termios(3type)

NAME
termios - terminal input/output settings

LIBRARY
Standard C library (libc)

SYNOPSIS
#include <termios.h>

    struct termios {
    	// At least these members, in no particular order
    	tcflag_t  c_iflag;
    	tcflag_t  c_oflag;
    	tcflag_t  c_cflag;
    	tcflag_t  c_lflag;
    	cc_t      c_cc[NCCS];
    };

DESCRIPTION

.c_iflag
    Input modes.

.c_oflag
    Output modes.

.c_cflag
    Control modes.

.c_lflag
    Local modes.

.c_cc
    Special characters.

The values that may be assigned to these fields are described below. In the case of the first four bit-mask fields, the definitions of some of the associated flags that may be set are exposed only if a specific feature test macro (see feature_test_macros(7)) is defined, as noted in brackets ("[]").

In the descriptions below, "not in POSIX" means that the value is not specified in POSIX.1-2001, and "XSI" means that the value is specified in POSIX.1-2001 as part of the XSI extension.

The below symbolic subscript values are all different, except that VTIME, VMIN may have the same value as VEOL, VEOF, respectively. In noncanonical mode, the special character meaning is replaced by the timeout meaning. For an explanation of VMIN and VTIME, see the description of noncanonical mode below.

  .c_iflag flag constants

IGNBRK
    Ignore BREAK condition on input.

BRKINT
    If IGNBRK is set, a BREAK is ignored. If it is not set but BRKINT is set, then a BREAK causes the input and output queues to be flushed, and if the terminal is the controlling terminal of a foreground process group, it will cause a SIGINT to be sent to this foreground process group. When neither IGNBRK nor BRKINT are set, a BREAK reads as a null byte ('\0'), except when PARMRK is set, in which case it reads as the sequence \377 \0 \0.

IGNPAR
    Ignore framing errors and parity errors.

PARMRK
    If this bit is set, input bytes with parity or framing errors are marked when passed to the program. This bit is meaningful only when INPCK is set and IGNPAR is not set. The way erroneous bytes are marked is with two preceding bytes, \377 and \0. Thus, the program actually reads three bytes for one erroneous byte received from the terminal. If a valid byte has the value \377, and ISTRIP (see below) is not set, the program might confuse it with the prefix that marks a parity error. Therefore, a valid byte \377 is passed to the program as two bytes, \377 \377, in this case.

    If neither IGNPAR nor PARMRK is set, read a character with a parity error or framing error as \0.

INPCK
    Enable input parity checking.

ISTRIP
    Strip off eighth bit.

INLCR
    Translate NL to CR on input.

IGNCR
    Ignore carriage return on input.

ICRNL
    Translate carriage return to newline on input (unless IGNCR is set).

IUCLC
    (not in POSIX) Map uppercase characters to lowercase on input.

IXON
    Enable XON/XOFF flow control on output.

IXANY
    (XSI) Typing any character will restart stopped output. (The default is to allow just the START character to restart output.)

IXOFF
    Enable XON/XOFF flow control on input.

IMAXBEL
    (not in POSIX) Ring bell when input queue is full. Linux does not implement this bit, and acts as if it is always set.

IUTF8 (since Linux 2.6.4)
    (not in POSIX) Input is UTF8; this allows character-erase to be correctly performed in cooked mode.

  .c_oflag flag constants

OPOST
    Enable implementation-defined output processing.

OLCUC
    (not in POSIX) Map lowercase characters to uppercase on output.

ONLCR
    (XSI) Map NL to CR-NL on output.

OCRNL
    Map CR to NL on output.

ONOCR
    Don't output CR at column 0.

ONLRET
    The NL character is assumed to do the carriage-return function; the kernel's idea of the current column is set to 0 after both NL and CR.

OFILL
    Send fill characters for a delay, rather than using a timed delay.

OFDEL
    Fill character is ASCII DEL (0177). If unset, fill character is ASCII NUL ('\0'). (Not implemented on Linux.)

