STREAMS Programming Guide

Line-Discipline Module

A STREAMS line-discipline module called ldterm (see ldterm(7M)) is a key part of the STREAMS-based terminal subsystem. Throughout this chapter, the terms line discipline and ldterm(7M) are used interchangeably and refer to the STREAMS version of the standard line discipline and not the traditional character version. ldterm performs the standard terminal I/O processing traditionally done through the linesw mechanism.

The termio(7I) and termios(3) specifications describe four flags that are used to control the terminal:

To process these flags elsewhere (for example, in the firmware or in another process), a mechanism is in place to turn on and off the processing of these flags. When ldterm(7M) is pushed, it sends an M_CTL message downstream that asks the driver which flags the driver will process. The driver sends back that message in response if it needs to change the ldterm default processing. By default, ldterm(7M) assumes that it must process all flags except c_cflag, unless it receives a message indicating otherwise.

Default Settings

When ldterm is pushed on the stream, the open routine initializes the settings of the termio flags. The default settings are:

c_oflag = OPOST|ONLCR|TAB3
c_cflag = CREAD|CS8|B9600

In canonical mode (ICANON flag in c_lflag is turned on), read from the terminal file descriptor is in message non-discard (RMSGN) mode (see streamio(7I)). This implies that in canonical mode, read on the terminal file descriptor always returns at most one line, regardles of how many characters have been requested. In non-canonical mode, read is in byte-stream (RNORM) mode. The flag ECHOCTL has been added for SunOS 4.1 compatibility.

User-Configurable Settings

See termio(7I) for more information on user-configurable settings.

Open and Close Routines

The open routine of the ldterm(7M) module allocates space for holding the TTY structure (see ldtermstd_state_t in ldterm.h) by allocating a buffer from the STREAMS buffer pool. The number of modules that can be pushed on one stream, as well as the number of TTY's in use, is limited. The number of instances of ldterm that have been pushed is limited only by available memory. The open also sends an M_SETOPTS message upstream to set the stream head high-water and low-water marks to 1024 and 200, respectively. These are the current values (although they may change over time).

The ldterm module identifies itself as a TTY to the stream head by sending an M_SETOPTS message upstream with the SO_ISTTY bit of so_flags set. The stream head allocates the controlling TTY on the open, if one is not already allocated.

To maintain compatibility with existing applications that use the O_NDELAY flag, the open routine sets the SO_NDELON flag on in the so_flags field of the stroptions(9S) structure in the M_SETOPTS message.

The open routine fails if there are no buffers available (cannot allocate the internal state structure) or when an interrupt occurs while waiting for a buffer to become available.

The close routine frees all the outstanding buffers allocated by this stream. It also sends an M_SETOPTS message to the stream head to undo the changes made by the open routine. The ldterm(7M) module also sends M_START messages downstream to undo the effect of any previous M_STOP messages.

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.

Write-Side Processing

Write side processing of the ldterm(7M) module is performed by the write-side put and service procedures.

The ldterm module supports the following ioctls:


All ioctl(2) not recognized by the ldterm(7M) module are passed downstream to the neighboring module or driver.

The following messages can be received on the write side:


On the write side, the ldterm module processes M_FLUSH, M_DATA, M_IOCTL, and M_READ messages, and all other messages are passed downstream unchanged.

An M_CTL message is generated by ldterm(7M) as a query to the driver for an intelligent peripheral and to decide on the functional split for termio(7I) processing. If all or part of termio(7I) processing is done by the intelligent peripheral, ldterm(7M) can turn off this processing to avoid computational overhead. This is done by sending an appropriate response to the M_CTL message, as follows:

One difference between AT&T STREAMS and SunOS 5 is that AT&T's line discipline module does not check if write side flow control is in effect before forwarding data downstream. It expects the downstream module or driver to add the messages to its queue until flow control is lifted. This is not true in SunOS 5.

EUC Handling in ldterm

The idea of letting post-processing (the o_flags) happen off the host processor is not recommended unless the board software is prepared to deal with international (EUC) character sets properly. The reason for this is that post-processing must take the EUC information into account. ldterm(7M) knows about the screen width of characters (that is, how many columns are taken by characters from each given code set on the current physical display) and it takes this width into account when calculating tab expansions. When using multi-byte characters or multi-column characters ldterm automatically handles tab expansion (when TAB3 is set) and does not leave this handling to a lower module or driver.

