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tty(7) [osf1 man page]

tty(7)							 Miscellaneous Information Manual						    tty(7)

NAME
tty - General terminal interface SYNOPSIS
#include <sys/termios.h> DESCRIPTION
This section describes both a particular special file /dev/tty and the terminal drivers used for conversational computing. Much of the terminal interface performance is governed by the settings of a terminal's termios structure. This structure provides definitions for ter- minal input and output processing, control and local modes, and so on. These definitions are found in the termios.h header file. Line Disciplines The operating system supports STREAMS-based and clist-based line disciplines, which allows for both STREAMS- and clist-based terminal driv- ers. Both line disciplines are POSIX compliant. The STREAMS-based line discipline is implemented as the STREAMS module ldterm. This line discipline performs most of the functions defined by the termios interface for session management and character processing but not some low level device control. In order to determine if a terminal device is implemented as a STREAMS device, use the libc function isastream on the open file descriptor. See the isastream(3) ref- erence page. Users can switch line disciplines by using the command strchg or the I_POP or I_PUSH STREAMS ioctls. The ldterm line discipline is the only STREAMS-based line discipline provided by the base operating system for use with terminals. However, additional STREAMS modules to support features needed in countries other than the United States can be pushed above and below the ldterm module. The operating system supports several clist-based line disciplines for controlling communication lines. By default, the Standard line dis- cipline, which supports full POSIX terminal semantics, is the only line discipline available for terminals. However, after the installation of worldwide portability subsets, two additional line disciplines are available for processing the multibyte and single-byte coded charac- ter sets used for Asian languages. The atty(7) and ttty(7) reference pages, which are also available after the worldwide portability sub- sets are installed, describe these additional line disciplines. Line discipline switching is done with the TIOCSETD ioctl for clist-based terminal drivers. The following example illustrates how to switch line disciplines: int ldisc = LDISC; ioctl(fd,TIOCSETD,&ldisc); In this example, LDISC is the index into the linesw table. (See the /sys/bsd/tty_conf.c file.) Other clist disciplines exist for special purposes, such as communication lines for network devices. The current line discipline can be obtained with the TIOCGETD ioctl for clist-based terminal drivers. Pending output is discarded when the line discipline is changed. When the Standard line discipline is used, NTTYDISC (value 2) is returned by default for BSD compatibility. TTYDISC (value 0) is returned only when you compile the application with the -D_USE_NEW_TTY switch. When the atty or ttty line discipline is used, ASYDISC (value 9) or TSB- DISC (value 10) is returned, respectively. The Controlling Terminal The operating system supports the concept of a controlling terminal. Any process in the system can have a controlling terminal associated with it. Certain events, such as the delivery of keyboard generated signals (for example, interrupt, quit, suspend), affect all the pro- cesses in the process group associated with the controlling terminal. The controlling terminal also determines the physical device that is accessed when the indirect device /dev/tty is opened. In earlier versions of UNIX systems, a controlling terminal was implicitly assigned to a process if, at the time an open was done on the terminal, the terminal was not the controlling terminal for any process, and if the process doing the open did not have a controlling ter- minal. In this version of UNIX, in accordance with POSIX 1003.1, a process must be a session leader to allocate a controlling terminal. In addition, the allocation is now done explicitly with a call to ioctl(). (This implies that the O_NOCTTY flag to the open() function is ignored.) The following example illustrates the correct sequence for obtaining a controlling tty (no error checking is shown). This code fragment calls the setsid() function to make the current process the group and session leader, and to remove any controlling tty that the process may already have. It then opens the console device and attaches it to the current session as the controlling terminal. Note that the process must not already be a session or process group leader, and the console must not already be the controlling tty of any other session. (void)setsid(); /* become session leader and */ /* lose controlling tty */ fd = open("/dev/console", O_RDWR); (void)ioctl(fd,TIOCSCTTY,0); A process can remove the association it has with its controlling terminal by opening the /dev/tty file and issuing the following call: ioctl(fd, TIOCNOTTY, 0); For example: fd = open("/dev/tty", O_RDWR); if (fd > = 0) { ioctl(fd,TIOCNOTTY,0); close(fd); } If the calling process is not the session leader, the process group of the calling process is set to 0. When a control terminal file is closed, pending input is removed, and pending output is sent to the receiving device. When a terminal file is opened, the process blocks until a carrier signal is detected. If the open() function is called with the O_NON- BLOCK flag set, however, the process does not wait. Instead, the first read() or write() call will wait for carrier to be established. If the CLOCAL mode is set in the termios structure, the driver assumes that modem control is not in effect, and open(), read(), and write() therefore proceed without waiting for a carrier signal to be established. Process Groups Each process belongs to a process group with a specific process group ID. Each process belongs to the process group of its creating process. This enables related processes to be signaled. Process group IDs are unique identifiers that cannot be used for other system process groups until the original process group is disbanded. Each process group also has a group leader process. A process group leader has the same process ID as its process group. Each process group belongs to a session. Each process in the process group also belongs to the process group's session. A process which is not the process group leader can create its own session and process group with a call to the setsid() function. That calling process then becomes the session leader of the new session and of the new process group. The new session has no controlling terminal until the session leader assigns one to it. The calling process's ID is assigned to the new process group. With the setpgid() function, other pro- cesses can be added to a process group. A controlling terminal can have a distinguished process group associated with it known as the foreground process group. The terminal's foreground process group is the one that receives signals generated by the INTR, QUIT, and SUSP special control characters. Certain opera- tions on the terminal are also restricted to processes in the terminal's foreground process group (see "Terminal Access Control"). A ter- minal's foreground process group may be changed by calling the tcsetpgrp() function. A terminal's current foreground process group may be obtained by calling the tcgetpgrp() function. Input Processing Modes The terminal drivers have two major modes, characterized by the kind of processing that takes place on the input characters: If a terminal is in canonical mode, input is collected and processed one line at a time. Lines are terminated by a newline ( ), End-of-File (EOF), or End-of-Line (EOL) character. A read request is not returned until the line has been terminated, or a signal has been received. The maxi- mum number of bytes of unread input allowed on an input terminal is MAX_INPUT bytes. If the maximum number of unread bytes exceeds MAX_INPUT bytes, the behavior of the driver depends on the setting of the IMAXBEL input flag (see "Input Editing"). Erase and kill processing is performed on input that has not been terminated by one of the line termination characters. Erase processing removes the last character in the line, kill processing removes the whole line. This mode eliminates erase and kill processing, making input characters available to the user program as they are typed. Input is not processed into lines. The received bytes are processed according to the values at the VMIN and VTIME indexes of the c_cc array in the termios structure. MIN is the minimum number of bytes the terminal can receive in noncanonical mode before a read is considered successful. TIME, measured in 0.1 second granularity, times out spo- radic input. These cases are summarized as follows: In this case, TIME is an interbyte timer that is activated after the first byte of the input line is received, and reset after each byte is received. The read operation is a success if MIN bytes are read before TIME runs out. If TIME runs out before MIN bytes have been received, the characters that were received are returned. In this case, only MIN is used. A queued read() waits until MIN bytes are received, or a signal is received. In this case, TIME is used as a read timer that starts when a read() call is made. The read() call is finished when one byte is read, or when TIME runs out. In this case, either the number of requested bytes or the number of currently available bytes is returned, depending on which is the lesser number. The read() function returns a zero if no