termios(4)termios(4)Nametermios - POSIX terminal interface
DescriptionInterface Characteristics
The POSIX terminal interface is provided to control asynchronous commu‐
nications ports, pseudoterminals, and the special file, The ULTRIX
operating system also provides a SVID termio terminal interface as
defined in and a Berkeley terminal interface as defined in The follow‐
ing sections describe the general terminal interface as defined by the
POSIX operating system specification. For a general overview of the
various terminal interfaces, refer to the subsection entitled Terminal
interface definitions in
The POSIX termios specification defines a set of terminal-related func‐
tions and attributes that facilitate the development of portable pro‐
grams. The specification allows for local extensions to the terminal
interface. Throughout this description, all local extensions to the
termios interface are noted. Programs that are written to be highly
portable should avoid the usage of local extensions.
Opening a Terminal Device File
When a terminal file is opened, it normally causes the process to wait
until a connection is established. In practice, application programs
seldom open these files; they are opened by special programs such as
and become a user's standard input, output, and error files.
As described in opening a terminal device file with the O_NONBLOCK flag
clear causes the process to block until the terminal device is ready
and available. If CLOCAL mode is not set, blocking occurs until a con‐
nection is established. If CLOCAL mode is set, or the O_NONBLOCK flag
is specified in the the returns a file descriptor, without waiting for
a connection to be established.
Process Groups
A terminal may have a foreground process group associated with it.
This foreground process group plays a special role in handling signal-
generating input characters, as discussed in the Special Characters
description.
A command interpreter that is capable of supporting job control, for
example, can allocate the terminal to different jobs or process groups
by placing related processes in a single process group and associating
this process group with the terminal. A terminal's foreground process
group can be set or examined by a process with sufficient privileges.
The terminal interface aids in this allocation by restricting access to
the terminal by processes that are not in the foreground process group.
See the Job Access Control description for more information.
The Controlling Terminal
A terminal can belong to a process as its controlling terminal. Each
process of a session that has a controlling terminal has the same con‐
trolling terminal. A terminal may be the controlling terminal for at
most one session. If a session leader has no controlling terminal and
opens a terminal device file that is not already associated with a ses‐
sion, without using the O_NOCTTY flag, the terminal then becomes the
controlling terminal of the session leader. If a process that is not a
session leader opens a terminal file, or the O_NOCTTY option is used
with that terminal cannot become the controlling terminal of the call‐
ing process. When a controlling terminal becomes associated with a
session, its foreground process group is set to the process group of
the session leader.
The controlling terminal is inherited by a child process during a func‐
tion. A process relinquishes its controlling terminal when it creates
a new session with the function, or when all file descriptors associ‐
ated with the controlling terminal have been closed.
When a controlling process terminates, the controlling terminal is dis‐
associated from the current session, allowing the terminal to be
acquired as a controlling terminal by a new session process group
leader. Subsequent access to the terminal by other processes in the
earlier session may be denied, with attempts to access the terminal
treated as if a modem disconnect had been sensed.
Closing a Terminal Device File
When the last process that has the terminal line open closes the termi‐
nal line, the process waits for all output to clear and any input is
discarded. The wait does not exceed four minutes, preventing the line
from becoming hung if a progress is not made in the final output.
After the final output has been transmitted, any pending input is
flushed. If HUPCL is set in the control modes, and the communications
port supports a disconnect function, the terminal device performs a
disconnect.
Modem Disconnect
When a modem disconnect is detected by the terminal interface for a
controlling terminal, and the CLOCAL bit is not set in the control
flag, the SIGHUP signal is sent to all the controlling processes asso‐
ciated with the terminal. Unless other arrangements have been made,
this signal causes the controlling processes to terminate. If SIGHUP
is ignored or caught, any subsequent read returns with an end-of-file
indication. Thus, programs that read a terminal file and test for end-
of-file can terminate appropriately after a disconnect. Any subsequent
to the terminal device returns -1 with errno set to [EIO], until the
device is closed.
Terminal Access Control
If a process is in the foreground process group (nonzero) of its con‐
trolling terminal (that is, if the process is a foreground process),
operations are allowed as described in the Input Processing and Reading
Characters description. Any attempts by a process in a background
process group to read from its controlling terminal causes its process
group to receive a SIGTTIN signal, unless one of the following is true:
· If the reading process is ignoring or blocking the SIGTTIN signal.
· If the process group of the reading process is orphaned, the
returns -1, with errno set to [EIO], and no signal is sent.
Attempts by a process in a background process group to write to its
controlling terminal causes the process group to receive a SIGTTOU sig‐
nal, unless one of the following is true:
· If TOSTOP is not set, or if TOSTOP is set and the process is
ignoring or blocking the SIGTTOU signal, the process is allowed to
write to the terminal and the SIGTTOU is not sent.
· If TOSTOP is set, and the process group of the writing process is
orphaned, and the writing process is not blocking SIGTTOU, the
returns -1, with errno set to [EIO], and no signal is sent.
Certain calls that set terminal parameters are treated in the same
fashion as except that TOSTOP is ignored. That is, the effect is iden‐
tical to that of terminal writes when TOSTOP is set. See the Control
Functions description for more information.
