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Limbo (programming language)
View on Wikipediafrom Wikipedia
 | This article needs additional citations for verification. (October 2013) |
| Limbo | |
|---|---|
| Paradigm | Concurrent |
| Designed by | Sean Dorward, Phil Winterbottom, Rob Pike |
| Developer | Bell Labs / Vita Nuova Holdings |
| First appeared | 1995 |
| Typing discipline | Strong |
| OS | Inferno |
| License | GNU GPL v2, see NOTICE in limbo subfolder of the tarball |
| Website | web |
| Major implementations | |
| Dis virtual machine | |
| Influenced by | |
| C, Pascal, CSP, Alef, Newsqueak | |
| Influenced | |
| Stackless Python, Go, Rust | |
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.[1]
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
[edit]Limbo supports the following features:
- modular programming
- concurrent programming
- strong type checking at compile and run-time
- interprocess communication over typed channels
- automatic garbage collection
- simple abstract data types
Virtual machine
[edit]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.[2] It uses a hybrid of reference counting and a real-time garbage-collector for cyclic data.[3]
Aspects of the design of Dis were inspired by the AT&T Hobbit microprocessor, as used in the original BeBox.
Examples
[edit]Limbo uses Ada-style definitions as in:
name := type value;
name0,name1 : type = value;
name2,name3 : type;
name2 = value;
Hello world
[edit]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
[edit]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, by Martin Atkins, Charles Forsyth, Rob Pike and Howard Trickey, was started, but never released.
See also
[edit]
- 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
[edit]- ^ "Inferno Application Programming". vitanuova. vitanuova. Retrieved January 26, 2021.
- ^ "Dis Virtual Machine Specification". Vita Nuova. 2000. Retrieved 2 February 2015.
- ^ Lorenz Huelsbergen and Phil Winterbottom (1998). "Very Concurrent Mark and Sweep Garbage Collection without Fine-Grain Synchronization" (PDF). 1998 International Symposium on Memory Management.
External links
[edit]- 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
- Dis source code, archived from the original on 2017-09-21, retrieved 2017-09-20
- The design of the Inferno virtual machine, Vita nuova.
- "Dis VM design", Inferno (4th ed.), Cat V.
- "Dis VM specification", Inferno (4th ed.), Cat V.
| International | |
|---|---|
| National | |
Limbo (programming language)
View on Grokipediafrom Grokipedia
.m files) and implementations (in .b files) that can be dynamically loaded at runtime, promoting reusability and encapsulation.[1] Concurrency is handled through lightweight processes spawned via the spawn keyword and synchronized using unbuffered channels with the alt statement for selective communication, avoiding explicit locks or semaphores.[1] The type system supports basic types like int, real, and string, as well as composite types such as arrays, lists, tuples, abstract data types (ADTs), and references, with compile-time inference and no pointer arithmetic to enhance safety.[1] Limbo compiles to architecture-independent bytecode interpreted by the Dis virtual machine, facilitating cross-platform deployment in embedded and distributed scenarios.[2]
History
Development Origins
Limbo's development began in 1995 at Bell Labs' Computing Sciences Research Center, led by Sean Dorward, Phil Winterbottom, and Rob Pike.[3] This effort emerged from the Computing Sciences Research Center's ongoing work on advanced operating systems and languages, building on the center's legacy of innovation in distributed computing.[4] The primary motivation for creating Limbo was to provide a programming language capable of supporting distributed applications on small, networked computers, while avoiding dependencies on specific hardware architectures.[1] At the time, existing languages struggled with portability across diverse, resource-constrained devices, prompting the need for a solution that could enable seamless execution in heterogeneous environments without low-level hardware bindings.[5] Limbo was thus designed with strong typing, garbage collection, and modularity to ensure safety and efficiency in such settings.[4] As a core component of the Inferno operating system initiative, Limbo aimed to facilitate a portable, lightweight environment tailored for ubiquitous computing across networks of embedded and desktop systems.[1] Inferno sought to extend the principles of distributed resource access to everyday devices, and Limbo served as its native application language, compiling to architecture-independent bytecode for the Dis interpreter.[4] The language's early design drew significant influence from the challenges encountered in Plan 9 and Alef, with the goal of overcoming their limitations in concurrency and portability.[5] Plan 9's file-based resource model informed Inferno's foundations, but Limbo addressed portability issues by virtualizing execution, while incorporating Alef's channel-based concurrency and abstract data types to enhance support for threaded, networked programs.[1] Inferno itself functions as Limbo's primary runtime environment, providing the networked filesystem and execution context essential for its applications.[4]Release and Evolution