data was read. Canonical mode is entered by setting the ICANON flag of the c_lflag field in the in the terminal's termios structure. Other input process- ing is performed according to the other flags set in the c_iflag and c_lflag fields. Input Editing A terminal ordinarily operates in full-duplex mode. Characters may be typed at any time, even while output is occurring. Characters are only lost when: The system's character input buffers become completely choked, which is rare. The user has accumulated the maximum allowed number of input characters (MAX_INPUT) that have not yet been read by some program. The MAX_INPUT limit is 512 characters. When this limit is reached, the terminal driver refuses to accept any further input and rings the terminal bell if IMAXBEL is set in the c_iflag field, or throws away all input and output without notice if this flag is not set. Input characters are normally accepted in either even or odd parity with the parity bit being stripped off before the character is given to the program. The ISTRIP mask of the c_iflag field controls whether the parity bit is stripped (ISTRIP set) or not stripped (ISTRIP not set). By setting the PARENB flag in the c_cflag field, and either setting (not setting) the PARODD flag, it is possible to have input characters with EVEN (ODD) parity discarded or marked (see "Input Modes"). In all of the line disciplines, it is possible to simulate terminal input using the TIOCSTI ioctl, which takes, as its third argument, the address of a character. The system pretends that this character was typed on the argument terminal, which must be the control terminal for the process, unless the process has superuser privileges. Input characters are normally echoed by putting them in an output queue as they arrive. This may be disabled by clearing the ECHO bit in the c_lflag word using the tcsetattr() call or the TIOCSETA, TIOCSETAW, or TIOCSETAF ioctls. In canonical mode, terminal input is processed in units of lines. A program attempting to read will normally be suspended until an entire line has been received (but see the description of SIGTTIN in "Terminal Access Control"). No matter how many characters are requested in the read call, at most one line will be returned. It is not, however, necessary to read a whole line at once; any number of characters may be requested in a read, even one, without losing information. In read() requests, the O_NONBLOCK flag affects the read() operation behav- ior. If O_NONBLOCK is not set, a read() request is blocked until data or a signal has been received. If the O_NONBLOCK flag is set, the read() request is not blocked, and one of the following situations holds: Some data may have been typed, but there may or may not be enough data to satisfy the entire read request. In either case, the read() function returns the data available, returning the number of bytes of data it read. If there is no data for the read operation, the read() returns a -1 with an error of EAGAIN. During input, line editing is normally done with the erase special control character (VERASE) logically erasing the last character typed and the kill special control character (VKILL) logically erasing the entire current input line. These characters never erase beyond the beginning of the current input line or an EOF (End-of-File). These characters, along with the other special control characters, may be entered literally by preceding them with the literal-next character (VLNEXT -- default ^V). The drivers normally treat either a newline character (` '), End-of-File character (EOF), or End-of-Line character (EOL) as terminating an input line, echoing a return and a line feed. If the ICRNL character bit is set in the c_iflag word then carriage returns are translated to newline characters on input, and are normally echoed as carriage return-linefeed sequences. If ICRNL is not set, this processing for carriage return is disabled, and it is simply echoed as a return, and does not terminate cooked mode input. The POSIX terminal driver also provides two other editing characters in normal mode. The word-erase character, normally <Ctrl-W>, is a c_cc structure special control character VWERASE. This character erases the preceding word, but not any spaces before it. For the pur- poses of <Ctrl-W>, a word is defined as a sequence of nonblank characters, with tabs counted as blanks. However, if the ALTWERASE flag is set in the c_lflag word, then a word is considered to be any sequence of alphanumerics or underscores bounded by characters that are not alphanumerics or underscores. Finally, the reprint character, normally <Ctrl-R>, is a c_cc structure special control character VREPRINT. This character retypes the pending input beginning on a new line. Retyping occurs automatically in canonical mode if characters which would normally