The stream head interacts between applications in the user space and the rest of the stream in kernel space. The stream head is responsible for configuring the plumbing of the stream through open, close, push, pop, link, and unlink operations. It also translates user data into messages to be passed down the stream, and translates messages that arrive at the stream head into user data. Any characteristics of the stream that can be modified by the user application or the underlying stream are controlled by the stream head, which also informs users of data arrival and events such as error conditions.
If an application makes a system call with a STREAMS file descriptor, the stream head routines are invoked, resulting in data copying, message generation, or control operations. Only the stream head can copy data between the user space and kernel space. All other parts of the stream pass data by way of messages and are thus isolated from direct interaction with users of the stream.