NLDLY
    Newline delay mask. Values are NL0 and NL1. [requires _BSD_SOURCE or _SVID_SOURCE or _XOPEN_SOURCE]

CRDLY
    Carriage return delay mask. Values are CR0, CR1, CR2, or CR3. [requires _BSD_SOURCE or _SVID_SOURCE or _XOPEN_SOURCE]

TABDLY
    Horizontal tab delay mask. Values are TAB0, TAB1, TAB2, TAB3 (or XTABS, but see the BUGS section). A value of TAB3, that is, XTABS, expands tabs to spaces (with tab stops every eight columns). [requires _BSD_SOURCE or _SVID_SOURCE or _XOPEN_SOURCE]

BSDLY
    Backspace delay mask. Values are BS0 or BS1. (Has never been implemented.) [requires _BSD_SOURCE or _SVID_SOURCE or _XOPEN_SOURCE]

VTDLY
    Vertical tab delay mask. Values are VT0 or VT1.

FFDLY
    Form feed delay mask. Values are FF0 or FF1. [requires _BSD_SOURCE or _SVID_SOURCE or _XOPEN_SOURCE]

  .c_cflag flag constants

CBAUD
    (not in POSIX) Baud speed mask (4+1 bits). [requires _BSD_SOURCE or _SVID_SOURCE]

CBAUDEX
    (not in POSIX) Extra baud speed mask (1 bit), included in CBAUD. [requires _BSD_SOURCE or _SVID_SOURCE]

    (POSIX says that the baud speed is stored in the termios structure without specifying where precisely, and provides cfgetispeed() and cfsetispeed() for getting at it. Some systems use bits selected by CBAUD in .c_cflag, other systems use separate fields, for example, sg_ispeed and sg_ospeed.)

CSIZE
    Character size mask. Values are CS5, CS6, CS7, or CS8.

CSTOPB
    Set two stop bits, rather than one.

CREAD
    Enable receiver.

PARENB
    Enable parity generation on output and parity checking for input.

PARODD
    If set, then parity for input and output is odd; otherwise even parity is used.

HUPCL
    Lower modem control lines after last process closes the device (hang up).

CLOCAL
    Ignore modem control lines.

LOBLK
    (not in POSIX) Block output from a noncurrent shell layer. For use by shl (shell layers). (Not implemented on Linux.)

CIBAUD
    (not in POSIX) Mask for input speeds. The values for the CIBAUD bits are the same as the values for the CBAUD bits, shifted left IBSHIFT bits. [requires _BSD_SOURCE or _SVID_SOURCE] (Not implemented in glibc, supported on Linux via TCGET* and TCSET* ioctls; see ioctl_tty(2))

CMSPAR
    (not in POSIX) Use "stick" (mark/space) parity (supported on certain serial devices): if PARODD is set, the parity bit is always 1; if PARODD is not set, then the parity bit is always 0. [requires _BSD_SOURCE or _SVID_SOURCE]

CRTSCTS
    (not in POSIX) Enable RTS/CTS (hardware) flow control. [requires _BSD_SOURCE or _SVID_SOURCE]

  .c_lflag flag constants

ISIG
    When any of the characters INTR, QUIT, SUSP, or DSUSP are received, generate the corresponding signal.

ICANON
    Enable canonical mode (described below).

XCASE
    (not in POSIX; not supported under Linux) If ICANON is also set, terminal is uppercase only. Input is converted to lowercase, except for characters preceded by \. On output, uppercase characters are preceded by \ and lowercase characters are converted to uppercase. [requires _BSD_SOURCE or _SVID_SOURCE or _XOPEN_SOURCE]

ECHO
    Echo input characters.

ECHOE
    If ICANON is also set, the ERASE character erases the preceding input character, and WERASE erases the preceding word.

ECHOK
    If ICANON is also set, the KILL character erases the current line.

ECHONL
    If ICANON is also set, echo the NL character even if ECHO is not set.

ECHOCTL
    (not in POSIX) If ECHO is also set, terminal special characters other than TAB, NL, START, and STOP are echoed as ^X, where X is the character with ASCII code 0x40 greater than the special character. For example, character 0x08 (BS) is echoed as ^H. [requires _BSD_SOURCE or _SVID_SOURCE]

ECHOPRT
    (not in POSIX) If ICANON and ECHO are also set, characters are printed as they are being erased. [requires _BSD_SOURCE or _SVID_SOURCE]

ECHOKE
    (not in POSIX) If ICANON is also set, KILL is echoed by erasing each character on the line, as specified by ECHOE and ECHOPRT. [requires _BSD_SOURCE or _SVID_SOURCE]

DEFECHO
    (not in POSIX) Echo only when a process is reading. (Not implemented on Linux.)