By default, multi-byte handling by ldterm is turned off. When ldterm receives an EUC_WSET ioctl(2), it turns multi-byte processing on, if it is essential to properly handle the indicated code set. Thus, if you use single byte 8-bit codes and has no special multi-column requirements, the special multi-column processing is not used at all. This means that multi-byte processing does not reduce the processing speed or efficiency of ldterm unless it is actually used.

The following describes how the EUC handling in ldterm works:

First, the multi-byte and multi-column character handling is only enabled when the EUC_WSET ioctl indicates that one of the following conditions is met:

Assuming that one or more of the previous conditions exists, EUC handling is enabled. At this point, a parallel array (see ldterm_mod structure) used for other information is allocated and a pointer to it is stored in t_eucp_mp. The parallel array that it holds is pointed to by t_eucp. The t_codeset field holds the flag that indicates which of the code sets is currently being processed on the read side. When a byte with the high bit arrives, it is checked to see if it is SS2 or SS3. If yes, it belongs to code set 2 or 3. Otherwise, it is a byte that comes from code set 1. Once the extended code set flag has been set, the input processor retrieves the subsequent bytes, as they arrive, to build one multi-byte character. The counter field t_eucleft tells the input processor how many bytes remain to be read for the current character. The parallel array t_eucp holds its display width for each logical character in the canonical buffer. During erase processing, positions in the parallel array are consulted to determine how many backspaces need to be send to erase each logical character. (In canonical mode, one backspace of input erases one logical character, no matter how many bytes or columns that character consumes.) This greatly simplifies erase processing for EUC.

The t_maxeuc field holds the maximum length, in memory bytes, of the EUC character mapping currently in use. The eucwioc field is a substructure that holds information about each extended code set.

The t_eucign field aids in output post-processing (tab expansion). When characters are output, ldterm(7M) keeps a column to indicate what the current cursor column is supposed to be. When it sends the first byte of an extended character, it adds the number of columns required for that character to the output column. It then subtracts one from the total width in memory bytes of that character and stores the result in t_eucign. This field tells ldterm(7M) how many subsequent bytes to ignore for the purposes of column calculation. (ldterm(7M) calculates the appropriate number of columns when it sees the first byte of the character.)

The field t_eucwarn is a counter for occurrences of bad extended characters. It is mostly useful for debugging. After receiving a certain number of illegal EUC characters (perhaps because of some problem on the line or with declared values), a warning is given on the system console.

There are two relevant files for handling multi-byte characters: euc.h and eucioctl.h. eucioctl.h contains the structure that is passed with EUC_WSET and EUC_WGET calls. The normal way to use this structure is to get CSWIDTH from the locale using a mechanism such as getwidth(3C) or setlocale(3C) and then copying the values into the structure in eucioctl.h, and sending the structure using an I_STR ioctl(2). The EUC_WSET call informs the ldterm(7M) module about the number of bytes in extended characters and how many columns the extended characters from each set consume on the screen. This allows ldterm(7M) to treat multi-byte characters as single units for the purpose of erase processing and to correctly calculate tab expansions for multi-byte characters.

Note -

LC_CTYPE (instead of CSWIDTH) should be used in the environment in SunOS 5 systems.

The file euc.h has the structure with fields for EUC width, screen width, and wide character width. The following functions are used to set and get EUC widths (these functions assume the environment where the eucwidth_t structure is needed and available):

Example 14-1 EUC

#include <eucioctl.h>								/* need others,like stropts.h*/

struct eucioc eucw;								/*for EUC_WSET/WGET to line disc*/
eucwidth_t width;								/* ret struct from _getwidth() */
 * set_euc					Send EUC code widths to line discipline.
set_euc(struct eucioc *e)
	struct strioctl sb;

	sb.ic_cmd = EUC_WSET;
	sb.ic_timout = 15;
	sb.ic_len = sizeof(struct eucioc);
	sb.ic_dp = (char *) e;

	if (ioctl(0, I_STR, &sb) < 0)
 * euclook.   Get current EUC code widths from line discipline.
euclook(struct eucioc *e)
	struct strioctl sb;

	sb.ic_cmd = EUC_WGET;
	sb.ic_timout = 15;
	sb.ic_len = sizeof(struct eucioc);
	sb.ic_dp = (char *) e;

	if (ioctl(0, I_STR, &sb) < 0)

			e->eucw[1], e->scrw[1],
			e->eucw[2], e->scrw[2],
			e->eucw[3], e->scrw[3]);

For more detailed descriptions, see System Interface Guide.