Limbo was initially released in 1996 alongside the beta version of the Inferno operating system, developed at Bell Labs under Lucent Technologies and distributed under a proprietary license.[6] The language was designed to support Inferno's distributed computing environment, with its first formal release following in April 1997 as part of Inferno 1.0.[6] The language evolved alongside Inferno through subsequent versions, culminating in the second edition in July 1999, which included enhancements to concurrency mechanisms such as channel-based communication and refinements to the module system for better modular programming support.[6] These updates improved Limbo's suitability for building concurrent, distributed applications on resource-constrained devices.[7] Following Lucent Technologies' restructuring, the Inferno project—including Limbo—was handed over to Vita Nuova Holdings in 2000, a company formed to continue its development.[8] Vita Nuova released the fourth edition as open source under the GNU GPL version 2 (dual-licensed with the MIT License) in 2005.[9][10] The third edition followed in June 2001, and the fourth in early 2005.[6][10] No major updates to Limbo occurred after 2005, as the project entered maintenance mode, with only minor patches issued for compatibility across platforms.[10]Design Principles
Goals and Influences
Limbo was designed primarily to support modular and concurrent programming for resource-constrained devices operating within heterogeneous networks, emphasizing strong static typing to catch errors at compile time and runtime while minimizing unexpected behaviors.[1] This focus addressed the needs of distributed systems where applications must run efficiently on small computers without relying on hardware memory protection, incorporating features like automatic garbage collection and typed channels for safe interprocess communication.[1] By prioritizing these elements, Limbo aimed to enable robust, safe execution in environments with varying computational resources and network topologies.[11] A key design goal was achieving architecture independence through the generation of bytecode for a virtual machine, which avoids direct dependencies on specific hardware or operating systems and facilitates portability across diverse platforms such as Intel x86, ARM, PowerPC, and others, as well as host OSes including Unix, Windows, and Plan 9 derivatives.[1] This bytecode approach, executed via the Dis interpreter, ensures consistent behavior and numeric type sizes (e.g., 32-bit integers) regardless of the underlying system, supporting Inferno's cross-platform deployment for distributed applications.[11] Limbo's syntax draws from C for expressions, statements, and control flow, while adopting Pascal's style for declarations to promote clarity and structure.[1] Its concurrency model is heavily influenced by Hoare's Communicating Sequential Processes (CSP), incorporating channel-based communication for synchronization, as refined in Pike's Newsqueak for process management.[11] The overall modular structure with abstract data types draws from Alef.[11] Developed as a successor to Alef to overcome its limitations in portability and thread safety—particularly Alef's tighter coupling to Plan 9's runtime and potential issues with shared memory in multithreaded contexts—Limbo provides a cleaner, more portable alternative better suited for Inferno's virtualized environment.[1] By reusing and enhancing Alef's abstract data types and channels while introducing bytecode and stricter safety guarantees, Limbo achieves greater flexibility for cross-architecture development without sacrificing concurrency performance.[11]Key Innovations
Limbo's key innovations center on enhancing safety, modularity, and efficiency for programming distributed and embedded systems, distinguishing it from languages like C or its predecessor Alef through built-in mechanisms for type-safe memory handling and concurrency. A foundational innovation is the "big" and "ref" types, which address challenges in managing dynamic data and references on resource-limited hardware. The "big" type represents 64-bit signed integers, enabling operations on larger numerical ranges than the standard 32-bit "int" without risking overflow in typical computations.[1] Complementing this, the "ref" type serves as a safe pointer to heap-allocated objects, such as abstract data types, with runtime type checking and automatic garbage collection to eliminate common memory errors like null pointer dereferences or leaks.[1] These types ensure that dynamic allocations remain verifiable and contained, promoting reliability in environments where manual memory management would introduce vulnerabilities. Limbo advances encapsulation through its module-based support for abstract data types (ADTs), eschewing traditional class hierarchies in favor of a lightweight, namespace-driven approach. Modules define ADTs using the "adt" declaration, which groups private fields and methods into a self-contained unit accessible only via a public interface, thereby enforcing information hiding at the language level.