This chapter describes functions that are specific to terminal devices.
You can use these functions to do things like turn off input echoing;
set serial line characteristics such as line speed and flow control; and
change which characters are used for end-of-file, command-line editing,
sending signals, and similar control functions.
Most of the functions in this chapter operate on file descriptors.
See section 13. Low-Level Input/Output, for more information about what a file
descriptor is and how to open a file descriptor for a terminal device.
The functions described in this chapter only work on files that
correspond to terminal devices. You can find out whether a file
descriptor is associated with a terminal by using the isatty
function.
Prototypes for the functions in this section are declared in the header
file `unistd.h'.
Function: int isatty(int filedes)
This function returns 1 if filedes is a file descriptor
associated with an open terminal device, and 0 otherwise.
If a file descriptor is associated with a terminal, you can get its
associated file name using the ttyname function. See also the
ctermid function, described in 27.7.1 Identifying the Controlling Terminal.
Function: char * ttyname(int filedes)
If the file descriptor filedes is associated with a terminal
device, the ttyname function returns a pointer to a
statically-allocated, null-terminated string containing the file name of
the terminal file. The value is a null pointer if the file descriptor
isn't associated with a terminal, or the file name cannot be determined.
Function: int ttyname_r(int filedes, char *buf, size_t len)
The ttyname_r function is similar to the ttyname function
except that it places its result into the user-specified buffer starting
at buf with length len.
The normal return value from ttyname_r is 0. Otherwise an
error number is returned to indicate the error. The following
errno error conditions are defined for this function:
EBADF
The filedes argument is not a valid file descriptor.
ENOTTY
The filedes is not associated with a terminal.
ERANGE
The buffer length len is too small to store the string to be
returned.
Many of the remaining functions in this section refer to the input and
output queues of a terminal device. These queues implement a form of
buffering within the kernel independent of the buffering
implemented by I/O streams (see section 12. Input/Output on Streams).
The terminal input queue is also sometimes referred to as its
typeahead buffer. It holds the characters that have been received
from the terminal but not yet read by any process.
The size of the input queue is described by the MAX_INPUT and
_POSIX_MAX_INPUT parameters; see 31.6 Limits on File System Capacity. You
are guaranteed a queue size of at least MAX_INPUT, but the queue
might be larger, and might even dynamically change size. If input flow
control is enabled by setting the IXOFF input mode bit
(see section 17.4.4 Input Modes), the terminal driver transmits STOP and START
characters to the terminal when necessary to prevent the queue from
overflowing. Otherwise, input may be lost if it comes in too fast from
the terminal. In canonical mode, all input stays in the queue until a
newline character is received, so the terminal input queue can fill up
when you type a very long line. See section 17.3 Two Styles of Input: Canonical or Not.
The terminal output queue is like the input queue, but for output;
it contains characters that have been written by processes, but not yet
transmitted to the terminal. If output flow control is enabled by
setting the IXON input mode bit (see section 17.4.4 Input Modes), the
terminal driver obeys START and STOP characters sent by the terminal to
stop and restart transmission of output.
Clearing the terminal input queue means discarding any characters
that have been received but not yet read. Similarly, clearing the
terminal output queue means discarding any characters that have been
written but not yet transmitted.
POSIX systems support two basic modes of input: canonical and
noncanonical.
In canonical input processing mode, terminal input is processed in
lines terminated by newline ('\n'), EOF, or EOL characters. No
input can be read until an entire line has been typed by the user, and
the read function (see section 13.2 Input and Output Primitives) returns at most a
single line of input, no matter how many bytes are requested.
In canonical input mode, the operating system provides input editing
facilities: some characters are interpreted specially to perform editing
operations within the current line of text, such as ERASE and KILL.
See section 17.4.9.1 Characters for Input Editing.
The constants _POSIX_MAX_CANON and MAX_CANON parameterize
the maximum number of bytes which may appear in a single line of
canonical input. See section 31.6 Limits on File System Capacity. You are guaranteed a maximum
line length of at least MAX_CANON bytes, but the maximum might be
larger, and might even dynamically change size.