Input Processing and Reading Characters
A terminal device associated with a terminal device file operates in
full-duplex mode, so that characters can arrive while output is occur‐
ring. Each terminal device file has associated with it an input queue,
in which incoming characters are stored by the system before being read
by a process. The system imposes a limit (MAX_INPUT), on the number of
bytes that can be stored in the input queue. MAX_INPUT is limited to
256 characters. When ICANON is not set and the system's character
input buffers become full, the input buffer is flushed without notice.
This causes all the characters in the input queue to be lost. If
ICANON is set and the system's character input buffers become full, the
driver discards additional characters and echos a bell (ASCII BEL) to
notify the user of the full condition.
Depending on whether or not the terminal device is in canonical mode or
noncanonical mode, two general types of input processing are available.
See the Canonical Mode Input Processing and Noncanonical Mode Input
Processing descriptions for more information. Additionally, input
characters are processed according to the c_iflag and c_lflag fields.
See the Input Modes and Local Modes descriptions. Such processing can
include echoing, which, in general, means transmitting input characters
immediately back to the terminal when they are received from the termi‐
nal. This is useful for terminals that can operate in full-duplex
mode. The manner in which characters are provided to a process reading
from a terminal device is dependent on whether the terminal file is in
canonical or noncanonical mode.
Another dependency is whether or not the O_NONBLOCK is set by or If the
O_NONBLOCK flag is clear, then the read request is blocked until data
is available or a signal has been received. If the O_NONBLOCK flag is
set, then the read request completes, without blocking, in one of the
following ways:
· If there is enough data available to satisfy the entire request,
the read completes successfully and returns the number of bytes
read.
· If there is not enough data available to satisfy the entire
request, the read completes successfully, having read as much data
as possible and returns the number of bytes it was able to read.
· If data is not available, the read returns a -1, with errno set to
EAGAIN.
As stated previously, the availability of data is dependent on the
input processing mode. The input processing mode can be either canoni‐
cal or non-canonical. The following sections discuss these modes in
detail.
Canonical Mode Input Processing
In canonical mode input processing, terminal input is processed in
units of lines. A line is delimited by a new-line (\n) character
(ASCII LF), an end-of-file character (ASCII EOF), or a user-defined
end-of-line character (EOL) . See the description of Special Charac‐
ters for more information on EOF and EOL.
A read request cannot be satisfied until an entire line has been typed
or a signal has been received. Regardless of the number of characters
requested in the read call, at most one line is returned. However, it
is not necessary to read a whole line at once; one or more characters
can be requested in a read without losing information.
MAX_CANON (256) defines the maximum number of input characters the sys‐
tem can buffer in canonical mode. When this limit is exceeded, the
system discards additional input.
Erase and kill processing occur when either the ERASE or KILL character
is received. This processing affects data in the input queue that has
not been delimited by a new-line (NL), end-of-file (EOF), or end-of-
line (EOL) character. The data that is not delimited creates the cur‐
rent line. The ERASE character deletes the last character in the cur‐
rent line, if there is any. The KILL character deletes all data in the
current line, if there is any. The ERASE and KILL characters have no
effect if there is no data in the current line. The ERASE and KILL
characters themselves are not placed in the input queue.
The reprint (RPRNT) character retypes pending input beginning on a new
line. Retyping is automatic, if characters that would normally be
erased from the screen are interspersed with program output.
Noncanonical Mode Input Processing
In noncanonical mode input processing, input characters are not assem‐
bled into lines, and erase and kill processing do not occur. The val‐
ues of the special characters MIN and TIME are used to determine how to
process the characters received. MIN and TIME are defined by the c_cc
array of special control characters.
MIN represents the minimum number of characters that should be received
when the read is satisfied (for example, the characters are returned to
the user). TIME is a timer of 0.1 second granularity that is used to
time out bursty and short term data transmissions. If MIN is greater
than MAX_INPUT (256), the value of MIN is truncated to be MAX_INPUT.
The four possible values for MIN and TIME and examples of their inter‐
actions are as follows:
1. MIN > 0, TIME > 0.
In this case, TIME serves as an intercharacter timer and is activated
after the first character is received. Because it is an intercharacter
timer, it is reset after a character is received. The interaction
between MIN and TIME provokes the intercharacter time to start as soon
as one character is received. If MIN characters are received before the
intercharacter timer expires, the read is satisfied.
If the timer expires before MIN characters are received, the characters
received to that point are returned to the user. If TIME expires, at
least one character is returned, because the timer would not have been
enabled unless a character was received. The read blocks until the MIN
and TIME mechanisms are activated by the receipt of the first charac‐
ter, or a signal is received.
2. MIN > 0, TIME = 0.
In this case, the value of TIME is zero, the timer is inactive. How‐
ever, MIN is significant. A pending read is not satisfied until MIN
characters or a signal are received. That is, the pending read sleeps
until MIN characters are received. A program that uses this example to
read record-based terminal I/O can indefinitely block the read opera‐
tion.
3. MIN = 0, TIME > 0.
In this case, because MIN = 0, TIME does not represent an intercharac‐
ter timer. Instead, it serves as a read timer that is activated as soon
as the function is processed. A read is satisfied as soon as a single
character is received or the read timer expires. Note, if the timer
expires, a character is not returned. If the timer does not expire, a
read is only satisfied if a character is received. For example, the
read cannot block indefinitely waiting for a character. If a character
is not received within TIME*0.1 seconds after the read is initiated,
the read returns a value of zero, having read no data.
