8.1 Introduction to Aliens

Because of Lisp's emphasis on dynamic memory allocation and garbage collection, Lisp implementations use unconventional memory representations for objects. This representation mismatch creates problems when a Lisp program must share objects with programs written in another language. There are three different approaches to establishing communication:

• The burden can be placed on the foreign program (and programmer) by requiring the use of Lisp object representations. The main difficulty with this approach is that either the foreign program must be written with Lisp interaction in mind, or a substantial amount of foreign ``glue'' code must be written to perform the translation.
• The Lisp system can automatically convert objects back and forth between the Lisp and foreign representations. This is convenient, but translation becomes prohibitively slow when large or complex data structures must be shared.
• The Lisp program can directly manipulate foreign objects through the use of extensions to the Lisp language. Most Lisp systems make use of this approach, but the language for describing types and expressing accesses is often not powerful enough for complex objects to be easily manipulated.

CMU Common Lisp relies primarily on the automatic conversion and direct manipulation approaches: Aliens of simple scalar types are automatically converted, while complex types are directly manipulated in their foreign representation. Any foreign objects that can't automatically be converted into Lisp values are represented by objects of type alien-value. Since Lisp is a dynamically typed language, even foreign objects must have a run-time type; this type information is provided by encapsulating the raw pointer to the foreign data within an alien-value object.

The Alien type language and operations are most similar to those of the C language, but Aliens can also be used when communicating with most other languages that can be linked with C.