In noncanonical input processing mode, characters are not grouped
into lines, and ERASE and KILL processing is not performed. The
granularity with which bytes are read in noncanonical input mode is
controlled by the MIN and TIME settings. See section 17.4.10 Noncanonical Input.
Most programs use canonical input mode, because this gives the user a
way to edit input line by line. The usual reason to use noncanonical
mode is when the program accepts single-character commands or provides
its own editing facilities.
The choice of canonical or noncanonical input is controlled by the
ICANON flag in the c_lflag member of struct termios.
See section 17.4.7 Local Modes.
This section describes the various terminal attributes that control how
input and output are done. The functions, data structures, and symbolic
constants are all declared in the header file `termios.h'.
Don't confuse terminal attributes with file attributes. A device special
file which is associated with a terminal has file attributes as described
in 14.9 File Attributes. These are unrelated to the attributes of the
terminal device itself, which are discussed in this section.
The entire collection of attributes of a terminal is stored in a
structure of type struct termios. This structure is used
with the functions tcgetattr and tcsetattr to read
and set the attributes.
Data Type:struct termios
Structure that records all the I/O attributes of a terminal. The
structure includes at least the following members:
An array specifying which characters are associated with various
control functions; see 17.4.9 Special Characters.
The struct termios structure also contains members which
encode input and output transmission speeds, but the representation is
not specified. See section 17.4.8 Line Speed, for how to examine and store the
speed values.
The following sections describe the details of the members of the
struct termios structure.
Data Type:tcflag_t
This is an unsigned integer type used to represent the various
bit masks for terminal flags.
Data Type:cc_t
This is an unsigned integer type used to represent characters associated
with various terminal control functions.
Macro: int NCCS
The value of this macro is the number of elements in the c_cc
array.
Function: int tcgetattr(int filedes, struct termios *termios-p)
This function is used to examine the attributes of the terminal
device with file descriptor filedes. The attributes are returned
in the structure that termios-p points to.
If successful, tcgetattr returns 0. A return value of -1
indicates an error. The following errno error conditions are
defined for this function:
EBADF
The filedes argument is not a valid file descriptor.
ENOTTY
The filedes is not associated with a terminal.
Function: int tcsetattr(int filedes, int when, const struct termios *termios-p)
This function sets the attributes of the terminal device with file
descriptor filedes. The new attributes are taken from the
structure that termios-p points to.
The when argument specifies how to deal with input and output
already queued. It can be one of the following values:
TCSANOW
Make the change immediately.
TCSADRAIN
Make the change after waiting until all queued output has been written.
You should usually use this option when changing parameters that affect
output.
TCSAFLUSH
This is like TCSADRAIN, but also discards any queued input.
TCSASOFT
This is a flag bit that you can add to any of the above alternatives.
Its meaning is to inhibit alteration of the state of the terminal
hardware. It is a BSD extension; it is only supported on BSD systems
and the GNU system.
Using TCSASOFT is exactly the same as setting the CIGNORE
bit in the c_cflag member of the structure termios-p points
to. See section 17.4.6 Control Modes, for a description of CIGNORE.
If this function is called from a background process on its controlling
terminal, normally all processes in the process group are sent a
SIGTTOU signal, in the same way as if the process were trying to
write to the terminal. The exception is if the calling process itself
is ignoring or blocking SIGTTOU signals, in which case the
operation is performed and no signal is sent. See section 27. Job Control.
If successful, tcsetattr returns 0. A return value of
-1 indicates an error. The following errno error
conditions are defined for this function:
EBADF
The filedes argument is not a valid file descriptor.
ENOTTY
The filedes is not associated with a terminal.
EINVAL
Either the value of the when argument is not valid, or there is
something wrong with the data in the termios-p argument.
Although tcgetattr and tcsetattr specify the terminal
device with a file descriptor, the attributes are those of the terminal
device itself and not of the file descriptor. This means that the
effects of changing terminal attributes are persistent; if another
process opens the terminal file later on, it will see the changed
attributes even though it doesn't have anything to do with the open file
descriptor you originally specified in changing the attributes.
