Session Management (Job Control)

A control terminal will distinguish one of the process groups in the session associated with it to be the foreground process group. All other process groups in the session are designated as background process groups. This foreground process group plays a special role in handling signal-generating input characters, as discussed below. By default, when a controlling terminal is allocated, the controlling process's process group is assigned as foreground process group.

Background process groups in the controlling process's session are subject to a job control line discipline when they attempt to access their controlling terminal. Process groups can be sent signals that will cause them to stop, unless they have made other arrangements. An exception is made for members of orphaned process groups.

An orphaned process group is one where the process group (see getpgid(2)) has no members with a parent in a different process group but sharing the same controlling terminal. When a member of an orphaned process group attempts to access its controlling terminal, EIO is returned because there would be no way to restart the process if it were stopped on one of these signals.

If a member of a background process group attempts to read its controlling terminal, its process group will be sent a SIGTTIN signal, which will normally cause the members of that process group to stop. If, however, the process is ignoring or holding SIGTTIN, or is a member of an orphaned process group, the read will fail with errno set to EIO, and no signal is sent.

If a member of a background process group attempts to write its controlling terminal and the TOSTOP bit is set in the c_lflag field, its process group is sent a SIGTTOU signal, which will normally cause the members of that process group to stop. If, however, the process is ignoring or holding SIGTTOU, the write will succeed. If the process is not ignoring or holding SIGTTOU and is a member of an orphaned process group, the write will fail with errno set to EIO, and no signal will be sent.

If TOSTOP is set and a member of a background process group attempts to ioctl() its controlling terminal, and that ioctl() will modify terminal parameters (for example, TCSETA, TCSETAW, TCSETAF, or TIOCSPGRP), its process group will be sent a SIGTTOU signal, which will normally cause the members of that process group to stop. If, however, the process is ignoring or holding SIGTTOU, the ioctl will succeed. If the process is not ignoring or holding SIGTTOU and is a member of an orphaned process group, the write will fail with errno set to EIO, and no signal will be sent.

Canonical Mode Input Processing

Normally, terminal input is processed in units of lines. A line is delimited by a newline (ASCII LF) character, an end-of-file (ASCII EOT) character, or an end-of-line character. This means that a program attempting to read will block until an entire line has been typed. Also, no matter how many characters are requested in the read call, at most one line will be returned. It is not necessary, however, to read a whole line at once; any number of characters may be requested in a read, even one, without losing information.

During input, erase, erase2, and kill processing is normally done. The ERASE and ERASE2 character (by default, the character DEL for ERASE and Control-h for ERASE2) erases the last character typed. The WERASE character (the character Control-w) erases the last "word" typed in the current input line (but not any preceding spaces or tabs). A “word” is defined as a sequence of non-blank characters, with tabs counted as blanks. None of ERASE or ERASE2 or WERASE will erase beyond the beginning of the line. The KILL character (by default, the character NAK) kills (deletes) the entire input line, and optionally outputs a newline character. All these characters operate on a key stroke basis, independent of any backspacing or tabbing that may have been done. The REPRINT character (the character Control-r) prints a newline followed by all characters that have not been read. Reprinting also occurs automatically if characters that would normally be erased from the screen are fouled by program output. The characters are reprinted as if they were being echoed; consequently, if ECHO is not set, they are not printed.

The ERASE, ERASE2, and KILL characters may be entered literally by preceding them with the escape character. In this case, the escape character is not read. The erase, erase2, and kill characters may be changed.

Non-canonical Mode Input Processing

In non-canonical mode input processing, input characters are not assembled into lines, and erase and kill processing does not occur. The MIN and TIME values are used to determine how to process the characters received.

MIN represents the minimum number of characters that should be received when the read is satisfied (that is, when the characters are returned to the user). TIME is a timer of 0.10-second granularity that is used to timeout bursty and short-term data transmissions. The four possible values for MIN and TIME and their interactions are described below.

