Reads data from a stream

Writes data to a stream

Controls a stream

getmsg(2)

Receives a message at the stream head

getpmsg(2)

Receives a priority message at the stream head

putmsg(2)

Sends a message downstream

Sends a priority message downstream

Identifies files on which a user can send or receive messages, or on which certain events have occurred (historically, it was unnecessarily restricted to streams)

pipe(2)

Creates a bidirectional channel that provides a communication path between multiple processes