STREAMS Programming Guide

Read-Side Processing

The ldterm(7M) module's read side processing has put and service procedures. High-waterand low-water marks for the read queue are 1024 and 200, respectively. These are the current values and may be subject to change.

ldterm(7M) can send the following messages upstream:


The ldterm(7M) module's read side processes M_BREAK, M_DATA, M_CTL, M_FLUSH, M_HANGUP, M_IOCACK and M_IOCNAK messages. All other messages are sent upstream unchanged.

The put procedure scans the message for flow-control characters (IXON), signal-generating characters, and after (possible) transformation of the message, queues the message for the service procedure. Echoing is handled completely by the service procedure.

In canonical mode, if the ICANON flag is on in c_lflag, canonical processing is performed. If the ICANON flag is off, non-canonical processing is performed (see termio(7I) for more details). Handling of VMIN/VTIME in the STREAMS environment is somewhat complicated, because read needs to activate a timer in the ldterm module in some cases; hence, read notification becomes necessary. When a user issues an ioctl(2) to put ldterm(7M) in non-canonical mode, the module sends an M_SETOPTS message to the Stream head to register read notification. Further reads on the terminal file descriptor will cause the Stream head to issue an M_READ message downstream and data will be sent upstream in response to the M_READ message. With read notification, buffering of raw data is performed by ldterm(7M). It is possible to canonize the raw data when the user has switched from raw to canonical mode. However, the reverse is not possible.

To summarize, in non-canonical mode, the ldterm(7M) module buffers all data until VMIN or VTIME criteria are met. For example, if VMIN=3 and VTIME=0, and three bytes have been buffered, these characters are sent to the stream head regardless of whether there is a pending M_READ, and no M_READ needs to be sent down. If an M_READ message is received, the number of bytes sent upstream is the argument of the M_READ message, unless VTIME is satisfied before VMIN (for example. the timer has expired) in which case whatever characters are available will be sent upstream.

The service procedure of ldterm(7M) handles STREAMS related flow control. Since the read side high-waterand low-water marks are 1024 and 200 respectively, placing 1024 characters or more on the read queue causes the QFULL flag be turned on, indicating that the module below should not send more data upstream.

Input flow control is regulated by the line-discipline module which generates M_STARTI and M_STOPI high priority messages. When sent downstream, receiving drivers or modules take appropriate action to regulate the sending of data upstream. Output flow control is activated when ldterm(7M) receives flow control characters in its data stream. The module then sets an internal flag indicating that output processing is to be restarted/stopped and sends an M_START/M_STOP message downstream.