FLUSHO
    (not in POSIX; not supported under Linux) Output is being flushed. This flag is toggled by typing the DISCARD character. [requires _BSD_SOURCE or _SVID_SOURCE]

NOFLSH
    Disable flushing the input and output queues when generating signals for the INT, QUIT, and SUSP characters.

TOSTOP
    Send the SIGTTOU signal to the process group of a background process which tries to write to its controlling terminal.

PENDIN
    (not in POSIX; not supported under Linux) All characters in the input queue are reprinted when the next character is read. (bash(1) handles typeahead this way.) [requires _BSD_SOURCE or _SVID_SOURCE]

IEXTEN
    Enable implementation-defined input processing. This flag, as well as ICANON must be enabled for the special characters EOL2, LNEXT, REPRINT, WERASE to be interpreted, and for the IUCLC flag to be effective.

  .c_cc array indices
The .c_cc array defines the terminal special characters. The symbolic indices (initial values) and meaning are:

VDISCARD
    (not in POSIX; not supported under Linux; 017, SI, Ctrl-O) Toggle: start/stop discarding pending output. Recognized when IEXTEN is set, and then not passed as input.

VDSUSP
    (not in POSIX; not supported under Linux; 031, EM, Ctrl-Y) Delayed suspend character (DSUSP): send SIGTSTP signal when the character is read by the user program. Recognized when IEXTEN and ISIG are set, and the system supports job control, and then not passed as input.

VEOF
    (004, EOT, Ctrl-D) End-of-file character (EOF). More precisely: this character causes the pending tty buffer to be sent to the waiting user program without waiting for end-of-line. If it is the first character of the line, the read(2) in the user program returns 0, which signifies end-of-file. Recognized when ICANON is set, and then not passed as input.

VEOL
    (0, NUL) Additional end-of-line character (EOL). Recognized when ICANON is set.

VEOL2
    (not in POSIX; 0, NUL) Yet another end-of-line character (EOL2). Recognized when ICANON is set.

VERASE
    (0177, DEL, rubout, or 010, BS, Ctrl-H, or also #) Erase character (ERASE). This erases the previous not-yet-erased character, but does not erase past EOF or beginning-of-line. Recognized when ICANON is set, and then not passed as input.

VINTR
    (003, ETX, Ctrl-C, or also 0177, DEL, rubout) Interrupt character (INTR). Send a SIGINT signal. Recognized when ISIG is set, and then not passed as input.

VKILL
    (025, NAK, Ctrl-U, or Ctrl-X, or also @) Kill character (KILL). This erases the input since the last EOF or beginning-of-line. Recognized when ICANON is set, and then not passed as input.

VLNEXT
    (not in POSIX; 026, SYN, Ctrl-V) Literal next (LNEXT). Quotes the next input character, depriving it of a possible special meaning. Recognized when IEXTEN is set, and then not passed as input.

VMIN
    Minimum number of characters for noncanonical read (MIN).

VQUIT
    (034, FS, Ctrl-\) Quit character (QUIT). Send SIGQUIT signal. Recognized when ISIG is set, and then not passed as input.

VREPRINT
    (not in POSIX; 022, DC2, Ctrl-R) Reprint unread characters (REPRINT). Recognized when ICANON and IEXTEN are set, and then not passed as input.

VSTART
    (021, DC1, Ctrl-Q) Start character (START). Restarts output stopped by the Stop character. Recognized when IXON is set, and then not passed as input.

VSTATUS
    (not in POSIX; not supported under Linux; status request: 024, DC4, Ctrl-T). Status character (STATUS). Display status information at terminal, including state of foreground process and amount of CPU time it has consumed. Also sends a SIGINFO signal (not supported on Linux) to the foreground process group.

VSTOP
    (023, DC3, Ctrl-S) Stop character (STOP). Stop output until Start character typed. Recognized when IXON is set, and then not passed as input.

