termios(3type)

💖 NAME 💖

termios — terminal input/output settings. This is the termios(3type) page, bestie, and it's literally all about how your terminal talks to your program. No cap.

📚 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 you're allowed to assign to these fields are described below. For the first four bit-mask fields, some associated flags only show up if a specific feature test macro is defined (see feature_test_macros(7)), and that's noted in brackets ([]). It's giving conditional visibility, fr fr.

In the descriptions below, "not in POSIX" means the value isn't specified in POSIX.1-2001, and "XSI" means it is specified in POSIX.1-2001 as part of the XSI extension. Slay.

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

🦋 .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 — (default 0, NUL) an additional end-of-line character, aka EOL. It only gets recognized when ICANON is set. 💖
• VEOL2 — (not in POSIX; default 0, NUL) yet another end-of-line character, aka EOL2. Again, only when ICANON is set. bestie there are SO many ways to end a line 😭
• VERASE — (default 0177, DEL, rubout, or 010, BS, Ctrl+H, or also #) the erase character, aka ERASE. It erases the previous not-yet-erased character, but it does not erase past EOF or the beginning-of-line. Recognized when ICANON is set — and when it is, it's not passed as input. 🧹
• VINTR — (default 003, ETX, Ctrl+C, or also 0177, DEL, rubout) the interrupt character, aka INTR. Sends a SIGINT signal. Recognized when ISIG is set — and then not passed as input. 💀
• VKILL — (default 025, NAK, Ctrl+U, or Ctrl+X, or also @) the kill character, aka KILL. This one 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; default 026, SYN, Ctrl+V) literal next, aka LNEXT. It quotes the next input character, depriving it of a possible special meaning — so your special char gets to be just a char, no main character energy for one whole keystroke. Recognized when IEXTEN is set — and then not passed as input.
• VMIN — the minimum number of characters for a noncanonical read, aka MIN.
• VQUIT — (default 034, FS, Ctrl+\) the quit character, aka QUIT. Sends a SIGQUIT signal. Recognized when ISIG is set — and then not passed as input.
• VREPRINT — (not in POSIX; default 022, DC2, Ctrl+R) reprint unread characters, aka REPRINT. Recognized when ICANON and IEXTEN are set — and then not passed as input.
• VSTART — (default 021, DC1, Ctrl+Q) the start character, aka START. It restarts output that got 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) the status character, aka STATUS. Displays status information at the terminal, including the state of the foreground process and the amount of CPU time it has consumed. It also sends a SIGINFO signal (not supported on Linux) to the foreground process group.
• VSTOP — (default 023, DC3, Ctrl+S) the stop character, aka STOP. Stops output until the Start character gets typed. Recognized when IXON is set — and then not passed as input. 🛑
• VSUSP — (default 032, SUB, Ctrl+Z) the suspend character, aka SUSP. Sends a SIGTSTP signal. Recognized when ISIG is set — and then not passed as input.
• VSWTCH — (not in POSIX; not supported under Linux; default 0, NUL) the switch character, aka SWTCH. Used in System V to switch shells in shell layers — a predecessor to shell job control. ancient lore fr fr 🏺
• VTIME — timeout in deciseconds for a noncanonical read, aka TIME.
• VWERASE — (not in POSIX; default 027, ETB, Ctrl+W) word erase, aka 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. 💅

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🌀 Canonical and noncanonical mode 🌀

The setting of the ICANON canon flag in .c_lflag decides whether your 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. 🫠

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🌈 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). yeah. that's the whole bug. iconic honestly 💀

👀 SEE ALSO 👀

• cc_t(3type)
• tcflag_t(3type)
• termios(7)

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okay that's the whole thing bestie, you did it, you read the entire page 🎀 this lil beauty came from Linux man-pages 6.20, dated 2026-07-02 — go forth and stty responsibly, no cap 💖✨🦋