Case A: MIN > 0, TIME > 0

In this case, TIME serves as an intercharacter timer and is activated after the first character is received. Since it is an intercharacter timer, it is reset after a character is received. The interaction between MIN and TIME is as follows: as soon as one character is received, the intercharacter timer is started. If MIN characters are received before the intercharacter timer expires (note that the timer is reset upon receipt of each character), the read is satisfied. If the timer expires before MIN characters are received, the characters received to that point are returned to the user. Note that if TIME expires, at least one character will be returned because the timer would not have been enabled unless a character was received. In this case (MIN > 0, TIME > 0), the read sleeps until the MIN and TIME mechanisms are activated by the receipt of the first character. If the number of characters read is less than the number of characters available, the timer is not reactivated and the subsequent read is satisfied immediately.

Case B: MIN > 0, TIME = 0

In this case, since the value of TIME is zero, the timer plays no role and only MIN is significant. A pending read is not satisfied until MIN characters are received (the pending read sleeps until MIN characters are received). A program that uses this case to read record based terminal I/O may block indefinitely in the read operation.

Case C: MIN = 0, TIME > 0

In this case, since MIN 0, TIME no longer represents an intercharacter timer: it now serves as a read timer that is activated as soon as a read(2) is done. A read is satisfied as soon as a single character is received or the read timer expires. Note that, in this case, if the timer expires, no character is returned. If the timer does not expire, the only way the read can be satisfied is if a character is received. In this case, the read will not block indefinitely waiting for a character; if no character is received within TIME *.10 seconds after the read is initiated, the read returns with zero characters.

Case D: MIN = 0, TIME = 0

In this case, return is immediate. The minimum of either the number of characters requested or the number of characters currently available is returned without waiting for more characters to be input.

Comparing Different Cases of MIN, TIME Interaction

Some points to note about MIN and TIME:

• In the following explanations, note that the interactions of MIN and TIME are not symmetric. For example, when MIN > 0 and TIME = 0, TIME has no effect. However, in the opposite case, where MIN = 0 and TIME > 0, both MIN and TIME play a role in that MIN is satisfied with the receipt of a single character.
• Also note that in case A (MIN > 0, TIME > 0), TIME represents an intercharacter timer, whereas in case C (MIN = 0, TIME > 0), TIME represents a read timer.

These two points highlight the dual purpose of the MIN/TIME feature. Cases A and B, where MIN > 0, exist to handle burst mode activity (for example, file transfer programs), where a program would like to process at least MIN characters at a time. In case A, the intercharacter timer is activated by a user as a safety measure; in case B, the timer is turned off.

Cases C and D exist to handle single character, timed transfers. These cases are readily adaptable to screen-based applications that need to know if a character is present in the input queue before refreshing the screen. In case C, the read is timed, whereas in case D, it is not.

Another important note is that MIN is always just a minimum. It does not denote a record length. For example, if a program does a read of 20 bytes, MIN is 10, and 25 characters are present, then 20 characters will be returned to the user.

Writing Characters

When one or more characters are written, they are transmitted to the terminal as soon as previously written characters have finished typing. nputt characters are echoed as they are typed if echoing has been enabled. If a process produces characters more rapidly than they can be typed, it will be suspended when its output queue exceeds some limit. When the queue is drained down to some threshold, the program is resumed.

Special Characters

Certain characters have special functions on input. These functions and their default character values are summarized as follows:

INTR

(Control-c or ASCII ETX) generates a SIGINT signal. SIGINT is sent to all foreground processes associated with the controlling terminal. Normally, each such process is forced to terminate, but arrangements may be made either to ignore the signal or to receive a trap to an agreed upon location. (See signal.h(3HEAD)).

QUIT

(Control-| or ASCII FS) generates a SIGQUIT signal. Its treatment is identical to the interrupt signal except that, unless a receiving process has made other arrangements, it will not only be terminated but a core image file (called core) will be created in the current working directory.

ERASE

(DEL) erases the preceding character. It does not erase beyond the start of a line, as delimited by a NL, EOF, EOL, or EOL2 character.

ERASE2

(Control-h or ASCII BS) erases the preceding character, with behaviour identical to that of ERASE.

WERASE

(Control-w or ASCII ETX) erases the preceding “word”. It does not erase beyond the start of a line, as delimited by a NL, EOF, EOL, or EOL2 character.

