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NAME
termios, tcgetattr, tcsetattr, tcsendbreak, tcdrain, tcflush, tcflow, cfmakeraw, cfgetospeed,
cfgetispeed, cfsetispeed, cfsetospeed, cfsetspeed - get and set terminal attributes, line control, get
and set baud rate
SYNOPSIS
#include <termios.h>
#include <unistd.h>
int tcgetattr(int fd, struct termios *termios_p);
int tcsetattr(int fd, int optional_actions,
const struct termios *termios_p);
int tcsendbreak(int fd, int duration);
int tcdrain(int fd);
int tcflush(int fd, int queue_selector);
int tcflow(int fd, int action);
void cfmakeraw(struct termios *termios_p);
speed_t cfgetispeed(const struct termios *termios_p);
speed_t cfgetospeed(const struct termios *termios_p);
int cfsetispeed(struct termios *termios_p, speed_t speed);
int cfsetospeed(struct termios *termios_p, speed_t speed);
int cfsetspeed(struct termios *termios_p, speed_t speed);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
cfsetspeed(), cfmakeraw(): _BSD_SOURCE
DESCRIPTION
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 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.
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 defined in POSIX.1:
OPOST Enable implementation-defined output processing.
The remaining c_oflag flag constants are defined in POSIX.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 ('\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]
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 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.
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.
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).
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(2) completes; there are
four distinct cases:
* MIN == 0; TIME == 0: 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: read(2) 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(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.
* 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(2) 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 started only after the initial byte becomes available, at
least one byte will be read.
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 that 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
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.
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.
RETURN VALUE
cfgetispeed() returns the input baud rate stored in the termios structure.
cfgetospeed() returns the output baud rate stored in the termios structure.
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.
CONFORMING TO
tcgetattr(), tcsetattr(), tcsendbreak(), tcdrain(), tcflush(), tcflow(), cfgetispeed(), cfgetospeed(),
cfsetispeed(), and cfsetospeed() are specified in POSIX.1-2001.
cfmakeraw() and cfsetspeed() are nonstandard, but available on the BSDs.
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.
The effect of a nonzero duration with tcsendbreak() varies. SunOS specifies a break of duration * N
seconds, where N is at least 0.25, and not more than 0.5. Linux, AIX, DU, Tru64 send a break of duration
milliseconds. FreeBSD and NetBSD and HP-UX and MacOS ignore the value of duration. Under Solaris and
UnixWare, tcsendbreak() with nonzero duration behaves like tcdrain().
SEE ALSO
stty(1), console_ioctl(4), tty_ioctl(4), setserial(8)
COLOPHON
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