I’ve been fighting with Linux serial port programming lately, and one thing I’ve realised is that the Linux termios manpage is terribly formatted.
Here’s a somewhat more nicely formatted quick reference.

In particular, in the standard man page it’s really easy to miss which field a particular flag belongs in…

Input Flags = c_iflag

Flag Meaning
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 IGNPAR is not set, prefix a character with a parity error or framing error with \377\0. 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.
IUTF81 (not in POSIX) Input is UTF8; this allows character-erase to be correctly performed in cooked mode.

Output Flags = c_oflag

Flag Meaning
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 Don’t output CR.
OFILL Send fill characters for a delay, rather than using a timed delay.
OFDEL (not in POSIX) 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). 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)

Control Flags = c_cflag

Flag Meaning
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 on Linux.))
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)

Local Modes = c_lflag

Flag Meaning
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.

Editorial Note

The IEXTEN flag would be better named the IFUCKWITHYOUINUNDOCUMENTEDWAYS flag. Reading the above might make you think it only has the effects described, and only when ICANON is enabled. This is not true. It does weird, implementation specific things, different from one serial adaptor to another, whether you are in Canonical mode or not. It also appears to be default-on.

For the sake of your sanity, clear the IEXTEN bit.

Special Character Processing

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

Index Meaning
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.

The above 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.

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 set.

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().
  • Line editing is enabled (ERASE, KILL; and if the IEXTEN flag is set: WERASE, REPRINT, LNEXT). A read() returns at most one line of input; if the read() 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).

Noncanonical mode

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 settings of MIN (c_cc[VMIN]) and TIME (c_cc[VTIME]) determine the circumstances in which a read() completes; there are four distinct cases:

  • MIN == 0; TIME == 0: If data is available, read() returns immediately, with the lesser of the number of bytes available, or the number of bytes requested. If no data is available, read() returns 0.
  • MIN > 0; TIME == 0: read() blocks until the lesser of MIN bytes or the number of bytes requested are available, and returns the lesser of these two values.
  • MIN == 0; TIME > 0: TIME specifies the limit for a timer in tenths of a second. The timer is started when read() is called. read() 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.
  • MIN > 0; TIME > 0: 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() returns either when the lesser of the number of bytes requested or MIN byte have been read, or when the inter-byte timeout expires. Because the timer is only started after the initial byte becomes available, at least one byte will be read.

  1. (since Linux 2.6.4) ↩︎