KILL

(Control-u or ASCII NAK) deletes the entire line, as delimited by a NL, EOF, EOL, or EOL2 character.

REPRINT

(Control-r or ASCII DC2) reprints all characters, preceded by a newline, that have not been read.

EOF

(Control-d or ASCII EOT) may be used to generate an end-of-file from a terminal. When received, all the characters waiting to be read are immediately passed to the program, without waiting for a newline, and the EOF is discarded. Thus, if no characters are waiting (that is, the EOF occurred at the beginning of a line) zero characters are passed back, which is the standard end-of-file indication. Unless escaped, the EOF character is not echoed. Because EOT is the default EOF character, this prevents terminals that respond to EOT from hanging up.

NL

(ASCII LF) is the normal line delimiter. It cannot be changed or escaped.

EOL

(ASCII NULL) is an additional line delimiter, like NL. It is not normally used.

EOL2

is another additional line delimiter.

SWTCH

(Control-z or ASCII EM) Header file symbols related to this special character are present for compatibility purposes only and the kernel takes no special action on matching SWTCH (except to discard the character).

SUSP

(Control-z or ASCII SUB) generates a SIGTSTP signal. SIGTSTP stops all processes in the foreground process group for that terminal.

DSUSP

(Control-y or ASCII EM). It generates a SIGTSTP signal as SUSP does, but the signal is sent when a process in the foreground process group attempts to read the DSUSP character, rather than when it is typed.

STOP

(Control-s or ASCII DC3) can be used to suspend output temporarily. It is useful with CRT terminals to prevent output from disappearing before it can be read. While output is suspended, STOP characters are ignored and not read.

START

(Control-q or ASCII DC1) is used to resume output. Output has been suspended by a STOP character. While output is not suspended, START characters are ignored and not read.

DISCARD

(Control-o or ASCII SI) causes subsequent output to be discarded. Output is discarded until another DISCARD character is typed, more input arrives, or the condition is cleared by a program.

STATUS

(Control-t or ASCII DC4) generates a SIGINFO signal. Processes with a handler will output status information when they receive SIGINFO, for example, dd(8). If a process does not have a SIGINFO handler, the signal will be ignored.

LNEXT

(Control-v or ASCII SYN) causes the special meaning of the next character to be ignored. This works for all the special characters mentioned above. It allows characters to be input that would otherwise be interpreted by the system (for example KILL, QUIT). The character values for INTR, QUIT, ERASE, ERASE2, WERASE, KILL, REPRINT, EOF, EOL, EOL2, SWTCH, SUSP, DSUSP, STOP, START, DISCARD, STATUS, and LNEXT may be changed to suit individual tastes. If the value of a special control character is _POSIX_VDISABLE (0), the function of that special control character is disabled. The ERASE, ERASE2, KILL, and EOF characters may be escaped by a preceding backslash (\) character, in which case no special function is done. Any of the special characters may be preceded by the LNEXT character, in which case no special function is done.

Modem Disconnect

When a modem disconnect is detected, a SIGHUP signal is sent to the terminal's controlling process. Unless other arrangements have been made, these signals cause the process to terminate. If SIGHUP is ignored or caught, any subsequent read returns with an end-of-file indication until the terminal is closed.

If the controlling process is not in the foreground process group of the terminal, a SIGTSTP is sent to the terminal's foreground process group. Unless other arrangements have been made, these signals cause the processes to stop.

Processes in background process groups that attempt to access the controlling terminal after modem disconnect while the terminal is still allocated to the session will receive appropriate SIGTTOU and SIGTTIN signals. Unless other arrangements have been made, this signal causes the processes to stop.

The controlling terminal will remain in this state until it is reinitialized ithh a successful open by the controlling process, or deallocated by the controlling process.