[1] This design facilitates reusable components without inheritance complexities, allowing implementations to evolve independently while maintaining strong compile-time and runtime type guarantees across distributed modules.[1] The language's integrated exception handling, via "raise" and "exception" constructs, provides a structured mechanism for error propagation tailored to distributed applications. Developers declare exceptions as types and use "raise" to signal failures, which can propagate across module boundaries or processes until intercepted by a matching "exception" block that handles recovery or logging.[1] This feature is particularly effective in Inferno's networked contexts, where it enables graceful degradation without halting unrelated computations.[1] To meet real-time constraints on embedded systems, Limbo incorporates lightweight threading and deterministic communication primitives for predictable performance. The "spawn" keyword initiates coroutines with minimal overhead, avoiding the context-switching costs of heavier OS threads and suiting small-footprint devices without memory protection hardware.[1] Coupled with unbuffered channels for inter-thread messaging, this model ensures bounded latency and avoids nondeterministic buffering, allowing applications to achieve real-time responsiveness in constrained environments.[1]Language Features
Data Types and Modules
Limbo's type system is centered on a set of primitive types designed for efficiency in distributed and resource-constrained environments. The primitive types includebyte, an 8-bit unsigned integer; int, a 32-bit signed integer in two's complement representation; big, a 64-bit signed integer also in two's complement; and real, a 64-bit IEEE floating-point number. Additionally, string serves as an immutable sequence of Unicode characters, providing a built-in mechanism for handling text data without modifiable buffers. For binary data, Limbo supports arrays of bytes (array of byte), which allow dynamic allocation and slicing for manipulation of raw data streams.[12][1]
Limbo also supports composite value types, including tuples and abstract data types (ADTs). Tuples are ordered collections of two or more values of potentially different types, denoted as (type1, type2, ...), and are commonly used for returning multiple values from functions, such as (int, list of [string](/page/String)) from a tokenizing routine. ADTs define structured data with methods, serving as value types that can be instantiated without references.[1]
Reference types in Limbo enable heap allocation and pointer-like semantics while maintaining type safety. The ref T construct denotes a reference to an object of type T, such as ref Draw->Image for graphical contexts, allowing indirect access to mutable structures without exposing raw pointers. These references are garbage-collected at runtime to prevent memory leaks. The language also supports composite reference types like arrays (array of T), lists (list of T), and channels (chan of T), which facilitate data sharing in concurrent programs. Unlike primitives, references can be nil, serving as a safe null value.[1][12]
Limbo organizes code into modules as first-class units to promote modularity and reusability across distributed applications. A module is declared as a named collection of constants, abstract data types (ADTs), global data, and functions, specified via a module interface such as MyModule: module { [init](/page/Init): fn(); add: fn(a, b: int): int; };. The implementation is provided using the implement keyword, as in implement MyModule;, followed by the concrete definitions of exported members. Modules export only explicitly named elements, encapsulating internals to enforce interfaces. To incorporate another module, the include directive is used, for example, include "sys.m";, which brings in the declaration file for standard library access like system calls. This structure supports separate compilation and linking, essential for Inferno's modular architecture.[1][12]
Type checking in Limbo is static and strong, performed primarily at compile time to catch errors early without runtime overhead in most cases. The compiler enforces exact type matching for assignments, function parameters, and returns, with no implicit conversions between types—explicit casts are required, such as int i := (int) 3.14;, to avoid subtle bugs. Local variables benefit from type inference: declarations like x := 42; deduce int from the initializer, reducing verbosity while preserving safety. This system extends to modules and ADTs, where interfaces define types contractually, and violations trigger compile errors. Runtime checks supplement for dynamic aspects like array bounds, but the static foundation ensures robust code. These types underpin Limbo's concurrency model by allowing typed channels for safe inter-module communication.[1][12]
Concurrency and Communication
Limbo provides concurrency through lightweight threads, known as processes, which are created using thespawn keyword. These processes share the same address space and are preemptively scheduled by the runtime system, enabling efficient parallelism without the overhead of traditional operating system threads.[1][13]