4. MIN = 0, TIME = 0.
In this case, only the minimum number of characters requested or the
number of characters currently available are returned without waiting
for more characters to be input. In this example, the return is imme‐
diate.
The following list summarizes the previous examples:
· The interactions of MIN and TIME are not symmetric. For example,
when MIN > 0 and TIME = 0, TIME has no effect. However, if MIN =
0 and TIME > 0, MIN is activated by the receipt of a single char‐
acter.
· When MIN > 0 and TIME > 0, TIME represents an intercharacter
timer; when MIN = 0, TIME > 0, TIME represents a read timer.
The previous summary highlights the dual purpose of the MIN and TIME
feature. Cases 1 and 2 handle burst mode activity (such as file trans‐
fer programs) where a program would like to process at least MIN char‐
acters at a time. In case 1, the intercharacter timer is activated by
a user as a safety precaution. However, in case 2, it is turned off.
Cases 3 and 4 exist to handle single character timed transfers. These
examples are readily adaptable to screen-based applications that need
to know if a character is present in the input queue before refreshing
the screen. In case 3, the read is timed. However, in case 4, it is
not.
Note that MIN is always a minimum. It does not denote a record length.
That is, if a program performs a read of 20 bytes, MIN has a value of
10, and there are 25 characters present, 20 characters are returned to
the user.
Writing Characters and Output Processing
When a process writes one or more characters to a terminal device file,
they are processed according to the c_oflag (see Output Modes). The
terminal interface provides a buffering mechanism. For example, when a
call to completes, all of the characters written have been scheduled
for transmission to the device, but the transmission is not necessarily
complete. The characters are transmitted to the terminal as soon as
previously written characters have output successfully.
Signal Handling
Signals caught during a or other operation on the file descriptor asso‐
ciated with the terminal file are handled appropriately, as described
in
Special Characters
Certain characters have special functions on input or output. These
functions are:
INTR Special character on input that is recognized if the ISIG
flag is enabled. Generates a SIGINT signal that is sent to
all processes in the foreground process group associated
with the terminal. If ISIG is set, the INTR character is
discarded when processed. The default value is octal 0177.
QUIT Special character on input that is recognized if the ISIG
flag is enabled. Generates a SIGQUIT signal that is sent to
all processes in the foreground process group associated
with the terminal. If ISIG is set, the QUIT character is
discarded when processed. The default value is CTRL/|
(ASCII FS).
ERASE Special character on input that is recognized if the ICANON
flag is set. Erases the last character in the current line.
It cannot erase beyond the start of a line, as delimited by
an NL, EOF, or EOL character. If ICANON is set, the ERASE
character is discarded when processed. The default value is
the number sign (#).
KILL Special character on input that is recognized if the ICANON
flag is set. Deletes the entire line, as delimited by an
NL, EOF, or EOL character. If ICANON is set, the KILL char‐
acter is discarded when processed. The default value is the
at sign (@).
EOF Special character on input that is recognized if the ICANON
flag is set. This character is used to generate an end of
file (EOF) from the terminal. When received, all the char‐
acters waiting to be read are immediately passed to the pro‐
gram, without waiting for a newline, and the EOF is dis‐
carded. Thus, if there are no characters waiting (that is,
if the EOF occurred at the beginning of a line), a byte
count of zero is returned from representing an end-of-file
indication. If ICANON is set, the EOF character is dis‐
carded when processed. The default value is CTRL/D (ASCII
EOT).
NL Special character on input that is recognized if the ICANON
flag is set. It is the assigned line delimiter (\n). It
cannot be changed.
EOL Special character on input that is recognized if the ICANON
flag is set. It is an additional line delimiter, like NL.
The default value is POSIX_V_DISABLE, which is used to spec‐
ify that this special character is ordinarily not used.
SUSP Special character on input that is recognized if the ISIG
flag is enabled. Generates a SIGTSTP signal that is sent to
all processes in the foreground process group associated
with the terminal. This signal is used by the job control
code to change from the current job to the controlling job.
If ISIG is set, the SUSP character is discarded when pro‐
cessed. The default value is CTRL/Z (ASCII SUB).
STOP Special character on both input and output that is recog‐
nized if the IXON (input) or IXOFF (output) flag is set.
Can be used to temporarily suspend output. It is useful
with CRT terminals to prevent output from disappearing
before it can be read. While output is suspended, STOP
characters are ignored and not read. If IXON is set, the
STOP character is discarded when processed. The default
value is CTRL/S (ASCII DC3).
START Special character on both input and output that is recog‐
nized if the IXON (input) or IXOFF (output) flag is set.
Can be used to resume output that has been suspended by a
STOP character. While output is not suspended, START char‐
acters are ignored and not read. If IXON is set, the STOP
character is discarded when processed. The default value is
CTRL/Q (ASCII DC1).
CR Special character on input that is recognized if the ICANON
flag is set. The value is (\r) and this value is not
changeable. When ICANON and ICRNL are set and IGNCR is not
set, this character is translated into an NL, and it has the
same effect as an NL character.
DSUSP Special character on input used as a delayed suspend charac‐
ter. DSUSP is only recognized if the IEXTEN flag is set.
Similar to the SUSP special character, a SIGTSTP signal is
issued. The process cannot actually stop until the next
character is input. If IEXTEN is set, the DSUSP character
is discarded when processed. The default value is CTRL/Y
(ASCII EM).
