STREAMS Programming Guide

Pseudo-TTY Emulation Module: ptem

Because the pseudo-TTY subsystem has no hardware driver downstream from the ldterm(7M) module to process the terminal ioctl(2) calls, another module that understands the ioctl commands is placed downstream from the ldterm(7M). This module, ptem(7M), processes all of the terminal ioctl(2)calls and mediates the passage of control information downstream.

ldterm(7M) and ptem(7M) together behave like a real terminal. Since there is no real terminal or modem in the pseudo-TTY subsystem, some of the ioctl(2) commands are ignored and cause only an acknowledgment of the command. ptem(7M) keeps track of the terminal parameters set by the various set commands such as TCSETA or TCSETAW but does not usually perform any action. For example, if a “set” ioctl is called, none of the bits in the c_cflag field of termio(7I) has any effect on the pseudo-terminal unless the baud rate is set to 0. Setting the baud rate to 0 has the effect of hanging up the pseudo-terminal.

The pseudo-terminal does not recognize parity, so none of the flags in the c_iflag that control the processing of parity errors have any effect. The delays specified in the c_oflag field are also not supported.

ptem(7M) does the following:

ptem Data Structure

Each instantiation of ptem(7M) is associated with a local area. These data are held in a structure called ptem that has the following format:

struct ptem
	long cflags;					/* copy of c_flags */
	mblk_t *dack_ptr;			/* pointer to preallocated msg blk
									   used to send disconnect */
	queue_t *q_ptr;				/* pointer to ptem's read queue */
	struct winsize wsz;			/*struct to hold windowing info*/
	unsigned short state;		/* state of ptem entry */

When ptem(7M) is pushed onto the slave side stream, a search of the ptem structure is made for a free entry (state is not set to INUSE). The c_cflags of the termio(7I) structure and the windowing variables are stored in cflags and wsz respectively. The dack_ptr is a pointer to a message block used to send a zero-length message whenever a hang-up occurs on the slave side.

Note –

ptem(7M) internal implementation might change. This structure should be relevant only to people wanting to change the module.

open and close Routines

Caution – Caution –

The following information is implementation-dependent.

In the open routine of ptem(7M) a STREAMS message block is allocated for a zero-length message for delivering a hangup message. This allocation of a buffer is done before it is needed to ensure that a buffer is available. An M_SETOPTS message is sent upstream to set the read-side stream head queues, to assign high-water and low-water marks (1024 and 256 respectively), and to establish a controlling terminal.

The same default values as for the line-discipline module are assigned to cflags, and INUSE to the state field.

Note –

These default values are currently being examined and may change in the future.

The open routine fails if:

The close routine is called on the last close of the slave-side stream. Pointers to read and write queues are cleared and the buffer for the zero-length message is freed.