Communication between processes occurs exclusively via channels, which are typed first-class objects declared as chan of T, where T is the type of messages to be exchanged. Channels support synchronous message passing using the send operator <-= and the receive operator <-, with sends blocking until a corresponding receive is ready on an unbuffered channel. For instance, a simple send might appear as c <-= value;, where c is the channel and value matches the declared type. Buffered channels, specified as chan [n] of T, allow up to n messages to be queued, but Limbo encourages unbuffered channels to enforce synchronization.[1][13]
Limbo's concurrency model draws inspiration from Communicating Sequential Processes (CSP), a formalism introduced by Tony Hoare for describing concurrent interactions through synchronous channels. This influence manifests in the language's emphasis on message passing over shared memory, promoting safe coordination without race conditions. To handle non-deterministic selection among multiple channels, Limbo uses the alt statement, which blocks until at least one communication operation (send or receive) is possible and then selects one at random if several are ready. An example alt construct might look like:
alt {
i := <- inputchan =>
# Handle received input
* =>
# Default case if no channels ready
}
alt {
i := <- inputchan =>
# Handle received input
* =>
# Default case if no channels ready
}
* clause provides a non-blocking default, ensuring the process does not deadlock.[1][13][14]
Process synchronization in Limbo relies entirely on channels for coordination, eschewing explicit locks or mutexes to avoid shared mutable state issues. For example, mutual exclusion can be implemented by funneling access through a single-element buffered channel acting as a semaphore, where a process sends a token to acquire the lock and receives it back to release. This channel-centric approach aligns with CSP principles, ensuring that concurrency remains composable and deadlock-prone patterns are explicit in the code.[1]
Virtual Machine
Dis Interpreter
The Dis virtual machine serves as the execution environment for Limbo programs within the Inferno operating system, executing bytecode generated from Limbo source code, typically via just-in-time compilation to native code for efficiency.[4] It employs a memory-to-memory, CISC-style architecture with three-operand instructions, where operations specify source operands, a destination, and sometimes additional modifiers.[15] The opcode field is 8 bits wide, allowing for up to 256 distinct opcodes, though the implemented set comprises approximately 100, covering arithmetic operations such as integer addition (addw for iadd), floating-point multiplication (mulf for fmul), control flow instructions like unconditional jumps (jmp) and function calls (call), and I/O primitives including message sending (send) and receiving (recv).[15] This design facilitates efficient handling of complex instructions in a compact form, optimized for resource-constrained environments.[4]
Limbo source code is compiled by the limbo compiler into architecture-independent bytecode stored in .dis files, which encapsulate the program's modules, symbols, and instructions in a portable binary format.[15] These files can be loaded and executed uniformly across different host architectures supported by Inferno, without modification, enabling seamless deployment in distributed systems.[16] The bytecode format includes a header with magic numbers, version information, and module metadata, followed by sections for code, data, and debugging symbols, ensuring self-contained executables that the Dis interpreter can process directly.[15]
The memory model of Dis utilizes a flat 32-bit virtual address space—although variants with 64-bit support have been developed[17]—divided into a non-addressable code segment for instructions, an addressable data segment (global and heap-allocated objects), and a stack (for local variables and call frames, dynamically allocated from the heap).[15] Addresses are represented as offsets relative to segment bases or module pointers, avoiding absolute references to maintain portability; nil (zero) denotes invalid or uninitialized pointers.[15] Dynamic loading of modules is supported through runtime resolution of external references, allowing programs to import and instantiate additional .dis modules on demand via the load instruction, which facilitates modular and extensible application design.[15]
Error handling in the Dis interpreter relies on trap mechanisms to detect runtime faults, such as type mismatches during operations or arithmetic overflows, which are captured and propagated as structured exceptions.[15] Instructions like tcmp enable explicit trap comparisons to check for error conditions, raising exceptions that can be caught and handled by the executing Limbo code, ensuring robust fault isolation without halting the entire virtual machine.[15]
Execution Model
