TERMIOS(3) Linux Programmer's Manual TERMIOS(3)
NAME
termios, tcgetattr, tcsetattr, tcsendbreak, tcdrain, tcflush, tcflow,
cfmakeraw, cfgetospeed, cfgetispeed, cfsetispeed, cfsetospeed - get and
set terminal attributes, line control, get and set baud rate
SYNOPSIS
#include<termios.h>
#include<unistd.h>
inttcgetattr(int fd,structtermios*termios_p);
inttcsetattr(int fd,int optional_actions,structtermios*termios_p);
inttcsendbreak(int fd,int duration);
inttcdrain(int fd);
inttcflush(int fd,int queue_selector);
inttcflow(int fd,int action);
intcfmakeraw(structtermios*termios_p);
speed_tcfgetispeed(structtermios*termios_p);
speed_tcfgetospeed(structtermios*termios_p);
intcfsetispeed(structtermios*termios_p,speed_t speed);
intcfsetospeed(structtermios*termios_p,speed_t speed);
DESCRIPTION
The termios functions describe a general terminal interface that is
provided to control asynchronous communications ports.
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]; /* control chars */
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 NUL character, 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 (not in POSIX.1; XSI) Enable any 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.
c_oflag flag constants defined in POSIX.1:
OPOST Enable implementation-defined output processing.
The remaining c_oflag flag constants are defined in POSIX 1003.1-2001,
unless marked otherwise.
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.
NLDLY Newline delay mask. Values are NL0 and NL1.
CRDLY Carriage return delay mask. Values are CR0, CR1, CR2, or CR3.
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).
BSDLY Backspace delay mask. Values are BS0 or BS1. (Has never been
implemented.)
VTDLY Vertical tab delay mask. Values are VT0 or VT1.
FFDLY Form feed delay mask. Values are FF0 or FF1.
c_cflag flag constants:
CBAUD (not in POSIX) Baud speed mask (4+1 bits).
CBAUDEX
(not in POSIX) Extra baud speed mask (1 bit), included in CBAUD.
(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, e.g. 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 Parity for input and output is odd.
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.)
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.
CRTSCTS
(not in POSIX) Enable RTS/CTS (hardware) flow control.
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. This enables the special characters EOF,
EOL, EOL2, ERASE, KILL, LNEXT, REPRINT, STATUS, and WERASE, and
buffers by lines.
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 char-
acters are preceded by \ and lowercase characters are converted
to uppercase.
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, ASCII control signals other
than TAB, NL, START, and STOP are echoed as ^X, where X is the
character with ASCII code 0x40 greater than the control signal.
For example, character 0x08 (BS) is echoed as ^H.
ECHOPRT
(not in POSIX) If ICANON and IECHO are also set, characters are
printed as they are being erased.
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.
DEFECHO
(not in POSIX) Echo only when a process is reading.
FLUSHO (not in POSIX; not supported under Linux) Output is being
flushed. This flag is toggled by typing the DISCARD character.
NOFLSH Disable flushing the input and output queues when generating the
SIGINT, SIGQUIT and SIGSUSP signals.
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 handles typeahead this way.)
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 special control characters. The symbolic
indices (initial values) and meaning are:
VINTR (003, ETX, Ctrl-C, or also 0177, DEL, rubout) Interrupt charac-
ter. Send a SIGINT signal. Recognized when ISIG is set, and
then not passed as input.
VQUIT (034, FS, Ctrl-\) Quit character. Send SIGQUIT signal. Recog-
nized when ISIG is set, and then not passed as input.
VERASE (0177, DEL, rubout, or 010, BS, Ctrl-H, or also #) Erase charac-
ter. 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.
VKILL (025, NAK, Ctrl-U, or Ctrl-X, or also @) Kill character. This
erases the input since the last EOF or beginning-of-line. Rec-
ognized when ICANON is set, and then not passed as input.
VEOF (004, EOT, Ctrl-D) End-of-file character. More precisely: this
character causes the pending tty buffer to be sent to the wait-
ing user program without waiting for end-of-line. If it is the
first character of the line, the read() in the user program
returns 0, which signifies end-of-file. Recognized when ICANON
is set, and then not passed as input.