be erased from the screen are fouled by program output. Input Modes The termios structure has an input mode field c_iflag, which controls basic terminal input characteristics. These characteristics are masks that can be bitwise inclusive ORed. The masks include: An interrupt is signaled on a break condition. All carriage returns are mapped to newline characters when input. Break conditions are ignored. Carriage returns are ignored. Characters with parity errors are ignored. Newline characters are mapped to carriage returns when input. Parity checks are enabled on input. The eighth bit (parity bit) is stripped on input characters. Stop/start characters are sent for input flow control enabled. Stop/start characters are recognized for output flow control. Any char will restart output after stop. Map upper case to lower case on input. Parity errors are marked with a three character sequence. The bell is rung when the input queue fills. The input mode mask bits can be combined for the following results: The setting of IGNBRK causes input break conditions to be ignored. If IGNBRK is not set, but BRKINT is set, the break condition has the same effect as if the VINTR control character had been typed. If neither IGNBRK nor BRKINT are set, then the break condition is input as a single character ''. If the PARMRK flag is set, then the input is read as three characters, '377', '', and ''. The setting of IGNPAR causes a byte with a parity or framing error, except for breaks, to be ignored (that is, discarded). If IGNPAR is not set, but PARMRK is set, a byte with parity or framing error, except for breaks, is passed as the three characters '377', '', and X, where X is the character data received in error. If the ISTRIP flag is not set, the valid character '377' is passed as '377', '377'. If both PARMRK and IGNPAR are not set, framing or parity errors, including breaks, are passed as the single character ''. The setting of INPCK enables input parity checking. If input parity checking is not enabled (INPCK not set), then characters with parity errors are simply passed through as is. The enabling/disabling of input parity checking is independent of the generation of parity on out- put. Setting ISTRIP causes the eighth bit of the eight valid input bits to be stripped before processing. If this mask is not set, all eight bits are processed. Setting INLCR causes a newline character to be read as a carriage return character. If the IGNCR flag is also set, the carriage return is ignored. If the IGNCR flag is not set, INLCR works as described earlier. The STOP character (normally <Ctrl-S>) suspends output and the START character (normally <Ctrl-Q>) restarts output. Setting IXON enables stop/start output control, in which the START and STOP characters are not read, but rather perform flow control functions. Extra stop characters typed when output is already stopped have no effect, unless the start and stop characters are made the same, in which case out- put resumes. Disabling IXON causes the START and STOP characters to be read. Setting IXOFF enables stop/start input control. When this flag is set, the terminal device will be sent STOP characters to halt the trans- mission of data when the input queue is in danger of overflowing (exceed MAX_INPUT). When enough characters have been read to reduce the amount of data queued to an acceptable level, a START character is sent to the device to allow it to continue transmitting data. This mode is useful when the terminal is actually another machine that obeys those conventions. Input Echoing and Redisplay The terminal driver has several modes for handling the echoing of terminal input, controlled by bits in the c_lflag field of the termios structure. Hardcopy Terminals When a hardcopy terminal is in use, the ECHOPRT bit is normally set in the local flags word. Characters which are logically erased are then printed out backwards preceded by (backslash) and followed by a / (slash) in this mode. Erasing Characters from a CRT When a CRT terminal is in use, the ECHOE bit may be set to cause input to be erased from the screen with a "backspace-space-backspace" sequence when character or word deleting sequences are used. The ECHOKE bit may be set as well, causing the input to be erased in this manner on line kill sequences as well. Echoing of Control Characters If the ECHOCTL bit is set in the local flags word, then nonprinting (control) characters are normally echoed as ^X (for some X) rather than being echoed unmodified; DELETE is echoed as ^?. Output Processing When one or more characters are written, they are actually transmitted to the terminal as soon as previously written characters have fin- ished typing. (As noted above, input characters are normally echoed by putting them in the output queue as they arrive.) When a process produces