RPRNT Special character on input used to force the present input
line to be re-echoed to the terminal after a newline charac‐
ter. RPRNT is only recognized if the IEXTEN flag is set.
If IEXTEN is set, the RPRNT character is discarded when pro‐
cessed. The default value is CTRL/R (ASCII DC2).
FLUSH Special character on input that causes output to the termi‐
nal to be flushed until another flush character is typed or
more input is received on the terminal. FLUSH is recognized
only if the IEXTEN flag is set. If IEXTEN is set, the FLUSH
character is discarded when processed. The default value is
CTRL/O (ASCII SI).
WERASE Special character on input used to erase the preceding word
of the input queue. The word erase operation erases charac‐
ters up to (and not including) a TAB, NL, space, or back‐
slash (\) character. Word erase cannot erase beyond the
start of a line as delimited by NL, EOF, or EOL. WERASE is
only recognized if the IEXTEN flag is set. If IEXTEN is
set, the WERASE character is discarded when processed. The
default value is CTRL/W (ASCII ETB).
LNEXT Special character on input used to disassociate any special
meaning that the next input character has. This allows for
the input of characters that would otherwise be interpreted
as special characters. LNEXT is only recognized if the IEX‐
TEN flag is set. If IEXTEN is set, the LNEXT character is
discarded when processed. The default value is CTRL/V
(ASCII SYN).
QUOTE Special character on input used to enter a literal ERASE or
KILL character. The same functionality could be achieved
through the use of the LNEXT character, but QUOTE is
included for backward compatibility. The default value is a
backslash (\). QUOTE is only recognized if the IEXTEN flag
is set. If IEXTEN is set, the QUOTE character is discarded
when processed.
The values for INTR, QUIT, ERASE, KILL, EOF, EOL, SUSP, START, STOP,
DSUSP, RPRNT, FLUSH WERASE, LNEXT, and QUOTE are changeable to suit
individual tastes. The following special characters are local exten‐
sions of DSUSP, RPRNT, FLUSH, WERASE, LNEXT, and QUOTE.
Special character functions can be disabled individually by setting
them to the constant POSIX_V_DISABLE, which is defined to be zero. The
POSIX_V_DISABLE character is always read if received, and never causes
a special character function. With the exception of NL and EOL, the
special characters cannot be passed up to the reading process.
If two or more special characters have the same value, the function
performed when that character is received is undefined. More than one
special character can be set to POSIX_V_DISABLE to disable the control
function normally associated with the special character.
Settable Parameters
Routines that need to control certain terminal I/O characteristics can
do so by using the termios structure as defined in the header The mem‐
bers of this structure include:
Member Member Description
Type Name
struct termios {
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 */
cc_t c_line; /* line discipline */
}
The types tcflag_t and cc_t are defined in the header
Input Modes
The c_iflag field describes the basic terminal input control:
──────────────────────────────────
Mask Name Description
®
──────────────────────────────────
IGNBRK Ignore the break
condition.
BRKINT Signal interrupt on
break.
IGNPAR Ignore characters
with parity errors.
PARMRK Mark parity errors.
INPCK Enable input parity
check.
ISTRIP Strip character.
INLCR Map NL to CR on
input.
IGNCR Ignore CR.
ICRNL Map CR to NL on
input.
IXON Enable start/stop
output control.
IXOFF Enable start/stop
input control.
──────────────────────────────────
A break condition is defined as a sequence of zero-valued bits that
continues for more than the time to send one byte. The entire sequence
of zero-valued bits is interpreted as a single break condition, even if
it continues for a time equivalent to more than one byte.
If IGNBRK is set, a break condition detected on input is ignored, that
is, not put on the input queue and therefore not read by any process.
Otherwise, if BRKINT is set, the break condition generates a single
SIGINT signal and flushes both the input and output queues. If neither
IGNBRK or BRKINT is set, a break condition is read as a single \0
(ASCII NUL), or, if PARMRK is set, as \377,\0,\0.
If IGNPAR is set, a byte with a framing or parity error (other than
break) is ignored.
If PARMRK is set and IGNPAR is not set, a character with a framing or
parity error (other than break) that is not ignored is given to the
application as the 3-character sequence \377,\0,X, where \377, \0 is a
2-character flag preceding each sequence and X is the data of the char‐
acter received in error. To avoid ambiguity in this case, if ISTRIP is
not set, a valid character of \377 is given to the application as \377,
\377. If either IGNPAR or PARMRK is set, a framing or parity error
(other than break) that is not ignored is given to the application as a
single character \0.
If INPCK is set, input parity checking is enabled. If INPCK is not
set, input parity checking is disabled, allowing output parity genera‐
tion without input parity errors. Parity checking can be enabled, even
if parity checking is not enabled. If parity detection is enabled,
but input parity checking is disabled, the hardware that connects to
the terminal recognizes the parity bit, but the terminal special file
does not check whether this bit is set correctly or not.
If ISTRIP is set, valid input characters are first stripped to seven
bits; otherwise, all eight bits are processed.
If INLCR is set, a received NL character is translated into a CR char‐
acter. If IGNCR is set, a received CR character is ignored (not read).
If IGNCR is not set and ICRNL is set, a received CR character is trans‐
lated into a NL character.
If IXON is set, start/stop output control is enabled. A received STOP
character suspends output and a received START character restarts out‐
put. When IXON is set, START and STOP characters are not read, but
merely perform flow control functions. When IXON is not set, the START
and STOP characters are read.