Similarly, if a single process has multiple or duplicated file
descriptors for the same terminal device, changing the terminal
attributes affects input and output to all of these file
descriptors. This means, for example, that you can't open one file
descriptor or stream to read from a terminal in the normal
line-buffered, echoed mode; and simultaneously have another file
descriptor for the same terminal that you use to read from it in
single-character, non-echoed mode. Instead, you have to explicitly
switch the terminal back and forth between the two modes.
When you set terminal modes, you should call tcgetattr first to
get the current modes of the particular terminal device, modify only
those modes that you are really interested in, and store the result with
tcsetattr.
It's a bad idea to simply initialize a struct termios structure
to a chosen set of attributes and pass it directly to tcsetattr.
Your program may be run years from now, on systems that support members
not documented in this manual. The way to avoid setting these members
to unreasonable values is to avoid changing them.
What's more, different terminal devices may require different mode
settings in order to function properly. So you should avoid blindly
copying attributes from one terminal device to another.
When a member contains a collection of independent flags, as the
c_iflag, c_oflag and c_cflag members do, even
setting the entire member is a bad idea, because particular operating
systems have their own flags. Instead, you should start with the
current value of the member and alter only the flags whose values matter
in your program, leaving any other flags unchanged.
Here is an example of how to set one flag (ISTRIP) in the
struct termios structure while properly preserving all the other
data in the structure:
int
set_istrip (int desc, int value)
{
struct termios settings;
int result;
result = tcgetattr (desc, &settings);
if (result < 0)
{
perror ("error in tcgetattr");
return 0;
}
settings.c_iflag &= ~ISTRIP;
if (value)
settings.c_iflag |= ISTRIP;
result = tcsetattr (desc, TCSANOW, &settings);
if (result < 0)
{
perror ("error in tcgetattr");
return;
}
return 1;
}
This section describes the terminal attribute flags that control
fairly low-level aspects of input processing: handling of parity errors,
break signals, flow control, and RET and LFD characters.
All of these flags are bits in the c_iflag member of the
struct termios structure. The member is an integer, and you
change flags using the operators &, | and ^. Don't
try to specify the entire value for c_iflag---instead, change
only specific flags and leave the rest untouched (see section 17.4.3 Setting Terminal Modes Properly).
Macro: tcflag_t INPCK
If this bit is set, input parity checking is enabled. If it is not set,
no checking at all is done for parity errors on input; the
characters are simply passed through to the application.
Parity checking on input processing is independent of whether parity
detection and generation on the underlying terminal hardware is enabled;
see 17.4.6 Control Modes. For example, you could clear the INPCK
input mode flag and set the PARENB control mode flag to ignore
parity errors on input, but still generate parity on output.
If this bit is set, what happens when a parity error is detected depends
on whether the IGNPAR or PARMRK bits are set. If neither
of these bits are set, a byte with a parity error is passed to the
application as a '\0' character.
Macro: tcflag_t IGNPAR
If this bit is set, any byte with a framing or parity error is ignored.
This is only useful if INPCK is also set.
Macro: tcflag_t 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 0377, and ISTRIP (see below)
is not set, the program might confuse it with the prefix that marks a
parity error. So a valid byte 0377 is passed to the program as
two bytes, 03770377, in this case.
Macro: tcflag_t ISTRIP
If this bit is set, valid input bytes are stripped to seven bits;
otherwise, all eight bits are available for programs to read.
Macro: tcflag_t IGNBRK
If this bit is set, break conditions are ignored.
A break condition is defined in the context of asynchronous
serial data transmission as a series of zero-value bits longer than a
single byte.
Macro: tcflag_t BRKINT
If this bit is set and IGNBRK is not set, a break condition
clears the terminal input and output queues and raises a SIGINT
signal for the foreground process group associated with the terminal.
If neither BRKINT nor IGNBRK are set, a break condition is
passed to the application as a single '\0' character if
PARMRK is not set, or otherwise as a three-character sequence
'\377', '\0', '\0'.