VSUSP
    (032, SUB, Ctrl-Z) Suspend character (SUSP). Send SIGTSTP signal. Recognized when ISIG is set, and then not passed as input.

VSWTCH
    (not in POSIX; not supported under Linux; 0, NUL) Switch character (SWTCH). Used in System V to switch shells in shell layers, a predecessor to shell job control.

VTIME
    Timeout in deciseconds for noncanonical read (TIME).

VWERASE
    (not in POSIX; 027, ETB, Ctrl-W) Word erase (WERASE). Recognized when ICANON and IEXTEN are set, and then not passed as input.

An individual terminal special character can be disabled by setting the value of the corresponding .c_cc element to _POSIX_VDISABLE.

  Canonical and noncanonical mode
The setting of the ICANON canon flag in .c_lflag determines whether the terminal is operating in canonical mode (ICANON set) or noncanonical mode (ICANON unset). By default, ICANON is set.

In canonical mode:

•   Input is made available line by line. An input line is available when one of the line delimiters is typed (NL, EOL, EOL2; or EOF at the start of line). Except in the case of EOF, the line delimiter is included in the buffer returned by read(2).

•   Line editing is enabled (ERASE, KILL; and if the IEXTEN flag is set: WERASE, REPRINT, LNEXT). A read(2) returns at most one line of input; if the read(2) requested fewer bytes than are available in the current line of input, then only as many bytes as requested are read, and the remaining characters will be available for a future read(2).

•   The maximum line length is 4096 chars (including the terminating newline character); lines longer than 4096 chars are truncated. After 4095 characters, input processing (e.g., ISIG and ECHO* processing) continues, but any input data after 4095 characters up to (but not including) any terminating newline is discarded. This ensures that the terminal can always receive more input until at least one line can be read.

In noncanonical mode input is available immediately (without the user having to type a line-delimiter character), no input processing is performed, and line editing is disabled. The read buffer will only accept 4095 chars; this provides the necessary space for a newline char if the input mode is switched to canonical. The settings of MIN (.c_cc[VMIN]) and TIME (.c_cc[VTIME]) determine the circumstances in which a read(2) completes; there are four distinct cases:

MIN == 0, TIME == 0 (polling read)
    If data is available, read(2) returns immediately, with the lesser of the number of bytes available, or the number of bytes requested. If no data is available, read(2) returns 0.

MIN > 0, TIME == 0 (blocking read)
    read(2) blocks until MIN bytes are available, and returns up to the number of bytes requested.

MIN == 0, TIME > 0 (read with timeout)
    TIME specifies the limit for a timer in tenths of a second. The timer is started when read(2) is called. read(2) returns either when at least one byte of data is available, or when the timer expires. If the timer expires without any input becoming available, read(2) returns 0. If data is already available at the time of the call to read(2), the call behaves as though the data was received immediately after the call.

MIN > 0, TIME > 0 (read with interbyte timeout)
    TIME specifies the limit for a timer in tenths of a second. Once an initial byte of input becomes available, the timer is restarted after each further byte is received. read(2) returns when any of the following conditions is met:

    •   MIN bytes have been received.

    •   The interbyte timer expires.

    •   The number of bytes requested by read(2) has been received. (POSIX does not specify this termination condition, and on some other implementations read(2) does not return in this case.)

    Because the timer is started only after the initial byte becomes available, at least one byte will be read. If data is already available at the time of the call to read(2), the call behaves as though the data was received immediately after the call.

POSIX does not specify whether the setting of the O_NONBLOCK file status flag takes precedence over the MIN and TIME settings. If O_NONBLOCK is set, a read(2) in noncanonical mode may return immediately, regardless of the setting of MIN or TIME. Furthermore, if no data is available, POSIX permits a read(2) in noncanonical mode to return either 0, or -1 with errno set to EAGAIN.

STANDARDS
POSIX.1-2024.

HISTORY
POSIX.1-2001.

BUGS
On the Alpha architecture before Linux 4.16 (and glibc before glibc 2.28), the XTABS value was different from TAB3 and it was ignored by the N_TTY line discipline code of the terminal driver as a result (because as it wasn't part of the TABDLY mask).

SEE ALSO
cc_t(3type), tcflag_t(3type), termios(7)