characters more rapidly than the terminal can accept them, it will be suspended when its output queue exceeds some limit. When the queue has drained down to some threshold the program is resumed. Even parity is normally generated on output. If the NOEOT bit is set in the c_oflag word of the termios structure, the EOT character (<Ctrl-D>) is not transmitted, to prevent terminals that respond to it from hanging up. The terminal drivers provide necessary processing for canonical and noncanonical mode output including delay generation for certain special characters and parity generation. Delays are available after backspaces (BSDLY), formfeeds (FFDLY), carriage returns (CRDLY), tabs (TABDLY) and newlines (NLDLY). The driver will also optionally expand tabs into spaces, where the tab stops are assumed to be set every eight columns, and optionally convert newlines to carriage returns followed by newline. Output process is controlled by bits in the c_oflag field of the termios structure. Refer to the write(2) reference page for a description of the O_NONBLOCK flag. The terminal drivers provide for mapping from lowercase to uppercase (OLCUC) for terminals lacking lower case, and for other special pro- cessing on deficient terminals. Finally, the terminal driver, supports an output flush character, normally <Ctrl-O>, which sets the FLUSHO bit in the local mode word, causing subsequent output to be flushed until it is cleared by a program or more input is typed. This character has effect in both canoni- cal and noncanonical modes and causes any pending input to be retyped. An ioctl to flush the characters in the input or output queues, TIOCFLUSH, is also available. Uppercase Terminals If the IUCLC bit in the c_iflag field is set in the tty flags, then all uppercase letters are mapped into the corresponding lowercase let- ter. The uppercase letter may be generated by preceding it by (backslash). Uppercase letters are preceded by a (backslash) when out- put. In addition, the following escape sequences will be generated on output and accepted on input if the XCASE bit is set in the c_lflag word: For: ` | ~ { } Use: ' ! ^ ( ) Line Control and Breaks There are several ioctl calls available to control the state of the terminal line. The TIOCSBRK ioctl will set the break bit in the hard- ware interface causing a break condition to exist; this can be cleared (usually after a delay with sleep(3)) by TIOCCBRK. The tcsend- break() can also be used to cause a break condition for a specified amount of time. Break conditions in the input are handled according to the c_iflag field settings for the termios structure. Refer to the section Input Modes" for a complete listing of the c_iflag field set- tings. The TIOCCDTR ioctl will clear the data terminal ready condition; it can be set again by TIOCSDTR. When the carrier signal from the dataset drops (usually because the user has hung up his terminal) a SIGHUP hangup signal is sent to the processes in the distinguished process group of the terminal; this usually causes them to terminate. The sending of SIGHUP does not take place if the CLOCAL bit is set in c_cflag field of the driver. Access to the terminal by other processes is then normally revoked, so any further reads will fail, and programs that read a terminal and test for End-of-File on their input will terminate appropriately. Interrupt Characters When the ISIG bit is set in the c_lflag word, there are several characters that generate signals in both canonical and noncanonical mode; all are sent to the processes in the foreground process group of the terminal. If the NOFLSH bit is not set in c_lflag, these characters also flush pending input and output when typed at a terminal. The characters shown here are the defaults; the symbolic names of the indices of these characters in the c_cc array of the termios structure are also shown. The characters may be changed. VINTR (in c_cc) generates a SIGINT signal. This is the normal way to stop a process which is no longer interesting, or to regain control in an interactive program. VQUIT (in c_cc) generates a SIGQUIT signal. This is used to cause a program to terminate and produce a core image, if possible, in the file core in the current directory. VSUSP (in c_cc) generates a SIGTSTP signal, which is used to suspend the current process group. VDSUSP (in c_cc) generates a SIGTSTP signal as <Ctrl-Z> does, but the signal is sent when a program attempts to read the <Ctrl-Y>, rather than when it is typed. Terminal Access Control If a process attempts to read from its controlling terminal when the process is not in the foreground process group of the terminal, that background process group is sent a SIGTTIN signal. This signal normally causes the members of that process group to stop. If, however, the process is ignoring SIGTTIN, has SIGTTIN blocked, or