termios(3) Library Functions Manual termios(3)
NAME
termios, tcgetattr, tcsetattr, tcsendbreak, tcdrain, tcflush, tcflow,
cfmakeraw, cfgetospeed, cfgetispeed, cfsetispeed, cfsetospeed, cfset-
speed - get and set terminal attributes, line control, get and set baud
rate
LIBRARY
Standard C library (libc, -lc)
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():
Since glibc 2.19:
_DEFAULT_SOURCE
glibc 2.19 and earlier:
_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 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 con-
fuse 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 The NL character is assumed to do the carriage-return function;
the kernel's idea of the current column is set to 0 after both
NL and CR.
OFILL Send fill characters for a delay, rather than using a timed de-
lay.
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 col-
umns). [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 struc-
ture 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 sepa-
rate 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 in-
put.
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 im-
plemented in glibc, supported on Linux via TCGET* and TCSET*
ioctls; see ioctl_tty(2))
CMSPAR (not in POSIX) Use "stick" (mark/space) parity (supported on
certain serial devices): if PARODD is set, the parity bit is al-
ways 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. [re-
quires _BSD_SOURCE or _SVID_SOURCE]
c_lflag flag constants:
ISIG When any of the characters INTR, QUIT, SUSP, or DSUSP are re-
ceived, 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 char-
acters 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 imple-
mented 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) Tog-
gle: start/stop discarding pending output. Recognized when IEX-
TEN is set, and then not passed as input.
VDSUSP (not in POSIX; not supported under Linux; 031, EM, Ctrl-Y) De-
layed 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 charac-
ter (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 charac-
ter (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 (RE-
PRINT). 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 infor-
mation 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 sig-
nal. Recognized when ISIG is set, and then not passed as input.
VSWTCH (not in POSIX; not supported under Linux; 0, NUL) Switch charac-
ter (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). Recog-
nized when ICANON and IEXTEN are set, and then not passed as in-
put.
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 de-
scription 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 trans-
mitted. This option should be used when changing parameters
that affect output.
TCSAFLUSH
the change occurs after all output written to the object re-
ferred 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 in-
put; 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. Af-
ter 4095 characters, input processing (e.g., ISIG and ECHO* process-
ing) 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 per-
formed, 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 avail-
able, 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 sta-
tus flag takes precedence over the MIN and TIME settings. If O_NON-
BLOCK 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 ei-
ther 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 charac-
ter, 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, tc-
sendbreak() 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 re-
ferred 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 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
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. 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.
Setting the baud rate to a value other than those defined by Bnnn con-
stants is possible via the TCSETS2 ioctl; see ioctl_tty(2).
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, which must be specified as one of the Bnnn constants listed
above for cfsetospeed(). If the input baud rate is set to the literal
constant 0 (not the symbolic constant B0), the input baud rate will be
equal to the output baud rate.
cfsetspeed() is a 4.4BSD extension. It takes the same arguments as cf-
setispeed(), and sets both input and output speed.
RETURN VALUE
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.
ATTRIBUTES
For an explanation of the terms used in this section, see at-
tributes(7).
┌────────────────────────────────────────────┬───────────────┬─────────┐
│Interface │ Attribute │ Value │
├────────────────────────────────────────────┼───────────────┼─────────┤
│tcgetattr(), tcsetattr(), tcdrain(), │ Thread safety │ MT-Safe │
│tcflush(), tcflow(), tcsendbreak(), │ │ │
│cfmakeraw(), cfgetispeed(), cfgetospeed(), │ │ │
│cfsetispeed(), cfsetospeed(), cfsetspeed() │ │ │
└────────────────────────────────────────────┴───────────────┴─────────┘
STANDARDS
tcgetattr(), tcsetattr(), tcsendbreak(), tcdrain(), tcflush(),
tcflow(), cfgetispeed(), cfgetospeed(), cfsetispeed(), and cfse-
tospeed() are specified in POSIX.1-2001.
cfmakeraw() and cfsetspeed() are nonstandard, but available on the BS-
Ds.
NOTES
UNIX V7 and several later systems have a list of baud rates where after
the values B0 through 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().
BUGS
On the Alpha architecture before Linux 4.16 (and glibc before glibc
2.28), the XTABS value was different from TAB3 and it was ignored by
the N_TTY line discipline code of the terminal driver as a result (be-
cause as it wasn't part of the TABDLY mask).
SEE ALSO
reset(1), setterm(1), stty(1), tput(1), tset(1), tty(1), ioctl_con-
sole(2), ioctl_tty(2), cc_t(3type), speed_t(3type), tcflag_t(3type),
setserial(8)
Linux man-pages 6.03 2023-02-05 termios(3)
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