Terminal Parameters

The parameters that control the behavior of devices and modules providing the termios interface are specified by the termios structure defined by <termios.h>. Several ioctl(2) system calls that fetch or change these parameters use this structure that contains the following members:

tcflag_t	c_iflag;	/* input modes */
tcflag_t	c_oflag;	/* output modes */
tcflag_t	c_cflag;	/* control modes */
tcflag_t	c_lflag;	/* local modes */
cc_t		c_cc[NCCS];	/* control chars */

The special control characters are defined by the array c_cc. The symbolic name NCCS is the size of the Control-character array and is also defined by <termios.h>. The relative positions, subscript names, and typical default values for each function are as follows:

Input Modes

The c_iflag field describes the basic terminal input control:

If IGNBRK is set, a break condition (a character framing error with data all zeros) detected on input is ignored, that is, not put on the input queue and therefore not read by any process. If IGNBRK is not set and BRKINT is set, the break condition shall flush the input and output queues and if the terminal is the controlling terminal of a foreground process group, the break condition generates a single SIGINT signal to that foreground process group. If neither IGNBRK nor BRKINT is set, a break condition is read as a single ‘\0’ (ASCII NULL) character, or if PARMRK is set, as ‘\377’, ‘\0’, c, where ‘\377’ is a single character with value 377 octal (0xff hex, 255 decimal), ‘\0’ is a single character with value 0, and c is the errored character received.

If IGNPAR is set, a byte with framing or parity errors (other than break) is ignored.

If PARMRK is set, and IGNPAR is not set, a byte with a framing or parity error (other than break) is given to the application as the three-character sequence: ‘\377’, ‘\0’, c, where ‘\377’ is a single character with value 377 octal (0xff hex, 255 decimal), ‘\0’ is a single character with value 0, and c is the errored character received. To avoid ambiguity in this case, if ISTRIP is not set, a valid character of ‘\377’ is given to the application as ‘\377’. If neither IGNPAR nor PARMRK is set, a framing or parity error (other than break) is given to the application as a single ‘\0’ (ASCII NULL) character.

If INPCK is set, input parity checking is enabled. If INPCK is not set, input parity checking is disabled. This allows output parity generation without input parity errors. Note that whether input parity checking is enabled or disabled is independent of whether parity detection is enabled or disabled. If parity detection is enabled but input parity checking is disabled, the hardware to which the terminal is connected will recognize the parity bit, but the terminal special file will not check whether this is set correctly or not.

If ISTRIP is set, valid input characters are first stripped to seven bits, otherwise all eight bits are processed.

If INLCR is set, a received NL character is translated into a CR character. If IGNCR is set, a received CR character is ignored (not read). Otherwise, if ICRNL is set, a received CR character is translated into a NL character.

If IUCLC is set, a received upper case, alphabetic character is translated into the corresponding lower case character.

If IXON is set, start/stop output control is enabled. A received STOP character suspends output and a received START character restarts output. The STOP and START characters will not be read, but will merely perform flow control functions. If IXANY is set, any input character restarts output that has been suspended.

If IXOFF is set, the system transmits a STOP character when the input queue is nearly full, and a START character when enough input has been read so that the input queue is nearly empty again.

If IMAXBEL is set, the ASCII BEL character is echoed if the input stream overflows. Further input is not stored, but any input already present in the input stream is not disturbed. If IMAXBEL is not set, no BEL character is echoed, and all input present in the input queue is discarded if the input stream overflows.

Output Modes

The c_oflag field specifies the system treatment of output:

If OPOST is set, output characters are post-processed as indicated by the remaining flags; otherwise, characters are transmitted without change.

If OLCUC is set, a lower case alphabetic character is transmitted as the corresponding upper case character. This function is often used in conjunction with IUCLC.

If ONLCR is set, the NL character is transmitted as the CR-NL character pair. If OCRNL is set, the CR character is transmitted as the NL character. If ONOCR is set, no CR character is transmitted when at column 0 (first position). If ONRET is set, the NL character is assumed to do the carriage-return function; the column pointer is set to 0 and the delays specified for CR are used. Otherwise, the NL character is assumed to do just the line-feed function; the column pointer remains unchanged. The column pointer is also set to 0 if the CR character is actually transmitted.

The delay bits specify how long transmission stops to allow for mechanical or other movement when certain characters are sent to the terminal. In all cases, a value of 0 indicates no delay. If OFILL is set, fill characters are transmitted for delay instead of a timed delay. This is useful for high baud rate terminals that need only a minimal delay. If OFDEL is set, the fill character is DEL; otherwise it is NULL.

