thread safety and Medley

[Larry Masinter]

We were talking about what features of Interlisp were important to development of the residential environment. I thought spaghetti stacks were poorly credited.

which implementations of common lisp are thread safe?

Common Lisp implementations vary in their threading support and thread safety:

Fully thread-safe with native threads:

• SBCL (Steel Bank Common Lisp) — has native threads on Linux, macOS, and Windows via sb-thread. Generally considered the most robust for multithreaded use, though some parts of the standard library are not thread-safe by default.
• CCL (Clozure CL) — designed with threading in mind from early on; native threads, good thread safety across its runtime.
• LispWorks — commercial implementation with mature, well-documented thread support (called "multiprocessing").
• Allegro CL — commercial, mature threading support with mp (multiprocessing) package.
• ABCL (Armed Bear Common Lisp) — runs on the JVM, so threading inherits Java's threading model; reasonably thread-safe.

Limited or cooperative threading:

• ECL (Embeddable Common Lisp) — has POSIX thread support, but thread safety can be inconsistent depending on the operation.
• CMUCL — has some thread support on certain platforms but is less mature than SBCL.

No real threading:

• CLISP — no native thread support; effectively single-threaded.
• GCL (GNU Common Lisp) — minimal threading support, not recommended for concurrent use.

Practical notes:

• The Bordeaux Threads library (bt) provides a portable threading API across implementations and is the standard way to write portable multithreaded CL code.
• SBCL + Bordeaux Threads is the most common production choice in the open-source ecosystem.
• Even in thread-safe implementations, the garbage collector and certain I/O operations may require locking. Libraries like lparallel and chanl build higher-level concurrency abstractions on top.
• Thread safety of your code is separate from the implementation's runtime safety — you still need to manage shared state carefully with locks or atomic operations.

If you're starting a new multithreaded project, SBCL or CCL with Bordeaux Threads is the most well-trodden path.

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