Macro: tcflag_t IGNCR
If this bit is set, carriage return characters ('\r') are
discarded on input. Discarding carriage return may be useful on
terminals that send both carriage return and linefeed when you type the
RET key.
Macro: tcflag_t ICRNL
If this bit is set and IGNCR is not set, carriage return characters
('\r') received as input are passed to the application as newline
characters ('\n').
Macro: tcflag_t INLCR
If this bit is set, newline characters ('\n') received as input
are passed to the application as carriage return characters ('\r').
Macro: tcflag_t IXOFF
If this bit is set, start/stop control on input is enabled. In other
words, the computer sends STOP and START characters as necessary to
prevent input from coming in faster than programs are reading it. The
idea is that the actual terminal hardware that is generating the input
data responds to a STOP character by suspending transmission, and to a
START character by resuming transmission. See section 17.4.9.3 Special Characters for Flow Control.
Macro: tcflag_t IXON
If this bit is set, start/stop control on output is enabled. In other
words, if the computer receives a STOP character, it suspends output
until a START character is received. In this case, the STOP and START
characters are never passed to the application program. If this bit is
not set, then START and STOP can be read as ordinary characters.
See section 17.4.9.3 Special Characters for Flow Control.
Macro: tcflag_t IXANY
If this bit is set, any input character restarts output when output has
been suspended with the STOP character. Otherwise, only the START
character restarts output.
This is a BSD extension; it exists only on BSD systems and the GNU system.
Macro: tcflag_t IMAXBEL
If this bit is set, then filling up the terminal input buffer sends a
BEL character (code 007) to the terminal to ring the bell.
This section describes the terminal flags and fields that control how
output characters are translated and padded for display. All of these
are contained in the c_oflag member of the struct termios
structure.
The c_oflag member itself is an integer, and you change the flags
and fields using the operators &, |, and ^. Don't
try to specify the entire value for c_oflag---instead, change
only specific flags and leave the rest untouched (see section 17.4.3 Setting Terminal Modes Properly).
Macro: tcflag_t OPOST
If this bit is set, output data is processed in some unspecified way so
that it is displayed appropriately on the terminal device. This
typically includes mapping newline characters ('\n') onto
carriage return and linefeed pairs.
If this bit isn't set, the characters are transmitted as-is.
The following three bits are BSD features, and they exist only BSD
systems and the GNU system. They are effective only if OPOST is
set.
Macro: tcflag_t ONLCR
If this bit is set, convert the newline character on output into a pair
of characters, carriage return followed by linefeed.
Macro: tcflag_t OXTABS
If this bit is set, convert tab characters on output into the appropriate
number of spaces to emulate a tab stop every eight columns.
Macro: tcflag_t ONOEOT
If this bit is set, discard C-d characters (code 004) on
output. These characters cause many dial-up terminals to disconnect.
This section describes the terminal flags and fields that control
parameters usually associated with asynchronous serial data
transmission. These flags may not make sense for other kinds of
terminal ports (such as a network connection pseudo-terminal). All of
these are contained in the c_cflag member of the struct
termios structure.
The c_cflag member itself is an integer, and you change the flags
and fields using the operators &, |, and ^. Don't
try to specify the entire value for c_cflag---instead, change
only specific flags and leave the rest untouched (see section 17.4.3 Setting Terminal Modes Properly).
Macro: tcflag_t CLOCAL
If this bit is set, it indicates that the terminal is connected
"locally" and that the modem status lines (such as carrier detect)
should be ignored.
On many systems if this bit is not set and you call open without
the O_NONBLOCK flag set, open blocks until a modem
connection is established.
If this bit is not set and a modem disconnect is detected, a
SIGHUP signal is sent to the controlling process group for the
terminal (if it has one). Normally, this causes the process to exit;
see 24. Signal Handling. Reading from the terminal after a disconnect
causes an end-of-file condition, and writing causes an EIO error
to be returned. The terminal device must be closed and reopened to
clear the condition.
Macro: tcflag_t HUPCL
If this bit is set, a modem disconnect is generated when all processes
that have the terminal device open have either closed the file or exited.