If a form-feed or vertical-tab delay is specified, it lasts for about 2 seconds.

Newline delay lasts about 0.10 seconds. If ONLRET is set, the carriage-return delays are used instead of the newline delays. If OFILL is set, two fill characters are transmitted.

Carriage-return delay type 1 is dependent on the current column position, type 2 is about 0.10 seconds, and type 3 is about 0.15 seconds. If OFILL is set, delay type 1 transmits two fill characters, and type 2 transmits four fill characters.

Horizontal-tab delay type 1 is dependent on the current column position. Type 2 is about 0.10 seconds. Type 3 specifies that tabs are to be expanded into spaces. If OFILL is set, two fill characters are transmitted for any delay.

Backspace delay lasts about 0.05 seconds. If OFILL is set, one fill character is transmitted.

The actual delays depend on line speed and system load.

Control Modes

The c_cflag field describes the hardware control of the terminal:

The CBAUD bits together with the CBAUDEXT bit specify the output baud rate. To retrieve the output speed from the termios structure pointed to by termios_p see the following code segment.

speed_t ospeed;
if (termios_p->c_cflag & CBAUDEXT)
 ospeed = (termios_p->c_cflag & CBAUD) + CBAUD + 1;
else
 ospeed = termios_p->c_cflag & CBAUD;

To store the output speed in the termios structure pointed to by termios_p see the following code segment.

speed_t ospeed;
if (ospeed > CBAUD) {
 termios_p->c_cflag |= CBAUDEXT;
 ospeed -= (CBAUD + 1);
} else {
 termios_p->c_cflag &= ~CBAUDEXT;
}
termios_p->c_cflag =
  (termios_p->c_cflag & ~CBAUD) | (ospeed & CBAUD);

The zero baud rate, B0, is used to hang up the connection. If B0 is specified, the data-terminal-ready signal is not asserted. Normally, this disconnects the line.

If the CIBAUDEXT or CIBAUD bits are not zero, they specify the input baud rate, with the CBAUDEXT and CBAUD bits specifying the output baud rate; otherwise, the output and input baud rates are both specified by the CBAUDEXT and CBAUD bits. The values for the CIBAUD bits are the same as the values for the CBAUD bits, shifted left IBSHIFT bits. For any particular hardware, impossible speed changes are ignored. To retrieve the input speed in the termios structure pointed to by termios_p see the following code segment.

speed_t ispeed;
if (termios_p->c_cflag & CIBAUDEXT) {
 ispeed = ((termios_p->c_cflag & CIBAUD) >> IBSHIFT)
 + (CIBAUD >> IBSHIFT) + 1;
} else {
 ispeed = (termios_p->c_cflag & CIBAUD) >> IBSHIFT;
}

To store the input speed in the termios structure pointed to by termios_p see the following code segment.

speed_t ispeed;
if (ispeed == 0) {
 ispeed = termios_p->c_cflag & CBAUD;
 if (termios_p->c_cflag & CBAUDEXT)
     ispeed += (CBAUD + 1);
}
if ((ispeed << IBSHIFT) > CIBAUD) {
  termios_p->c_cflag |= CIBAUDEXT;
  ispeed -= ((CIBAUD >> IBSHIFT) + 1);
} else {
  termios_p->c_cflag &= ~CIBAUDEXT;
}
termios_p->c_cflag =
  (termios_p->c_cflag & ~CIBAUD) | ((ispeed << IBSHIFT) & CIBAUD);

The CSIZE bits specify the character size in bits for both transmission and reception. This size does not include the parity bit, if any. If CSTOPB is set, two stop bits are used; otherwise, one stop bit is used. For example, at 110 baud, two stops bits are required.

If PARENB is set, parity generation and detection is enabled, and a parity bit is added to each character. If parity is enabled, the PARODD flag specifies odd parity if set; otherwise, even parity is used.

If CREAD is set, the receiver is enabled. Otherwise, no characters are received.

If HUPCL is set, the line is disconnected when the last process with the line open closes it or terminates. That is, the data-terminal-ready signal is not asserted.

If CLOCAL is set, the line is assumed to be a local, direct connection with no modem control; otherwise, modem control is assumed.