VMIN Minimum number of characters for non-canonical read.
VEOL (0, NUL) Additional end-of-line character. Recognized when
ICANON is set.
VTIME Timeout in deciseconds for non-canonical read.
VEOL2 (not in POSIX; 0, NUL) Yet another end-of-line character. Rec-
ognized when ICANON is set.
VSWTCH (not in POSIX; not supported under Linux; 0, NUL) Switch charac-
ter. (Used by shl only.)
VSTART (021, DC1, Ctrl-Q) Start character. Restarts output stopped by
the Stop character. Recognized when IXON is set, and then not
passed as input.
VSTOP (023, DC3, Ctrl-S) Stop character. Stop output until Start char-
acter typed. Recognized when IXON is set, and then not passed
as input.
VSUSP (032, SUB, Ctrl-Z) Suspend character. Send SIGTSTP signal. Rec-
ognized when ISIG is set, and then not passed as input.
VDSUSP (not in POSIX; not supported under Linux; 031, EM, Ctrl-Y)
Delayed suspend character: send SIGTSTP signal when the charac-
ter 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.
VLNEXT (not in POSIX; 026, SYN, Ctrl-V) Literal next. Quotes the next
input character, depriving it of a possible special meaning.
Recognized when IEXTEN is set, and then not passed as input.
VWERASE
(not in POSIX; 027, ETB, Ctrl-W) Word erase. Recognized when
ICANON and IEXTEN are set, and then not passed as input.
VREPRINT
(not in POSIX; 022, DC2, Ctrl-R) Reprint unread characters.
Recognized when ICANON and IEXTEN are set, and then not passed
as input.
VDISCARD
(not in POSIX; not supported under Linux; 017, SI, Ctrl-O) Tog-
gle: start/stop discarding pending output. Recognized when IEX-
TEN is set, and then not passed as input.
VSTATUS
(not in POSIX; not supported under Linux; status request: 024,
DC4, Ctrl-T).
These symbolic subscript values are all different, except that VTIME,
VMIN may have the same value as VEOL, VEOF, respectively. (In non-
canonical mode the special character meaning is replaced by the timeout
meaning. MIN represents the minimum number of characters that should be
received to satisfy the read. TIME is a decisecond-valued timer. When
both are set, a read will wait until at least one character has been
received, and then return as soon as either MIN characters have been
received or time TIME has passed since the last character was received.
If only MIN is set, the read will not return before MIN characters have
been received. If only TIME is set, the read will return as soon as
either at least one character has been received, or the timer times
out. If neither is set, the read will return immediately, only giving
the currently already available characters.)
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 pro-
cess.
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 trans-
mitted. This function 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.
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 that 0.5 seconds. If duration is
not zero, it sends zero-valued bits for duration*N seconds, where N is
at least 0.25, and not more than 0.5.
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.
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.
cfmakeraw sets the terminal attributes 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;
cfgetospeed() returns the output baud rate stored in the termios struc-
ture 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
The zero baud rate, B0, is used to terminate the connection. If B0 is
specified, the modem control lines shall no longer be asserted. Nor-
mally, 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.
cfgetispeed() returns the input baud rate stored in the termios struc-
ture.
cfsetispeed() sets the input baud rate stored in the termios structure
to speed. If the input baud rate is set to zero, the input baud rate
will be equal to the output baud rate.
RETURNVALUE
cfgetispeed() returns the input baud rate stored in the termios struc-
ture.
cfgetospeed() returns the output baud rate stored in the termios struc-
ture.
All other functions return:
0 on success.
-1 on failure and set errno to indicate the error.
Note that tcsetattr() returns success if any of the requested changes
could be successfully carried out. Therefore, when making multiple
changes it may be necessary to follow this call with a further call to
tcgetattr() to check that all changes have been performed successfully.
NOTES
Unix V7 and several later systems have a list of baud rates where after
the fourteen values B0, ..., B9600 one finds the two constants EXTA,
EXTB ("External A" and "External B"). Many systems extend the list
with much higher baud rates.
SEEALSO
stty(1), setserial(8)
Linux 2001-12-17 TERMIOS(3)
Take our 
