The termios functions describe a general terminal interface that
is provided to control asynchronous communications ports.
The termios structure
Many of the functions described here have a termios_p argument
that is a pointer to a termios structure. This structure
contains at least the following members:
tcflag_t c_iflag; /* input modes */
tcflag_t c_oflag; /* output modes */
tcflag_t c_cflag; /* control modes */
tcflag_t c_lflag; /* local modes */
cc_t c_cc[NCCS]; /* 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.
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 Don't output 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 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]
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.
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.
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.
Retrieving and changing terminal settings
tcgetattr() gets the parameters associated with the object
referred by fd and stores them in the termios structure
referenced by termios_p. This function may be invoked from a
background process; however, the terminal attributes may be
subsequently changed by a foreground process.
tcsetattr() sets the parameters associated with the terminal
(unless support is required from the underlying hardware that is
not available) from the termios structure referred to by
termios_p. optional_actions specifies when the changes take
effect:
TCSANOW
the change occurs immediately.
TCSADRAIN
the change occurs after all output written to fd has been
transmitted. This option should be used when changing
parameters that affect output.
TCSAFLUSH
the change occurs after all output written to the object
referred by fd has been transmitted, and all input that
has been received but not read will be discarded before
the change is made.
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.
Raw mode
cfmakeraw() sets the terminal to something like the "raw" mode of
the old Version 7 terminal driver: input is available character
by character, echoing is disabled, and all special processing of
terminal input and output characters is disabled. The terminal
attributes are set as follows:
termios_p->c_iflag &= ~(IGNBRK | BRKINT | PARMRK | ISTRIP
| INLCR | IGNCR | ICRNL | IXON);
termios_p->c_oflag &= ~OPOST;
termios_p->c_lflag &= ~(ECHO | ECHONL | ICANON | ISIG | IEXTEN);
termios_p->c_cflag &= ~(CSIZE | PARENB);
termios_p->c_cflag |= CS8;
Line control
tcsendbreak() transmits a continuous stream of zero-valued bits
for a specific duration, if the terminal is using asynchronous
serial data transmission. If duration is zero, it transmits
zero-valued bits for at least 0.25 seconds, and not more than 0.5
seconds. If duration is not zero, it sends zero-valued bits for
some implementation-defined length of time.
If the terminal is not using asynchronous serial data
transmission, tcsendbreak() returns without taking any action.
tcdrain() waits until all output written to the object referred
to by fd has been transmitted.
tcflush() discards data written to the object referred to by fd
but not transmitted, or data received but not read, depending on
the value of queue_selector:
TCIFLUSH
flushes data received but not read.
TCOFLUSH
flushes data written but not transmitted.
TCIOFLUSH
flushes both data received but not read, and data written
but not transmitted.
tcflow() suspends transmission or reception of data on the object
referred to by fd, depending on the value of action:
TCOOFF suspends output.
TCOON restarts suspended output.
TCIOFF transmits a STOP character, which stops the terminal
device from transmitting data to the system.
TCION transmits a START character, which starts the terminal
device transmitting data to the system.
The default on open of a terminal file is that neither its input
nor its output is suspended.
Line speed
The baud rate functions are provided for getting and setting the
values of the input and output baud rates in the termios
structure. The new values do not take effect until tcsetattr()
is successfully called.
Setting the speed to B0 instructs the modem to "hang up". The
actual bit rate corresponding to B38400 may be altered with
setserial(8).
The input and output baud rates are stored in the termios
structure.
cfgetospeed() returns the output baud rate stored in the termios
structure pointed to by termios_p.
cfsetospeed() sets the output baud rate stored in the termios
structure pointed to by termios_p to speed, which must be one of
these constants:
B0
B50
B75
B110
B134
B150
B200
B300
B600
B1200
B1800
B2400
B4800
B9600
B19200
B38400
B57600
B115200
B230400
B460800
B500000
B576000
B921600
B1000000
B1152000
B1500000
B2000000
These constants are additionally supported on the SPARC
architecture:
B76800
B153600
B307200
B614400
These constants are additionally supported on non-SPARC
architectures:
B2500000
B3000000
B3500000
B4000000
Due to differences between architectures, portable applications
should check if a particular Bnnn constant is defined prior to
using it.
The zero baud rate, B0, is used to terminate the connection. If
B0 is specified, the modem control lines shall no longer be
asserted. Normally, this will disconnect the line. CBAUDEX is a
mask for the speeds beyond those defined in POSIX.1 (57600 and
above). Thus, B57600 & CBAUDEX is nonzero.
Setting the baud rate to a value other than those defined by Bnnn
constants is possible via the TCSETS2 ioctl; see ioctl_tty(2).
cfgetispeed() returns the input baud rate stored in the termios
structure.
cfsetispeed() sets the input baud rate stored in the termios
structure to speed, which must be specified as one of the Bnnn
constants listed above for cfsetospeed(). If the input baud rate
is set to zero, the input baud rate will be equal to the output
baud rate.
cfsetspeed() is a 4.4BSD extension. It takes the same arguments
as cfsetispeed(), and sets both input and output speed.
