STREAMS device drivers have processing routines that are registered with the framework through the streamtab structure. The put procedure is a driver's entry point, but it is a message (not system) interface. STREAMS drivers and STREAMS modules implement these entry points similarly, as described in Entry Points.
The stream head translates write(2) and ioctl(2) calls into messages and sends them downstream to be processed by the driver's write queue put(9E) procedure. read is seen directly only by the stream head, which contains the functions required to process system calls. A STREAMS driver does not check system interfaces other than open and close, but it can detect the absence of a read indirectly if flow control propagates from the stream head to the driver and affects the driver's ability to send messages upstream.
For read-side processing, when the driver is ready to send data or other information to a user process, it prepares a message and sends it upstream to the read queue of the appropriate (minor device) stream. The driver's open routine generally stores the queue address corresponding to this stream.
For write-side (or output) processing, the driver receives messages in place of a write call. If the message cannot be sent immediately to the hardware, it may be stored on the driver's write message queue. Subsequent output interrupts can remove messages from this queue.
A driver is at the end of a stream. As a result, drivers must include standard processing for certain message types that a module might be able to pass to the next component. For example, a driver must process all M_IOCTL messages; otherwise, the stream head blocks for an M_IOCNAK, M_IOCACK, or until the timeout (potentially infinite) expires. If a driver does not understand an ioctl(2), an M_IOCNAK message is sent upstream.
Messages that are not understood by the drivers should be freed.
The stream head locks up the stream when it receives an M_ERROR message, so driver developers should be careful when using the M_ERROR message.