► |  | The Racket Reference

1 | Language Model
2 | Syntactic Forms
3 | Datatypes
4 | Structures
5 | Classes and Objects
6 | Units
7 | Contracts
8 | Pattern Matching
9 | Control Flow
10 | Concurrency and Parallelism
11 | Macros
12 | Input and Output
13 | Reflection and Security
14 | Operating System
15 | Memory Management
16 | Unsafe Operations
17 | Running Racket
| Bibliography
| Index

► | 10 | Concurrency and Parallelism

10.1 | Threads
10.2 | Synchronization
10.3 | Thread-Local Storage
10.4 | Futures
10.5 | Places
10.6 | Distributed Places
10.7 | Engines

▼ | 10.2 | Synchronization

10.2.1 | Events
10.2.2 | Channels
10.2.3 | Semaphores
10.2.4 | Buffered Asynchronous Channels

On this page:

channel?
make-channel
channel-get
channel-try-get
channel-put
channel-put-evt
channel-put-evt?

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##### 10.2.2`` Channels

A channel both synchronizes a pair of threads and passes a value from one to the other. Channels are synchronous; both the sender and the receiver must block until the (atomic) transaction is complete. Multiple senders and receivers can access a channel at once, but a single sender and receiver is selected for each transaction.

Channel synchronization is fair: if a thread is blocked on a channel and transaction opportunities for the channel occur infinitely often, then the thread eventually participates in a transaction.

In addition to its use with channel-specific procedures, a channel can be used as a synchronizable event (see Events). A channel is ready for synchronization when make-channel is ready when channel-get would not block; the channel’s synchronization result is the same as the channel-get result.

For buffered asynchronous channels, see Buffered Asynchronous Channels.

> > procedure

> > (channel? v) → boolean?

> v : any/c

Returns #t if v is a channel, #f otherwise.

> > procedure

> > (make-channel) → channel?

Creates and returns a new channel. The channel can be used with channel-get, with channel-try-get, or as a synchronizable event (see Events) to receive a value through the channel. The channel can be used with channel-put or through the result of channel-put-evt to send a value through the channel.

> > procedure

> > (channel-get ch) → any

> ch : channel?

Blocks until a sender is ready to provide a value through ch. The result is the sent value.

> > procedure

> > (channel-try-get ch) → any

> ch : channel?

Receives and returns a value from ch if a sender is immediately ready, otherwise returns #f.

> > procedure

> > (channel-put ch v) → void?

> ch : channel?
> v : any/c

Blocks until a receiver is ready to accept the value v through ch.

> > procedure

> > (channel-put-evt ch v) → channel-put-evt?

> ch : channel?
> v : any/c

Returns a fresh synchronizable event for use with sync. The event is ready for synchronization when (channel-put ch v) would not block, and the event’s synchronization result is the event itself.

> > procedure

> > (channel-put-evt? v) → boolean?

> v : any/c

Returns #t if v is a channel-put event produced by channel-put-evt, #f otherwise.
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