If IXOFF is set, start/stop input control is enabled. The system
transmits STOP characters which cause the terminal device to stop
transmitting data when the number of characters in the input queue
attempt to exceed MAX_INPUT (256). As soon as the device can continue
transmitting data without risk of an overflow, START characters are
transmitted which cause the terminal device to resume transmitting
data.
The initial input control value after is 0 (all settings off).
Output Modes
The c_oflag field specifies the terminal interface's treatment of out‐
put. Because OPOST is the only output flag defined by the POSIX stan‐
dard, all of the other definitions are local extensions to the stan‐
dard.
──────────────────────────────────
Mask Name Description
®
──────────────────────────────────
OPOST Postprocess output.
OLCUC Map lower case to
upper on output.
ONLCR Map NL to CR-NL on
output.
OCRNL Map CR to NL on out‐
put.
ONOCR No CR output at col‐
umn 0.
ONLRET NL performs CR func‐
tion.
OFILL Use fill characters
for delay.
OFDEL Fill is DEL, else
NUL.
NLDLY Select new-line
delays:
NL0 New-line delay type
0.
NL1 New-line delay type
1.
CRDLY Select carriage-
return delays:
CR0 Carriage-return
delay type 0.
CR1 Carriage-return
delay type 1.
CR2 Carriage-return
delay type 2.
CR3 Carriage-return
delay type 3.
TABDLY Select horizontal-
tab delays:
TAB0 Horizontal-tab delay
type 0.
TAB1 Horizontal-tab delay
type 1.
TAB2 Horizontal-tab delay
type 2.
TAB3 Expand tabs to spa‐
ces.
BSDLY Select backspace
delays:
BS0 Backspace delay type
0.
BS1 Backspace delay type
1.
VTDLY Select vertical-tab
delays:
VT0 Vertical-tab delay
type 0.
VT1 Vertical-tab delay
type 1.
FFDLY Select form-feed
delays:
FF0 Form-feed delay type
0.
FF1 Form-feed delay type
1.
──────────────────────────────────
If OPOST is set, output characters are post-processed as indicated by
the remaining flags. Otherwise, characters are transmitted without
change.
If OLCUC is set, a lowercase letter is transmitted as the corresponding
uppercase letter. This function is often used in conjunction with
IUCLC.
If ONLCR is set, the NL character is transmitted as the CR-NL character
pair. If OCRNL is set, the CR character is transmitted as the NL char‐
acter. If ONOCR is set, no CR character is transmitted when at column
0 (first position). If ONLRET is set, the NL character is assumed to
do the carriage-return function; the column pointer is set to 0, and
the delays specified for CR are used. Otherwise the NL character is
assumed to do just the line-feed function; the column pointer remains
unchanged. The column pointer is also set to 0 if the CR character is
actually transmitted.
Delay bits specify transmission stops that allow mechanical or other
movement when certain characters are sent to the terminal. In all
cases, a value of 0 indicates no delay. If OFILL is set, fill charac‐
ters are transmitted for delay instead of a timed delay. This is use‐
ful for high baud rate terminals which need only a minimal delay. If
OFDEL is set, the fill character is DEL. Otherwise, it is NUL.
The delay specifications for NLDLY, CRDLY, TABDLY, BSDLY, VTDLY and
FFDLY are meant to serve as masks for the respective delay field.
If a form-feed or vertical-tab delay is specified, either delay lasts
approximately two seconds.
New-line delay lasts about 0.10 seconds. If ONLRET is set, the car‐
riage-return delays are used instead of the new-line delays. If OFILL
is set, two fill characters are transmitted.
Carriage-return delay for type 1 depends on the current column posi‐
tion, type 2 is approximately 0.10 seconds, and type 3 is approximately
0.15 seconds. If OFILL is set, delay type 1 transmits two fill charac‐
ters, and type 2, four fill characters.
Horizontal-tab delay type 1 depends on the current column position.
Type 2 is approximately 0.10 seconds. Type 3 specifies that tabs are
to be expanded into spaces. If OFILL is set, two fill characters are
transmitted for any delay.
Backspace delay lasts about 0.05 seconds. If OFILL is set, one fill
character is transmitted.
The actual delays depend on line speed and system load.
The initial output control value after open(2) is 0 (all settings off).
Control Modes
The c_cflag field describes the hardware control of the terminal:
──────────────────────────────────
Mask Name Description
®
──────────────────────────────────
CSIZE Character size:
CS5 5 bits
CS6 6 bits
CS7 7 bits
CS8 8 bits
CSTOPB Send two stop bits,
else one.
CREAD Enable receiver.
PARENB Parity enable.
PARODD Odd parity, else
even.
HUPCL Hung up on last
close.
CLOCAL Ignore modem status
lines.
TAUTOFLOW Use hardware moni‐
tored flow control.