if the reading process' process group is orphaned, the read will return -1 and set errno to [EIO]. The operation will then not send a signal. If a process attempts to write to its controlling terminal when the process is not in the foreground process group of the terminal, and the TOSTOP bit is set in the c_lflag word of the termios structure, that background process group is sent a SIGTTOU signal and the process is prohibited from writing. If TOSTOP is not set, or if TOSTOP is set and the process is blocking or ignoring the SIGTTOU signal, process writes to the terminal are allowed, and the SIGTTOU signal is not sent. If TOSTOP is set, if the writing process' process group is orphaned, and if SIGTTOU is not blocked by the writing process, the write operation returns a -1 with errno set to [EIO], and does not a send a signal. Terminal/Window Sizes To accommodate terminals and workstations with variable-sized windows, the terminal driver provides a mechanism for obtaining and setting the current terminal size. The driver does not use this information internally, but only stores it and provides a uniform access mecha- nism. When the size is changed, a SIGWINCH signal is sent to the terminal's process group so that knowledgeable programs may detect size changes. tty Parameters In contrast to earlier versions of the tty driver, the POSIX terminal parameters and structures are contained in a single structure, the termios structure defined in the sys/termios.h file. Refer to the termios(4) reference page for a complete summary of this file. Basic ioctl Calls A large number of ioctl(2) calls apply to terminals. Some have the general form: #include <sys/termios.h> ioctl(fildes, code, arg) struct termios *arg; The applicable codes are: Gets the termios structure and all its associated parameters. The interface delays until output is quiescent, then throws away any unread characters. Sets the parameters according to the termios structure. Drains the output before setting the parameters according to the termios structure. Sets the parameters like TIOCSETA. Drains the output and flushes the input before setting the parameters according to the termios structure. Sets the parameters like TIOCSETA. With the following codes arg is ignored: Set "exclusive-use" mode: no further opens are permitted until the file has been closed. Turn off "exclusive-use" mode. With the following codes arg is a pointer to an int: If the int pointed to by arg has a zero value, all characters waiting in input or out- put queues are flushed. Otherwise, the value of the int is for the FREAD and FWRITE bits defined in the sys/file.h file; if the FREAD bit is set, all characters waiting in input queues are flushed, and if the FWRITE bit is set, all characters waiting in output queues are flushed. ioctls for Controlling Terminals Sets the terminal as the controlling terminal for the calling process. Voids the terminal as a controlling terminal for the calling process. If the calling process is not the session leader, the process group of the calling process is seet to 0. With the following codes, arg is a pointer to an int: The arg parameter is a pointer to the value representing the process group ID of the process group, and is returned by the controlling terminal specified by the filedes parameter. See tcgetpgrp(3) for more information on error codes that can occur if this ioctl fails. The arg parameter is a pointer to the value to which the process group ID for the terminal specified by the filedes parameter will be set. This terminal must be the controlling terminal and must be associated with the calling process's session. The process group value must match a process group ID of a process in the same session as the calling process. See tcsetpgrp for more information on error codes that can occur if this ioctl fails. Miscellaneous tty ioctl Codes The following are miscellaneous ioctl terminal commands. In cases where arguments are required, they are described; arg should otherwise be given as 0. The argument points to a character that the system pretends had been typed on the terminal. The break bit is set in the terminal. The break bit is cleared. Data terminal ready is set. Data terminal ready is cleared. Output is stopped as if the ``stop'' character had been typed. Output is restarted as if the ``start'' character had been typed. Returns in the int pointed to by arg the num- ber of characters queued for output to the terminal. Sets the terminal for remote input editing. Sets or clears the virtual console. The arg parameter is a pointer to an integer. A non zero value sets the virtual console to the requesting tty. A zero value clears the vir- tual console. Set this to write console messages to the virtual console rather than /dev/console. Clearing a virtual console causes con- sole messages to