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.