──────────────────────────────────
In addition, the input and output baud rates are also stored in the
c_cflag field. The following values are supported:
─────────────────────
Name Description
®
─────────────────────
B0 Hang up
B50 50 baud
B75 75 baud
B110 110 baud
B134 134.5 baud
B150 150 baud
B300 300 baud
B600 600 baud
B1200 1200 baud
B1800 1800 baud
B2400 2400 baud
B4800 4800 baud
B9600 9600 baud
B19200 19200 baud
B38400 38400 baud
─────────────────────
The following interfaces are provided for getting and setting the val‐
ues of the input and output baud rates in the structure. The effects
on the terminal device do not become effective until the function is
successfully called.
speed_t cfgetospeed (termios_p)
struct termios *termios_p;
int cfsetospeed (termios_p, speed)
struct termios *termios_p;
speed_t speed;
speed_t cfgetispeed (termios_p)
struct termios *termios_p;
int cfsetispeed (termios_p, speed)
struct termios *termios_p;
speed_t speed;
The type speed_t is defined in
The termios_p argument is a pointer to a termios structure that allows
the c_cflag field to be manipulated. The cfgetospeed() returns the
output baud rate stored in cflag.
Additionally, the cfsetospeed() sets the output baud rate stored in the
cflag to speed. The zero baud rate, B0, is used to terminate the con‐
nection. If B0 is specified, the modem control line can no longer be
asserted. Normally, this disconnects the line. Both cfsetispeed() and
cfsetospeed() return a value of zero if successful and -1 to indicate
error.
The cfgetispeed() returns the input baud rate stored in cflag.
The cfsetispeed() sets the input baud rate stored in cflag to speed.
If the input baud rate is set to zero, the input baud rate is specified
by the value of the output baud rate. For any particular hardware,
unsupported baud rates are ignored. This refers to changes and baud
rates not supported by the hardware, and to changes setting the input
and output baud rates to different values if the hardware does not sup‐
port this.
The CSIZE bits specify the character size in bits for both transmission
and reception. This size does not include the parity bit, if any. If
CSTOPB is set, two stop bits are used, otherwise one stop bit. For
example, at 110 baud, two stop bits are normally used.
If CREAD is set, the receiver is enabled. Otherwise, no characters are
received.
If PARENB is set, parity generation and detection is enabled and a par‐
ity bit is added to each character. If parity is enabled, PARODD spec‐
ifies odd parity, if set. Otherwise, even parity is used.
If TAUTOFLOW is set, hardware monitored flow control is performed, if
the hardware supports this functionality. In this mode, the hardware
terminal multiplexer suspends output upon receipt of a STOP character
(ASCII DC3). The hardware resumes output after a START character
(ASCII DC1) has been received. The advantage of this mode is that it
provides quick response to flow control characters which would be use‐
ful in preventing overflow of the terminal device's input buffer.
TAUTOFLOW is a local extension to the termios specification.
If HUPCL is set, the modem control lines for the port are lowered when
the last process with the port open closes the port or the process ter‐
minates. The modem connection is broken. If HUPCL is not set, the
control lines are not altered.
If CLOCAL is set, a connection does not depend on the state of the
modem status lines. If CLOCAL is clear, the modem status lines are
monitored. CLOCAL is typically used by direct connect terminal lines.
Under normal circumstances, a call to the open(2) function waits for
the modem connection to complete. However, if the O_NONBLOCK flag is
set, or, if CLOCAL has been set, the function returns immediately with‐
out waiting for the connection. For further information, see For those
files on which the connection has not been established, or on which a
modem disconnect has occurred, and for which CLOCAL is not set, both
and return a zero character count. For this is equivalent to an end-
of-file condition.
The initial hardware control value after open(2) is CS8, CREAD, HUPCL,
B300.
Local Modes
The c_lflag field of the argument structure is used to control various
functions.
─────────────────────────────────────────────────────────────────
Mask Name Description
®
─────────────────────────────────────────────────────────────────
ISIG Enable signals
ICANON Canonical input (erase and kill processing).
NOFLSH Disable flush after interrupt, quit, or suspend.
TOSTOP Send SIGTTOU for background output.
ECHO Enable echo.
ECHOE Echo ERASE as an error-correcting backspace.
ECHOK Echo KILL.
ECHONL Echo \n.
IEXTEN Enable extended (implementation defined) functions.
TCTLECH Echo input control chars as ^char, delete as ^?.
TCRTKIL BS-space-BS erase entire line on kill.
TPRTERA Hard-copy terminal erase mode using
─────────────────────────────────────────────────────────────────
If ISIG is set, each input character is checked against the special
control characters INTR, QUIT, and SUSP. If an input character matches
one of these control characters, the function associated with that
character is performed. If ISIG is not set, no checking is done.
Thus, these special input functions are possible only if ISIG is set.
If ICANON is set, canonical processing is enabled. This enables the
erase, word erase, reprint, and kill edit functions, and the assembly
of input characters into lines delimited by NL, EOF, and EOL, as
described in Canonical Mode Input Processing.
If ICANON is not set, requests are satisfied directly from the input
queue. A is not satisfied until at least MIN characters have been
received or the timeout value TIME expired between characters. The
time value represents tenths of seconds. See the Noncanonical Mode
Input Processing section for more details.
If NOFLSH is set, the normal flush of the input and output queues asso‐
ciated with the INTR, QUIT, and SUSP characters is not done.
If TOSTOP is set, the signal SIGTTOU is sent to the process group of a
process that tries to write to its controlling terminal, if it is not
in the foreground process group for that terminal. This signal, by
default, stops the members of the process group. Otherwise, the output
generated by that process is output to the current output stream. Pro‐
cesses that are holding or ignoring SIGTTOU signals are accepted and
allowed to produce output and the SIGTTOU signal is not sent.