Limbo programs execute within the Dis virtual machine, where concurrency is supported through lightweight threads implemented as coroutines, each with its own dynamically allocated stack from the heap. These coroutines enable efficient multitasking by allowing the program to yield control voluntarily or preemptively, facilitating responsive distributed applications. The VM's internal scheduler multiplexes all coroutines onto a single underlying OS thread, handling preemptive scheduling independently of the host platform's threading model to ensure portability and low overhead.[15] Memory allocation for reference types, such as arrays, strings, lists, and channels, occurs dynamically on the heap, with objects referenced via pointers that maintain a reference count. This reference counting mechanism increments the count when a reference is created or copied and decrements it upon destruction, enabling immediate deallocation of objects when the count reaches zero and promoting efficient memory usage in resource-constrained environments. Cyclic references, which evade detection by reference counting alone, are addressed through a supplementary periodic mark-sweep garbage collection process that periodically scans for and reclaims unreachable cycles.[1][18] The garbage collector operates as a real-time, incremental mark-sweep system designed to minimize execution pauses, triggered automatically when heap allocation thresholds are met to prevent memory exhaustion. It employs a tri-color marking algorithm, classifying objects as white (potentially unreachable), gray (marked but with unscanned pointers), or black (fully marked and scanned), allowing the collection to proceed in small increments interleaved with mutator activity. This approach ensures low-latency behavior suitable for interactive and embedded systems, with the sweeper phase reclaiming white objects after marking completes, effectively handling both acyclic and cyclic garbage while complementing reference counting.[18][15] Unhandled exceptions in Limbo trigger stack unwinding in the affected coroutine, propagating upward through the call chain until intercepted by an exception handler or causing the thread to terminate, thereby deallocating its stack resources.[1]Implementations
Supported Platforms
Limbo programs, interpreted by the Dis virtual machine, were designed for high portability across diverse hardware and operating systems as part of the Inferno environment. Originally developed in the mid-1990s, Inferno supported native execution on bare hardware for architectures including Intel x86, MIPS, SPARC, ARM (particularly for embedded systems), and PowerPC, allowing Limbo applications to run without an underlying host OS.[1][19][20] In hosted mode, Inferno—and thus Limbo—operated as an emulated environment atop existing operating systems, with early ports targeting Plan 9, Windows 95, 98, NT, and 2000, Linux, Solaris, FreeBSD, Irix, and Mac OS X in the early 2000s.[19][20] These ports enabled cross-platform development by compiling Limbo to Dis bytecode, which the Dis interpreter then executed on the host system.[1] As of 2025, community-maintained versions continue to support modern environments such as Linux on x86-64 processors, FreeBSD, and ARM-based devices like the Raspberry Pi through dedicated ports, though official support remains focused on legacy configurations without extensions to mobile operating systems or web browsers.[21][22] The reliance on the Dis virtual machine ensures bytecode portability but introduces interpretation overhead, limiting performance on high-end systems compared to native compilation.[1][20]| Mode | Architectures | Host Operating Systems |
|---|---|---|
| Native | x86, ARM, MIPS, SPARC, PowerPC | None (bare hardware) |
| Emulated | x86, ARM, MIPS, SPARC, PowerPC | Plan 9, Linux, FreeBSD, Solaris, Windows 95/98/NT/2000, Irix, Mac OS X, AIX |
Tools and Compilers
The Limbo compiler, invoked via thelimbo tool, translates source files ending in .b into architecture-independent bytecode files with the .dis extension, suitable for execution on the Dis virtual machine. This process supports options for optimization and debugging, such as -g to include symbolic debugging information in companion .sbl files and -w to report warnings during compilation. The resulting bytecode maintains portability across Inferno-supported platforms without requiring host-specific recompilation.[23][24]
Inferno's mk serves as the primary build tool and linker, employing a declarative mkfile to automate the combination of multiple Limbo modules into cohesive executables while managing dependencies and installation to directories like /dis. This toolstreamlines the development workflow by handling incremental builds and cleanup operations, such as removing intermediate files with the clean target.[24]
For debugging, the wm-deb tool provides an interactive graphical interface to load and monitor Dis bytecode, enabling developers to inspect program state, step through execution, and examine threads and data structures in running Limbo applications.[23]
Limbo's standard library consists of reusable modules that extend core functionality, including draw for low-level graphics primitives such as image compositing and alpha blending, and tk for constructing window-based graphical user interfaces. Networking capabilities are supported through modules interfacing with the Styx protocol, which facilitates file-system-like access to distributed resources, alongside built-in support for TCP/IP, UDP, and other protocols.[12][1]
Examples
Basic Program