If CRTSXOFF is set, inbound hardware flow control is enabled.

If CRTSCTS is set, outbound hardware flow control is enabled.

The four possible combinations for the state of CRTSCTS and CRTSXOFF bits and their interactions are described below.

Case A:

CRTSCTS off, CRTSXOFF off. In this case the hardware flow control is disabled.

Case B:

CRTSCTS on, CRTSXOFF off. In this case only outbound hardware flow control is enabled. The state of CTS signal is used to do outbound flow control. It is expected that output will be suspended if CTS is low and resumed when CTS is high.

Case C:

CRTSCTS off, CRTSXOFF on. In this case only inbound hardware flow control is enabled. The state of RTS signal is used to do inbound flow control. It is expected that input will be suspended if RTS is low and resumed when RTS is high.

Case D:

CRTSCTS on, CRTSXOFF on. In this case both inbound and outbound hardware flow control are enabled. Uses the state of CTS signal to do outbound flow control and RTS signal to do inbound flow control.

Local Modes

The c_lflag field of the argument structure is used by the line discipline to control terminal functions. The basic line discipline provides the following:

If ISIG is set, each input character is checked against the special control characters INTR, QUIT, SWTCH, SUSP, STATUS, and DSUSP. If an input character matches one of these control characters, the function associated with that character is performed. (Note: If SWTCH is set and the character matches, the character is simply discarded. No other action is taken.) If ISIG is not set, no checking is done. Thus, these special input functions are possible only if ISIG is set.

If ICANON is set, canonical processing is enabled. This enables the erase and kill edit functions, and the assembly of input characters into lines delimited by NL-c, EOF, EOL, and EOL. If ICANON is not set, read requests are satisfied directly from the input queue. A read is not satisfied until at least MIN characters have been received or the timeout value TIME has expired between characters. This allows fast bursts of input to be read efficiently while still allowing single character input. The time value represents tenths of seconds.

If XCASE is set and ICANON is set, an upper case letter is accepted on input if preceded by a backslash ‘\’ character, and is output preceded by a backslash ‘\’ character. In this mode, the following escape sequences are generated on output and accepted on input:

For example, input A as \a, \n as \\n, and \N as \\\n.

If ECHO is set, characters are echoed as received.

When ICANON is set, the following echo functions are possible.

• If ECHO and ECHOE are set, and ECHOPRT is not set, the ERASE, ERASE2, and WERASE characters are echoed as one or more ASCII BS SP BS, which clears the last character(s) from a CRT screen.
• If ECHO, ECHOPRT, and IEXTEN are set, the first ERASE, ERASE2, and WERASE character in a sequence echoes as a backslash ‘\’, followed by the characters being erased. Subsequent ERASE and WERASE characters echo the characters being erased, in reverse order. The next non-erase character causes a ‘/’ (slash) to be typed before it is echoed. ECHOPRT should be used for hard copy terminals.
• If ECHOKE and IEXTEN are set, the kill character is echoed by erasing each character on the line from the screen (using the mechanism selected by ECHOE and ECHOPR).
• If ECHOK is set, and ECHOKE is not set, the NL character is echoed after the kill character to emphasize that the line is deleted. Note that a ‘\’ (escape) character or an LNEXT character preceding the erase or kill character removes any special function.
• If ECHONL is set, the NL character is echoed even if ECHO is not set. This is useful for terminals set to local echo (so called half-duplex).

If ECHOCTL and IEXTEN are set, all control characters (characters with codes between 0 and 37 octal) other than ASCII TAB, ASCII NL, the START character, and the STOP character, ASCII CR, and ASCII BS are echoed as ^X, where X is the character given by adding ‘100’ octal to the code of the control character (so that the character with octal code ‘1’ is echoed as ^A), and the ASCII DEL character, with code ‘177’ octal, is echoed as ^?.

If NOFLSH is set, the normal flush of the input and output queues associated with the INTR, QUIT, STATUS, and SUSP characters is not done. This bit should be set when restarting system calls that read from or write to a terminal (see sigaction(2)).

