The implementation-dependent features which exist on most implementations but not a part of ANSI Common Lisp standard.
Since there are no agreed upon interfaces to these facilities, each implementation does these things differently. To make things simpler for the user, oftentimes compatibility packages are maintained to provide a standard interface.
- Debugging - debugging Lisp systems
- OS - communicating with the underlying OS (shell access, etc.)
- Streams - user-defined streams (a.k.a. Gray Streams)
- Sockets - using sockets (using IOLib)
- MOP - Meta Object Protocol (using closer-mop)
- FFI - Foreign Function interface (using CFFI compatibility layer)
- Threads - writing multi-threaded programs (using bordeaux-threads)
- User Control of GC - opening the black box of the Garbage Collector (using trivial-garbage)
- CLX — X Window Programming in Common Lisp