If ECHO is set, input characters are echoed back to the terminal. If
ECHO is not set, input characters are not echoed.
The echo functions (ECHOE, ECHOK, ECHONL, TCTLECH, TCRTKIL, and
TPRTERA) are performed if ICANON is set.
If ECHOE and ICANON are set, the ERASE character causes the terminal to
erase the last character in the current line from the display, if pos‐
sible.
If ECHOK and ICANON are set, the KILL character, either the terminal
erases the line from the display or echoes the \n character after the
KILL character.
If ECHONL and ICANON are set, the \n character is echoed even if ECHO
is not set.
If IEXTEN is set, implementation defined functions are recognized from
the input data. In this manner the DSUSP, RPRNT, FLUSH, WERASE, LNEXT,
and QUOTE special characters in the c_cc array are only recognized if
the IEXTEN flag is set.
If TCTLECH is set, all control characters are echoed as ^X, where X is
the character obtained by adding the octal value of the character A
(100) to the octal code for the control character. In this context, a
control character is defined to be a character whose octal value is
less than 37. The following control characters are excluded from
TCTLECH operations: ASCII NL, ASCII TAB, as well as control characters
that are defined in the c_cc array but are not returned to user pro‐
grams (such as START and STOP). TCTLECH is a local extension to the
local modes.
If TCRTKIL is set, the response to a kill character is to erase the
present input line through a sequence of backspace-space-backspace.
TCRTKIL is a local extension to the local modes.
If TCRTERA is set, characters that are logically erased are printed out
backwards preceded by a backslash (\) and followed by a slash (/).
This mode is useful when a hard-copy terminal is in use. TCRTERA is a
local extension to the local modes.
The initial local control value after open (2) is 0 (all bits clear).
Special Control Characters
The special control characters values are defined by the array c_cc.
The subscript name and description for each element in both canonical
and noncanonical modes are as follows.
──────────────────────────────────────────────
Subscript Description
®
──────────────────────────────────────────────
VINTR (INTR) Interrupt character
VQUIT (QUIT) Quit character
VERASE (ERASE) Erase character
VKILL (KILL) Kill character
VEOF (EOF) End-of-file character
VEOL (EOL) End-of-line character
VMIN (MIN) Value for noncanonical
reads
VTIME (TIME) Value for noncanonical
reads
VSTART (START) Start character
VSTOP (STOP) Stop character
VSUSP (SUSP) Suspend character
VDSUSP (DSUSP) Delayed suspend character
VRPRNT (RPRNT) Reprint character
VFLUSH (FLUSH) Flush character
VWERASE (WERASE) Word erase character
VLNEXT (LNEXT) Literal next character
VQUOTE (QUOTE) Erase and kill quoting
character
──────────────────────────────────────────────
The following subscripts are local extensions to the c_cc array:
VDSUSP, VRPRNT, VFLUSH, VWERASE, VLNEXT, and VQUOTE. The constant NCCS
defines the total number of elements in the c_cc array.
Setting the value of a special character to POSIX_V_DISABLE causes that
function to be disabled; that is, no input data will be recognized as
the disabled special character. If ICANON is not set, the value of
POSIX_V_DISABLE has no special meaning for the VMIN and VTIME entries
of the c_cc array.
Line Discipline
The c_line field of the termios data structure is used to specify the
line discipline. Support is provided for the basic termios line disci‐
pline only. For this reason, the value of this field should be set to
the value TERMIODISC (the default value) by convention. The value of
c_line is reset to TERMIODISC by the system, if attempts are made to
set c_line to other values. This field is a local extension.
Control Functions
The functions that are used to control the general terminal function
are described in this section. Unless otherwise noted for a specific
command, these functions are restricted from use by background pro‐
cesses. Attempts to perform these operations cause the process group
to be sent a SIGTTOU signal. If the calling process is blocking or
ignoring SIGTTOU signals, the process is allowed to perform the opera‐
tion and the SIGTTOU signal is not sent.
In all the functions, fildes is an open file descriptor. However, the
functions affect the underlying terminal file, not just the open
instance associated with the file descriptor.
Get and Set State
Functions:
Syntax
#include <termios.h>
int tcgetattr (fildes, termios_p)
int fildes;
struct termios *termios_p;
int tcsetattr (fildes, optional_actions, termios_p)
int fildes;
int optional_actions;
struct termios *termios_p;
Description
The function retrieves the parameters associated with the object
referred to by fildes and store them in the structure referenced by
termios_p . This function is allowed from a background process; how‐
ever, the information can be subsequently changed by a foreground
process.
The function sets the parameters associated with the terminal from the
structure referenced by termios_p as follows:
· If optional_actions is TCSANOW, the change occurs immediately.
· If optional_actions is TCSADRAIN, the change occurs after all out‐
put written to fildes has been transmitted. This function should
be used when changing parameters that affect output.
· If optional_actions is TCSADFLUSH, the change occurs after all
output written to the object referred to by fildes has been trans‐
mitted, and all input that has been received but not read is dis‐
carded before the change is made.
Returns
Upon successful completion, a value of zero is returned. Otherwise, a
value of -1 is returned and errno is set to indicate the error.
Errors
If any of the following conditions occur, the function returns -1 and
sets errno to the corresponding value:
[EBADF] The fildes argument is not a valid file descriptor.
[EINVAL] The device does not support the function.