If TOSTOP and IEXTEN are set, the signal SIGTTOU is sent to a process that tries to write to its controlling terminal if it is not in the foreground process group for that terminal. This signal normally stops the process. Otherwise, the output generated by that process is output to the current output stream. Processes that are blocking or ignoring SIGTTOU signals are excepted and allowed to produce output, if any.

If FLUSHO and IEXTEN are set, data written to the terminal is discarded. This bit is set when the FLUSH character is typed. A program can cancel the effect of typing the FLUSH character by clearing FLUSHO.

If PENDIN and IEXTEN are set, any input that has not yet been read is reprinted when the next character arrives as input. PENDIN is then automatically cleared.

If IEXTEN is set, the following implementation-defined functions are enabled: special characters ( WERASE, REPRINT, DISCARD, and LNEXT) and local flags ( TOSTOP, ECHOCTL, ECHOPRT, ECHOKE, FLUSHO, and PENDIN).

Minimum and Timeout

The MIN and TIME values were described previously, in the subsection, Non-canonical Mode Input Processing. The initial value of MIN is 1, and the initial value of TIME is 0.

Terminal Size

The number of lines and columns on the terminal's display is specified in the winsize structure defined by <sys/termios.h> and includes the following members:

unsigned	short ws_row;  /* rows, in characters */
unsigned short	ws_col;        /* columns, in characters */
unsigned short	ws_xpixel;     /* horizontal size, in pixels */
unsigned short	ws_ypixel;     /* vertical size, in pixels */

Termio Structure

The SunOS/SVR4 termio structure is used by some ioctl()s; it is defined by <sys/termio.h> and includes the following members:

unsigned	short	c_iflag;    /* input modes */
unsigned	short	c_oflag;    /* output modes */
unsigned	short	c_cflag;    /* control modes */
unsigned	short	c_lflag;    /* local modes */
char			c_line;     /* line discipline */
unsigned	char	c_cc[NCC];  /* control chars */

The special control characters are defined by the array c_cc. The symbolic name NCC is the size of the Control-character array and is also defined by <termio.h>. The relative positions, subscript names, and typical default values for each function are as follows:

The MIN values is stored in the VMIN element of the c_cc array; the TIME value is stored in the VTIME element of the c_cc array. The VMIN element is the same element as the VEOF element; the VTIME element is the same element as the VEOL element.

The calls that use the termio structure only affect the flags and control characters that can be stored in the termio structure; all other flags and control characters are unaffected.

Modem Lines

On special files representing serial ports, modem control lines can be read. Control lines (if the underlying hardware supports it) may also be changed. Status lines are read-only. The following modem control and status lines may be supported by a device; they are defined by <sys/termios.h>:

TIOCM_CD is a synonym for TIOCM_CAR, and TIOCM_RI is a synonym for TIOCM_RNG. Not all of these are necessarily supported by any particular device; check the manual page for the device in question.

The software carrier mode can be enabled or disabled using the TIOCSSOFTCAR ioctl(). If the software carrier flag for a line is off, the line pays attention to the hardware carrier detect (DCD) signal. The tty device associated with the line cannot be opened until DCD is asserted. If the software carrier flag is on, the line behaves as if DCD is always asserted.

The software carrier flag is usually turned on for locally connected terminals or other devices, and is off for lines with modems.

To be able to issue the TIOCGSOFTCAR and TIOCSSOFTCAR ioctl() calls, the tty line should be opened with O_NDELAY so that the open(2) will not wait for the carrier.

Default Values

The initial termios values upon driver open is configurable. This is accomplished by setting the "ttymodes" property in the file /kernel/drv/options.conf. Since this property is assigned during system initialization, any change to the "ttymodes" property will not take effect until the next reboot. The string value assigned to this property should be in the same format as the output of the stty(1) command with the -g option.

If this property is undefined, the following termios modes are in effect. The initial input control value is BRKINT, ICRNL, IXON, IMAXBEL. The initial output control value is OPOST, ONLCR, TAB3. The initial hardware control value is B9600, CS8, CREAD. The initial line-discipline control value is ISIG, ICANON, IEXTEN, ECHO, ECHOK, ECHOE, ECHOKE, ECHOCTL.