In computing, a channel is a model for interprocess communication and synchronization via message passing. A message may be sent over a channel, and another process or thread is able to synchronously receive messages sent over a channel it has a reference to, as a stream.
Channels are fundamental to the process calculus approach to concurrency, and first originated in communicating sequential processes (CSP), a formal model for concurrency, and has been used in many derived languages, such as occam, and Limbo programming language (via Newsqueak and the Alef programming language). They are also used in the C programming language threading library libthread, and in Plan 9 from Bell Labs, which uses libthread, as well as in Stackless Python and the Go programming language.
Most Channel implementations are modeled close to the CSP model. Channels are inherently synchronous: a process waiting to receive an object from a channel will block until the object is sent. This is also called rendezvous behaviour. Typical supported operations are presented below using the example of the libthread channel API.
- Channel creation of fixed or variable size, returning a reference or handle
Channel* chancreate(int elemsize, int bufsize)
- sending to a channel
int chansend(Channel *c, void *v)
- receiving from a channel
int chanrecv(Channel *c, void *v)
The Multithreading library, libthread, which was first created for the operating system Plan 9 and later adopted by several Unix systems as well, offers inter-thread communication based on fixed-size channels.
The OCaml event module offers typed channels for synchronization. When the module's send and receive functions are called, they create corresponding send and receive events which can be synchronized.
In addition to their fundamental use for interprocess communication, channels can be used as a primitive to implement various other concurrent programming constructs which can be realized as streams. For example, channels can be used to construct futures and promises, where a future is a one-element channel, and a promise is a process that sends to the channel, fulfilling the future. Similarly, iterators can be constructed directly from channels.
- – Libthread Channel Implementation
- Bell Labs and CSP Threads
- Limbo – Inferno Application Programming
- Stackless.com – Channels
- – OCaml Events
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