A basic "Hello, World!" program in Limbo demonstrates the language's module-based structure, dynamic module loading, and lightweight concurrency through thread spawning. The following example defines a module namedHello with an entry point init function, loads the standard Sys module for input/output operations, spawns a separate thread to perform the printing, and then exits the main thread.[11]
limbo
implement Hello;
include "sys.m";
include "draw.m";
sys: Sys;
Hello: module {
init: fn(nil: ref Draw->Context, nil: list of string);
};
init(nil: ref Draw->Context, nil: list of string) {
sys = load Sys Sys->PATH;
spawn printer();
exit;
}
printer() {
sys->print("Hello, world!\n");
}
implement Hello;
include "sys.m";
include "draw.m";
sys: Sys;
Hello: module {
init: fn(nil: ref Draw->Context, nil: list of string);
};
init(nil: ref Draw->Context, nil: list of string) {
sys = load Sys Sys->PATH;
spawn printer();
exit;
}
printer() {
sys->print("Hello, world!\n");
}
implement Hello; declares the implementation of the Hello module, while include "sys.m"; and include "draw.m"; import the interfaces for the Sys and Draw modules, respectively; the Draw module provides the Context type for the init signature but is unused here.[11] The variable sys: Sys; declares a reference to the Sys module, which is dynamically loaded inside init using load Sys Sys->PATH to access system services like printing.[11] The init function serves as the program's entry point, spawning the printer function as a new thread with spawn printer();—this creates an independent thread of control that executes concurrently—and then calls exit to terminate the main thread without waiting.[11] The printer function uses sys->print to output the string literal to standard output, illustrating Limbo's support for simple I/O via the Sys module.[11]
To compile the program, save the code in a file such as hello.b and invoke the Limbo compiler with the command limbo -g hello.b, which generates the Dis virtual machine bytecode file hello.dis; the -g flag enables debugging information.[1] On a standard platform like Unix hosting the Inferno environment, execute the program using the emulator with emu hello.dis or the Dis interpreter directly with dis hello.dis.[1] The runtime behavior involves the Dis virtual machine loading the module, initializing the init function in the main thread, spawning the print thread (which shares the address space but runs independently and preemptively scheduled), outputting "Hello, world!" followed by a newline to stdout, and then both threads completing, resulting in the program exiting promptly with no further interaction.[25]
Concurrent Example
Limbo's concurrency model leverages lightweight threads spawned via thespawn keyword and typed channels for communication, enabling efficient producer-consumer patterns without explicit locks. A representative example implements a producer thread that generates integers and sends them over a channel to a consumer thread, which receives and prints them, incorporating non-blocking selection via the alt statement for handling potential multiple inputs or cases where no data is immediately available.[1]
The code structure begins by creating a channel of integers (chan of int), spawning the producer and consumer threads, and ensuring clean termination. The producer runs a loop sending values from 1 to 10, while the consumer uses alt to receive from the channel or execute the default case if no data is ready, demonstrating selective, non-blocking operations. Upon completion, the producer signals end-of-data with a sentinel value (e.g., -1), allowing the consumer to exit gracefully without errors.[11]
limbo
implement Main;
include "sys.m";
include "draw.m";
sys: Sys;
Main: module {
init: fn(nil: ref Draw->Context, nil: list of string);
};
init(nil: ref Draw->Context, nil: list of string) {
sys = load Sys Sys->PATH;
c: chan of int;
c = chan of int;
spawn producer(c);
spawn consumer(c);
}
producer(c: chan of int) {
for(i := 1; i <= 10; i++) {
c <-= i;
}
c <-= -1; // Sentinel for termination
}
consumer(c: chan of int) {
for(;;) {
alt {
i := <-c =>
if(i == -1) break;
sys->print("Received: %d\n", i);
* =>
sys->print("No data ready\n");
break;
}
}
}
implement Main;
include "sys.m";
include "draw.m";
sys: Sys;
Main: module {
init: fn(nil: ref Draw->Context, nil: list of string);
};
init(nil: ref Draw->Context, nil: list of string) {
sys = load Sys Sys->PATH;
c: chan of int;
c = chan of int;
spawn producer(c);
spawn consumer(c);
}
producer(c: chan of int) {
for(i := 1; i <= 10; i++) {
c <-= i;
}
c <-= -1; // Sentinel for termination
}
consumer(c: chan of int) {
for(;;) {
alt {
i := <-c =>
if(i == -1) break;
sys->print("Received: %d\n", i);
* =>
sys->print("No data ready\n");
break;
}
}
}
alt for concurrent selection across channels, where the default clause (* =>) executes if no receive is ready, preventing indefinite blocking and supporting responsive distributed systems. The producer-consumer interaction ensures error-free termination through the sentinel value, avoiding resource leaks. In the Dis virtual machine, such threading incurs low overhead, making it ideal for networked applications like those in the Inferno operating system, where channels facilitate inter-module communication across processes or machines.[1]