About Clozure CL

Clozure Associates develops, hosts, and maintains Clozure CL (aka CCL), formerly known as OpenMCL (see NamingHistory). Clozure CL is a fast, mature, open source Common Lisp implementation that runs on the following platforms:

  • Mac OS X 10.4 and later (PowerPC and x86)
  • Linux (PowerPC and x86)
  • FreeBSD (x86)
  • Solaris (x86)
  • Microsoft Windows XP and later (x86)

There are both 32- and 64-bit versions for all platforms.

Clozure CL was forked from MCL in 1998 and the development has been entirely separate since.


  • Very fast compilation speed.
  • A fast, precise, compacting, generational garbage collector written in hand-optimized C. The sizes of the generations are fully configurable. Typically, a generation can be collected in a millisecond on modern systems.
  • Fast execution speed, competitive with other Common Lisp implementations on most benchmarks.
  • Robust and stable. Customers report that their CPU-intensive, multi-threaded applications run for extended periods on Clozure CL without difficulty.
  • Full native OS threads on all platforms. Threads are automatically distributed across multiple cores. The API includes support for shared memory, locking, and blocking for OS operations such as I/O.
  • Full Unicode support.
  • Full SLIME integration.
  • An IDE on the Mac OS X, fully integrated with the Macintosh window system and User Interface standards.
  • Excellent debugging facilities. The names of all local variables are available in a backtrace.
  • A complete, mature foreign function interface. This includes a powerful bridge to Objective-C and Cocoa on Mac OS X.
  • Many extensions including: files mapped to Common Lisp vectors for fast file I/O; thread local hash tables and streams to eliminate locking overhead; cons hashing support; etc.
  • Very efficient use of memory


Clozure CL is used for many commercial projects. One project of note is ACL2 at the University of Texas. This is a CPU intensive theorem proving system used to prove the correctness of large commercial software projects. Clozure CL fully supports ACL2 which greatly benefits from its stability and its native threads that are automatically distributed to multiple cores. Some of the development of Clozure CL was funded by DARPA and the NSF under subcontract from the University of Texas; contact Professor Warren A. Hunt, Jr. for additional information. This development includes a number of special extensions and optimizations to make ACL2 perform better. Any opinions, findings and conclusions or recomendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation (NSF).


Clozure CL is fully supported by Clozure Associates. We have made specific extensions and compiler modifications in response to customer needs. Clozure is committed to the ongoing maintenance and evolution of Clozure CL.

Clozure Associates offers full support and development services for Clozure CL.

Last modified 11 years ago Last modified on Oct 19, 2009, 9:30:00 PM