A Linux Termios Reference
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
; orEOF
at the start of line). Except in the case ofEOF
, the line delimiter is included in the buffer returned byread()
. - Line editing is enabled (
ERASE
,KILL
; and if theIEXTEN
flag is set:WERASE
,REPRINT
,LNEXT
). Aread()
returns at most one line of input; if theread()
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 ofMIN
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 whenread()
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 orMIN
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.
-
(since Linux 2.6.4) ↩︎