaces remain intact.

• Incompatible updates. These updates have changed the existing interface in such a way that existing users of the interface can fail or behave incorrectly.

The following table categorizes some common object changes.

Table 5–1 Interface Compatibility Examples

Object Change	Update Type
The addition of a symbol	Compatible
The removal of a symbol	Incompatible
The addition of an argument to a non-varargs(3HEAD) function	Incompatible
The removal of an argument from a function	Incompatible
The change of size, or content, of a data item to a function or as an external definition	Incompatible
A bug fix, or internal enhancement to a function, providing the semantic properties of the object remain unchanged	Compatible
A bug fix, or internal enhancement to a function when the semantic properties of the object change	Incompatible

Because of interposition, the addition of a symbol can constitute an incompatible update, such that the new symbol might conflict with an applications use of that symbol. However, this does seem rare in practice as source-level name space management is commonly used.

Compatible updates can be accommodated by maintaining version definitions internal to the object being generated. Incompatible updates can be accommodated by producing a new object with a new external versioned name. Both of these versioning techniques enable the selective binding of applications. They also enable verification of correct version binding at runtime. These two techniques are explored in more detail in the following sections.