Macro: tcflag_t CREAD
If this bit is set, input can be read from the terminal. Otherwise,
input is discarded when it arrives.
Macro: tcflag_t CSTOPB
If this bit is set, two stop bits are used. Otherwise, only one stop bit
is used.
Macro: tcflag_t PARENB
If this bit is set, generation and detection of a parity bit are enabled.
See section 17.4.4 Input Modes, for information on how input parity errors are handled.
If this bit is not set, no parity bit is added to output characters, and
input characters are not checked for correct parity.
Macro: tcflag_t PARODD
This bit is only useful if PARENB is set. If PARODD is set,
odd parity is used, otherwise even parity is used.
The control mode flags also includes a field for the number of bits per
character. You can use the CSIZE macro as a mask to extract the
value, like this: settings.c_cflag & CSIZE.
Macro: tcflag_t CSIZE
This is a mask for the number of bits per character.
Macro: tcflag_t CS5
This specifies five bits per byte.
Macro: tcflag_t CS6
This specifies six bits per byte.
Macro: tcflag_t CS7
This specifies seven bits per byte.
Macro: tcflag_t CS8
This specifies eight bits per byte.
The following four bits are BSD extensions; this exist only on BSD
systems and the GNU system.
Macro: tcflag_t CCTS_OFLOW
If this bit is set, enable flow control of output based on the CTS wire
(RS232 protocol).
Macro: tcflag_t CRTS_IFLOW
If this bit is set, enable flow control of input based on the RTS wire
(RS232 protocol).
Macro: tcflag_t MDMBUF
If this bit is set, enable carrier-based flow control of output.
Macro: tcflag_t CIGNORE
If this bit is set, it says to ignore the control modes and line speed
values entirely. This is only meaningful in a call to tcsetattr.
The c_cflag member and the line speed values returned by
cfgetispeed and cfgetospeed will be unaffected by the
call. CIGNORE is useful if you want to set all the software
modes in the other members, but leave the hardware details in
c_cflag unchanged. (This is how the TCSASOFT flag to
tcsettattr works.)
This bit is never set in the structure filled in by tcgetattr.
This section describes the flags for the c_lflag member of the
struct termios structure. These flags generally control
higher-level aspects of input processing than the input modes flags
described in 17.4.4 Input Modes, such as echoing, signals, and the choice
of canonical or noncanonical input.
The c_lflag member itself is an integer, and you change the flags
and fields using the operators &, |, and ^. Don't
try to specify the entire value for c_lflag---instead, change
only specific flags and leave the rest untouched (see section 17.4.3 Setting Terminal Modes Properly).
If this bit is set, echoing of input characters back to the terminal
is enabled.
Macro: tcflag_t ECHOE
If this bit is set, echoing indicates erasure of input with the ERASE
character by erasing the last character in the current line from the
screen. Otherwise, the character erased is re-echoed to show what has
happened (suitable for a printing terminal).
This bit only controls the display behavior; the ICANON bit by
itself controls actual recognition of the ERASE character and erasure of
input, without which ECHOE is simply irrelevant.
Macro: tcflag_t ECHOPRT
This bit is like ECHOE, enables display of the ERASE character in
a way that is geared to a hardcopy terminal. When you type the ERASE
character, a `\' character is printed followed by the first
character erased. Typing the ERASE character again just prints the next
character erased. Then, the next time you type a normal character, a
`/' character is printed before the character echoes.
This is a BSD extension, and exists only in BSD systems and the
GNU system.
Macro: tcflag_t ECHOK
This bit enables special display of the KILL character by moving to a
new line after echoing the KILL character normally. The behavior of
ECHOKE (below) is nicer to look at.
If this bit is not set, the KILL character echoes just as it would if it
were not the KILL character. Then it is up to the user to remember that
the KILL character has erased the preceding input; there is no
indication of this on the screen.
This bit only controls the display behavior; the ICANON bit by
itself controls actual recognition of the KILL character and erasure of
input, without which ECHOK is simply irrelevant.