[ENOTTY] The file associated with fildes is not a terminal.
If any of the following conditions occur, the function returns -1 and
sets errno to the corresponding value:
[EBADF] The fildes argument is not a valid file descriptor.
[EINVAL] The device does not support the function, or the
optional_actions argument is not a proper value.
[ENOTTY] The file associated with fildes is not a terminal.
Line Control Functions
Functions:
Syntax
#include <termios.h>
int tcsendbreak (fildes, duration)
int fildes;
int duration;
int tcdrain (fildes)
int fildes;
int tcflush (fildes, queue_selector)
int fildes;
int queue_selector;
int tcflow (fildes, action)
int fildes;
int action;
Description
The function sends a ``break'' that is a continuous stream of zero-val‐
ued bits for a specific duration. If duration is zero, it sends zero-
valued bits for 0.25 seconds. If duration is greater than zero, it
sends zero-valued bits for duration*0.1 seconds. If the object
referred to by fildes no break sequence is generated.
The function waits until all output written to the object referred to
by fildes has been transmitted.
The function discards data written to the object referred to by fildes
but not transmitted, or data received but not read, depending on the
value of queue_selector:
· If queue_selector is TCIFLUSH, it flushes data received but not
read.
· If queue_selector is TCOFLUSH, it flushes data written but not
transmitted.
· If queue_selector is TCIOFLUSH, it flushes both data received but
not read, and data written but not transmitted.
The function suspends transmission or reception of data on the object
referred to by fildes , depending on the value of action:
· If action is TCOOFF, it suspends output.
· If action is TCOON, it restarts suspended output.
· If action is TCIOFF, the system transmits a STOP character, which
is intended to cause the terminal device to stop transmitting data
to the system.
· If action is TCION, the system transmits a START character, which
is intended to cause the terminal device to resume transmitting
data to the system.
· The default on open of a terminal file is that neither its input
nor its output is suspended.
Returns
Upon successful completion, a value of zero is returned. Otherwise, a
value of -1 is returned and errno is set to indicate the error.
Errors
If any of the following conditions occur, the function returns -1 and
sets errno to the corresponding value:
[EBADF] The fildes argument is not a valid file descriptor.
[EINVAL] The device does not support the function.
[ENOTTY] The file associated with fildes is not a terminal.
If any of the following conditions occur, the function returns -1 and
sets errno to the corresponding value:
[EBADF] The fildes argument is not a valid file descriptor.
[EINVAL] The device does not support the function.
[ENOTTY] The file associated with fildes is not a terminal.
[EINTR] A signal interrupted the function.
If any of the following conditions occur, the function returns -1 and
sets errno to the corresponding value:
[EBADF] The fildes argument is not a valid file descriptor.
[EINVAL] The device does not support the function, or the
queue_selector argument is not a proper value.
[ENOTTY] The file associated with fildes is not a terminal.
If any of the following conditions occur, the function returns -1 and
sets errno to the corresponding value:
[EBADF] The fildes argument is not a valid file descriptor.
[EINVAL] The device does not support the function, or the action
argument is not a proper value.
[ENOTTY] The file associated with fildes is not a terminal.
Get Foreground Process Group Id
Function:
Synopsis
#include <sys/types.h>
#include <termios.h>
pid_t tcgetpgrp (fildes)
int fildes;
Description
The function returns the value of the process group ID of the fore‐
ground process group associated with the terminal.
The function is allowed from a background process; however, the infor‐
mation can be subsequently changed by a foreground process.
Returns
Upon successful completion, retuns the process group ID of the fore‐
ground process group associated with the terminal. Otherwise, a value
of -1 is returned and errno is set to indicate the error.
Errors
If any of the following conditions occur, the function returns -1 and
sets errno to the corresponding value:
[EBADF] The fildes argument is not a valid file descriptor.
[EINVAL] This function is not allowed for the device associated with
the fildes argument.
[ENOTTY] The calling process does not have a controlling terminal or
the file is not the controlling terminal.
Set Foreground Process Group ID
Function:
Synopsis
#include <sys/types.h>
#include <termios.h>
int tcsetpgrp (fildes, pgrp_id)
int fildes;
pid_t pgrp_id;
Description
If the process has a controlling terminal, the function sets the fore‐
ground process group ID associated with the terminal to pgrp_id. The
file associated with fildes must be the controlling terminal of the
calling process, and the controlling terminal must be currently associ‐
ated with the session of the calling process. The value of pgrp_id
must match a process group ID of a process in the same session as the
calling process.
Returns
Upon successful completion, returns a value of zero. Otherwise, a
value of -1 is returned and errno is set to indicate the error.
Errors
If any of the following conditions occur, the function returns -1 and
sets errno to the corresponding value:
[EBADF] The fildes argument is not a valid file descriptor.
[EINVAL] This function is not allowed for the device associated with
the fildes argument or the value of pgrp_id argument is
less than or equal to zero, or exceeds {PID_MAX}.
[ENOTTY] The calling process does not have a controlling terminal or
the file is not the controlling terminal, or the control‐
ling terminal is no longer associated with the session of
the calling process.
[EPERM] The value of the pgrp_id argument does not match the
process group ID of a process in the same session as the
calling process.
See Alsocsh(1), stty(1), tset(1), ioctl(2), sigvec(2), setsid(2), termio(4),
getty(8)termios(4)
