--- name: "termios" section: "3type" platform: "linux" package: "man-pages" version: "6.19-1" release: "Arch Linux 2026-11-01" platforms: ["linux"] url: "https://new.carta.tech/termios/3type" --- # termios(3type) terminal input/output settings ## NAME termios - terminal input/output settings ## LIBRARY Standard C library (*libc*) ## SYNOPSIS ``` #include ``` ``` 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)](/feature_test_macros/7.md)) 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)](/ioctl_tty/2.md)) - **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)](/bash/1.md) 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)](/read/2.md) 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)](/read/2.md). - • Line editing is enabled (ERASE, KILL; and if the **IEXTEN** flag is set: WERASE, REPRINT, LNEXT). A [**read**(2)](/read/2.md) returns at most one line of input; if the [**read**(2)](/read/2.md) 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)](/read/2.md). - • 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)](/read/2.md) completes; there are four distinct cases: - MIN == 0, TIME == 0 (polling read) If data is available, [**read**(2)](/read/2.md) returns immediately, with the lesser of the number of bytes available, or the number of bytes requested. If no data is available, [**read**(2)](/read/2.md) returns 0. - MIN \> 0, TIME == 0 (blocking read) [**read**(2)](/read/2.md) 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)](/read/2.md) is called. [**read**(2)](/read/2.md) 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)](/read/2.md) returns 0. If data is already available at the time of the call to [**read**(2)](/read/2.md), 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)](/read/2.md) 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)](/read/2.md) has been received. (POSIX does not specify this termination condition, and on some other implementations [**read**(2)](/read/2.md) 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)](/read/2.md), 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)](/read/2.md) 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)](/read/2.md) 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)](/cc_t/3type.md), [**tcflag\_t**(3type)](/tcflag_t/3type.md), [**termios**(7)](/termios/7.md)