Macro: tcflag_t ECHOKE
This bit is similar to ECHOK. It enables special display of the
KILL character by erasing on the screen the entire line that has been
killed. This is a BSD extension, and exists only in BSD systems and the
GNU system.
Macro: tcflag_t ECHONL
If this bit is set and the ICANON bit is also set, then the
newline ('\n') character is echoed even if the ECHO bit
is not set.
Macro: tcflag_t ECHOCTL
If this bit is set and the ECHO bit is also set, echo control
characters with `^' followed by the corresponding text character.
Thus, control-A echoes as `^A'. This is usually the preferred mode
for interactive input, because echoing a control character back to the
terminal could have some undesired effect on the terminal.
This is a BSD extension, and exists only in BSD systems and the
GNU system.
Macro: tcflag_t ISIG
This bit controls whether the INTR, QUIT, and SUSP characters are
recognized. The functions associated with these characters are performed
if and only if this bit is set. Being in canonical or noncanonical
input mode has no affect on the interpretation of these characters.
You should use caution when disabling recognition of these characters.
Programs that cannot be interrupted interactively are very
user-unfriendly. If you clear this bit, your program should provide
some alternate interface that allows the user to interactively send the
signals associated with these characters, or to escape from the program.
Normally, the INTR, QUIT, and SUSP characters cause input and output
queues for the terminal to be cleared. If this bit is set, the queues
are not cleared.
Macro: tcflag_t TOSTOP
If this bit is set and the system supports job control, then
SIGTTOU signals are generated by background processes that
attempt to write to the terminal. See section 27.4 Access to the Controlling Terminal.
The following bits are BSD extensions; they exist only in BSD systems
and the GNU system.
Macro: tcflag_t ALTWERASE
This bit determines how far the WERASE character should erase. The
WERASE character erases back to the beginning of a word; the question
is, where do words begin?
If this bit is clear, then the beginning of a word is a nonwhitespace
character following a whitespace character. If the bit is set, then the
beginning of a word is an alphanumeric character or underscore following
a character which is none of those.
This is the bit that toggles when the user types the DISCARD character.
While this bit is set, all output is discarded. See section 17.4.9.4 Other Special Characters.
If this bit is set, it indicates that there is a line of input that
needs to be reprinted. Typing the REPRINT character sets this bit; the
bit remains set until reprinting is finished. See section 17.4.9.1 Characters for Input Editing.
The terminal line speed tells the computer how fast to read and write
data on the terminal.
If the terminal is connected to a real serial line, the terminal speed
you specify actually controls the line--if it doesn't match the
terminal's own idea of the speed, communication does not work. Real
serial ports accept only certain standard speeds. Also, particular
hardware may not support even all the standard speeds. Specifying a
speed of zero hangs up a dialup connection and turns off modem control
signals.
If the terminal is not a real serial line (for example, if it is a
network connection), then the line speed won't really affect data
transmission speed, but some programs will use it to determine the
amount of padding needed. It's best to specify a line speed value that
matches the actual speed of the actual terminal, but you can safely
experiment with different values to vary the amount of padding.
There are actually two line speeds for each terminal, one for input and
one for output. You can set them independently, but most often
terminals use the same speed for both directions.
The speed values are stored in the struct termios structure, but
don't try to access them in the struct termios structure
directly. Instead, you should use the following functions to read and
store them:
This function returns the input line speed stored in the structure
*termios-p.
Function: int cfsetospeed(struct termios *termios-p, speed_t speed)
This function stores speed in *termios-p as the output
speed. The normal return value is 0; a value of -1
indicates an error. If speed is not a speed, cfsetospeed
returns -1.
Function: int cfsetispeed(struct termios *termios-p, speed_t speed)
This function stores speed in *termios-p as the input
speed. The normal return value is 0; a value of -1
indicates an error. If speed is not a speed, cfsetospeed
returns -1.
Function: int cfsetspeed(struct termios *termios-p, speed_t speed)
This function stores speed in *termios-p as both the
input and output speeds. The normal return value is 0; a value
of -1 indicates an error. If speed is not a speed,
cfsetspeed returns -1. This function is an extension in
4.4 BSD.
Data Type:speed_t
The speed_t type is an unsigned integer data type used to
represent line speeds.
