Packet mode

The STREAMS-based pseudo-terminal subsystem also supports a feature called packet mode. This is used to inform the process on the master-side when ``state'' changes have occurred in the pseudo-tty. Packet mode is enabled by pushing the pckt module on the master-side. Data written on the master-side is processed normally. When data is written on the slave-side or when other messages are encountered by the pckt module, a header is added to the message so it can be retrieved later by the master-side with a getmsg operation.

The pckt module does the following:

• When a message is passed to this module on its write queue, the module does no processing and passes the message to the next module or driver.

• The pckt module creates an M_PROTO message when one of the following messages is passed to it:

  M_DATA, M_IOCTL, M_PROTO/M_PCPROTO, M_FLUSH, M_START/M_STOP, M_STARTI/M_STOPI, and M_READ.

  All other messages are passed through. The M_PROTO message is passed upstream and retrieved when the user issues getmsg(2).

• If the message is an M_FLUSH message, pckt does the following:

  If the flag is FLUSHW, it is changed to FLUSHR (because FLUSHR was the original flag before the pts driver changed it), packetized into an M_PROTO message, and passed upstream. To prevent the Stream head's read queue from being flushed, the original M_FLUSH message must not be passed upstream.

  If the flag is FLUSHR, it is changed to FLUSHW, packetized into an M_PROTO message, and passed upstream. To flush the write queues properly, an M_FLUSH message with the FLUSHW flag set is also sent upstream.

  If the flag is FLUSHRW, the message with both flags set is packetized and passed upstream. An M_FLUSH message with the FLUSHW flag set is also sent upstream.