Limbo (programming language)

Limbo is a programming language for writing distributed systems and is the language used to write applications for the Inferno operating system. It was designed at Bell Labs by Sean Dorward, Phil Winterbottom, and Rob Pike.

The Limbo compiler generates architecture-independent object code which is then interpreted by the Dis virtual machine or compiled just before runtime to improve performance. Therefore all Limbo applications are completely portable across all Inferno platforms.

Limbo's approach to concurrency was inspired by Hoare's communicating sequential processes (CSP), as implemented and amended in Pike's earlier Newsqueak language and Winterbottom's Alef.

Language features

Limbo supports the following features:

• modular programming
• concurrent computing
• strong type checking at compile and run-time
• interprocess communication over typed channels
• automatic garbage collection
• simple abstract data types

Virtual machine

The Dis virtual machine that executes Limbo code is a CISC-like VM, with instructions for arithmetic, control flow, data motion, process creation, synchronizing and communicating between processes, loading modules of code, and support for higher-level data-types: strings, arrays, lists, and communication channels. It uses a hybrid of reference counting and a real-time garbage-collector for cyclic data.

Aspects of the design of Dis were inspired by the AT&T Hobbit microprocessor, as used in the prototype BeBox. A possible production of the formal logical specification for the Dis Virtual machine has been discussed by Charles Forsyth of Terzarima Systems. This is possible due to the regular structure and size of the virtual Machine compared to similar virtual machines.

Examples

Limbo uses Ada-style definitions as in:

name := type value;
name0,name1 : type = value;
name2,name3 : type;
name2 = value;

implement Command;

include "sys.m";
    sys: Sys;

include "draw.m";

include "sh.m";

init(nil: ref Draw->Context, nil: list of string)
{
    sys = load Sys Sys->PATH;
    sys->print("Hello World!\n");
}

Books

The 3rd edition of the Inferno operating system and Limbo programming language are described in the textbook Inferno Programming with Limbo ISBN 0-470-84352-7 (Chichester: John Wiley & Sons, 2003), by Phillip Stanley-Marbell.

Another textbook The Inferno Programming Book: An Introduction to Programming for the Inferno Distributed System was started by Martin Atkins, Rob Pike, Howard Trickey and Charles Forsyth but never released.

See also

• The Inferno operating system
• Alef, the predecessor of Limbo
• Plan 9 from Bell Labs, operating system
• Go, similar language from Google
• AT&T Hobbit, a processor architecture which inspired the Dis VM

References

External links

• Vita Nuova page on Limbo
• A Descent into Limbo by Brian Kernighan
• The Limbo Programming Language by Dennis M. Ritchie and Addendum by Vita Nuova.
• Inferno Programming with Limbo by Phillip Stanley-Marbell
• Threaded programming in the Bell Labs CSP style
• Historic Inferno 3rd Edition Dis source code, retrieved 2026-08-15{{citation}}: CS1 maint: url-status (link)
• The Current Inferno 4th Edition Dis source code as maintained by Charles Forsyth
• The 64-bit source code of the forked Inferno Dis virtual machine as used by the Inferno64 project
• A implementation of Limbo written in Rust as used by the RiceVM project
• The design of the Inferno virtual machine, Vita nuova, archived from the original on 2025-02-11, retrieved 2025-04-28.
• "Dis VM design", Inferno (4th ed.), Cat V.
• "Dis VM specification", Inferno (4th ed.), Cat V.