The functions cfsetospeed and cfsetispeed report errors
only for speed values that the system simply cannot handle. If you
specify a speed value that is basically acceptable, then those functions
will succeed. But they do not check that a particular hardware device
can actually support the specified speeds--in fact, they don't know
which device you plan to set the speed for. If you use tcsetattr
to set the speed of a particular device to a value that it cannot
handle, tcsetattr returns -1.
Portability note: In the GNU library, the functions above
accept speeds measured in bits per second as input, and return speed
values measured in bits per second. Other libraries require speeds to
be indicated by special codes. For POSIX.1 portability, you must use
one of the following symbols to represent the speed; their precise
numeric values are system-dependent, but each name has a fixed meaning:
B110 stands for 110 bps, B300 for 300 bps, and so on.
There is no portable way to represent any speed but these, but these are
the only speeds that typical serial lines can support.
In canonical input, the terminal driver recognizes a number of special
characters which perform various control functions. These include the
ERASE character (usually DEL) for editing input, and other editing
characters. The INTR character (normally C-c) for sending a
SIGINT signal, and other signal-raising characters, may be
available in either canonical or noncanonical input mode. All these
characters are described in this section.
The particular characters used are specified in the c_cc member
of the struct termios structure. This member is an array; each
element specifies the character for a particular role. Each element has
a symbolic constant that stands for the index of that element--for
example, VINTR is the index of the element that specifies the INTR
character, so storing '=' in termios.c_cc[VINTR]
specifies `=' as the INTR character.
On some systems, you can disable a particular special character function
by specifying the value _POSIX_VDISABLE for that role. This
value is unequal to any possible character code. See section 31.7 Optional Features in File Support, for more information about how to tell whether the operating
system you are using supports _POSIX_VDISABLE.
This is the subscript for the EOF character in the special control
character array. termios.c_cc[VEOF] holds the character
itself.
The EOF character is recognized only in canonical input mode. It acts
as a line terminator in the same way as a newline character, but if the
EOF character is typed at the beginning of a line it causes read
to return a byte count of zero, indicating end-of-file. The EOF
character itself is discarded.
Usually, the EOF character is C-d.
Macro: int VEOL
This is the subscript for the EOL character in the special control
character array. termios.c_cc[VEOL] holds the character
itself.
The EOL character is recognized only in canonical input mode. It acts
as a line terminator, just like a newline character. The EOL character
is not discarded; it is read as the last character in the input line.
You don't need to use the EOL character to make RET end a line.
Just set the ICRNL flag. In fact, this is the default state of
affairs.
Macro: int VEOL2
This is the subscript for the EOL2 character in the special control
character array. termios.c_cc[VEOL2] holds the character
itself.
The EOL2 character works just like the EOL character (see above), but it
can be a different character. Thus, you can specify two characters to
terminate an input line, by setting EOL to one of them and EOL2 to the
other.
The EOL2 character is a BSD extension; it exists only on BSD systems
and the GNU system.
Macro: int VERASE
This is the subscript for the ERASE character in the special control
character array. termios.c_cc[VERASE] holds the
character itself.
The ERASE character is recognized only in canonical input mode. When
the user types the erase character, the previous character typed is
discarded. (If the terminal generates multibyte character sequences,
this may cause more than one byte of input to be discarded.) This
cannot be used to erase past the beginning of the current line of text.
The ERASE character itself is discarded.
Usually, the ERASE character is DEL.
Macro: int VWERASE
This is the subscript for the WERASE character in the special control
character array. termios.c_cc[VWERASE] holds the character
itself.
The WERASE character is recognized only in canonical mode. It erases an
entire word of prior input, and any whitespace after it; whitespace
characters before the word are not erased.
The definition of a "word" depends on the setting of the
ALTWERASE mode; see section 17.4.7 Local Modes.
If the ALTWERASE mode is not set, a word is defined as a sequence
of any characters except space or tab.