resume being sent to /dev/console. Only one virtual console may be in effect at a time. This ioctl requires superuser privilege. Returns in the int pointed to by arg the number of characters immediately readable from the argument descriptor. This works for files, pipes, and terminals. Controlling Terminal Modems The following ioctls apply to modems: The arg parameter is a pointer to an int, which is the value of the modem control state. The arg parameter is a pointer to an int, which is the value to which the modem control state is to be set. Sets all modem bits. The arg parameter is a pointer to an int, which specifies the modem bits to be set. arg is a pointer to an int, which specifies the modem bits to be cleared. Gets all the modem bits and returns them in the int point to by arg. Window/Terminal Sizes Each terminal has provision for storage of the current terminal or window size in a winsize structure, which has the following format: struct winsize { unsigned short ws_row; /* rows, in characters */ unsigned short ws_col; /* columns, in characters */ unsigned short ws_xpixel; /* horizontal size, pixels */ unsigned short ws_ypixel; /* vertical size, pixels */ }; A value of 0 (zero) in any field is interpreted as ``undefined;'' the entire structure is zeroed on final close. The applicable ioctl functions are: The arg parameter is a pointer to a struct winsize into which will be placed the current terminal or window size information. The arg parameter is a pointer to a struct winsize, which will be used to set the current terminal or window size information. If the new information is different than the old information, a SIGWINCH signal will be sent to the terminal's process group. NOTES
The following System V and BSD ioctls are currently supported as a compatibility interface for System V and older BSD system programs. They should not be used in new applications. The ioctl call has the following format: ioctl (fdes, cmd, arg) The System V termio ioctls use a pointer to a termio structure for the arg parameter. The following include is required for the System V termio ioctls: #include <sys/termio> The parameters of the tty driver are set/returned by translating the termio structure to/from a termios structure. Gets termio structure. Sets termio structure. Drains output and then sets termio structure. Drains output, flushes input, and then sets termio structure. The following group of System V ioctls take an integer value for the arg parameter. Sends break. Sends break. Sets flow control. The arg parameter uses same values as the POSIX function tcflow() (must include <sys/termios.h>) Flushes queue(s). You must include <sys/fcntl.h> for valid arg parameters (for example, FREAD for flushing input side). The following group of BSD compatibility ioctls use a pointer to a sgttyb structure for the arg parameter. The tty parameters of the tty driver are set/returned by translating the sgttyb structure to/from a termios structure. Returns tty parameters. Sets tty parameters. Sets tty parameter without flushing. The next group uses a pointer to the local mode bits (an integer value) as their arg parameter. The local mode bits are converted to the appropriate POSIX termios flag settings. The arg value is used as a mask to set the local mode bits. The arg value is used as a mask to clear the local mode bits. The arg value is used to replace the current setting of the local mode bits. The arg parameter is used to return the current value of the local mode bits. The following group uses a pointer to a ltchars structure as their arg parameter. The local special characters are translated to POSIX termios control characters. Set local control characters. Get local control characters. The following group of BSD compatibility ioctls sets/clears the POSIX CLOCAL and HUPCL flags. The CLOCAL flag turns modem control on/off in the tty driver. If the arg parameter (a pointer to an integer) for TIOCMODEM/TIOCNMODEM is nonzero and the requesting process has super user privileges the change is made permanent. Otherwise after the last close, the tty line reverts back to the default value. The HUPCL flag determines if the terminal line should be disconnected (that is, DTR is non asserted) after the last close. The TIOCHPCL ioctl ignores the arg parameter. Clears CLOCAL. Sets CLOCAL. Sets HUPCL. FILES
Special file for tty. Special files for ttys, where the * (asterisk) sign represents the tty number. Device special file for console. RELATED INFORMATION
Functions: ioctl(2), sigvec(2), tcsetattr(3), tcgetattr(3), tcdrain(3), tcflush(3), tcsendbreak(3), tcgetpgrp(3), tcsetpgrp(3). Commands: csh(1), strchg(1), tset(1), getty(8). Files: termios(4) Interfaces: atty(7), ldterm(7), modem(7), ttty(7). IEEE Std POSIX 1003.1-1988 delim off tty(7)

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