If the ALTWERASE mode is set, a word is defined as a sequence of
characters containing only letters, numbers, and underscores, optionally
followed by one character that is not a letter, number, or underscore.
The WERASE character is usually C-w.
This is a BSD extension.
Macro: int VKILL
This is the subscript for the KILL character in the special control
character array. termios.c_cc[VKILL] holds the character
itself.
The KILL character is recognized only in canonical input mode. When the
user types the kill character, the entire contents of the current line
of input are discarded. The kill character itself is discarded too.
The KILL character is usually C-u.
Macro: int VREPRINT
This is the subscript for the REPRINT character in the special control
character array. termios.c_cc[VREPRINT] holds the character
itself.
The REPRINT character is recognized only in canonical mode. It reprints
the current input line. If some asynchronous output has come while you
are typing, this lets you see the line you are typing clearly again.
These special characters may be active in either canonical or noncanonical
input mode, but only when the ISIG flag is set (see section 17.4.7 Local Modes).
Macro: int VINTR
This is the subscript for the INTR character in the special control
character array. termios.c_cc[VINTR] holds the character
itself.
The INTR (interrupt) character raises a SIGINT signal for all
processes in the foreground job associated with the terminal. The INTR
character itself is then discarded. See section 24. Signal Handling, for more
information about signals.
Typically, the INTR character is C-c.
Macro: int VQUIT
This is the subscript for the QUIT character in the special control
character array. termios.c_cc[VQUIT] holds the character
itself.
The QUIT character raises a SIGQUIT signal for all processes in
the foreground job associated with the terminal. The QUIT character
itself is then discarded. See section 24. Signal Handling, for more information
about signals.
Typically, the QUIT character is C-\.
Macro: int VSUSP
This is the subscript for the SUSP character in the special control
character array. termios.c_cc[VSUSP] holds the character
itself.
The SUSP (suspend) character is recognized only if the implementation
supports job control (see section 27. Job Control). It causes a SIGTSTP
signal to be sent to all processes in the foreground job associated with
the terminal. The SUSP character itself is then discarded.
See section 24. Signal Handling, for more information about signals.
Typically, the SUSP character is C-z.
Few applications disable the normal interpretation of the SUSP
character. If your program does this, it should provide some other
mechanism for the user to stop the job. When the user invokes this
mechanism, the program should send a SIGTSTP signal to the
process group of the process, not just to the process itself.
See section 24.6.2 Signaling Another Process.
Macro: int VDSUSP
This is the subscript for the DSUSP character in the special control
character array. termios.c_cc[VDSUSP] holds the character
itself.
The DSUSP (suspend) character is recognized only if the implementation
supports job control (see section 27. Job Control). It sends a SIGTSTP
signal, like the SUSP character, but not right away--only when the
program tries to read it as input. Not all systems with job control
support DSUSP; only BSD-compatible systems (including the GNU system).
These special characters may be active in either canonical or noncanonical
input mode, but their use is controlled by the flags IXON and
IXOFF (see section 17.4.4 Input Modes).
Macro: int VSTART
This is the subscript for the START character in the special control
character array. termios.c_cc[VSTART] holds the
character itself.
The START character is used to support the IXON and IXOFF
input modes. If IXON is set, receiving a START character resumes
suspended output; the START character itself is discarded. If
IXANY is set, receiving any character at all resumes suspended
output; the resuming character is not discarded unless it is the START
character. IXOFF is set, the system may also transmit START
characters to the terminal.
The usual value for the START character is C-q. You may not be
able to change this value--the hardware may insist on using C-q
regardless of what you specify.
Macro: int VSTOP
This is the subscript for the STOP character in the special control
character array. termios.c_cc[VSTOP] holds the character
itself.
The STOP character is used to support the IXON and IXOFF
input modes. If IXON is set, receiving a STOP character causes
output to be suspended; the STOP character itself is discarded. If
IXOFF is set, the system may also transmit STOP characters to the
terminal, to prevent the input queue from overflowing.
The usual value for the STOP character is C-s. You may not be
able to change this value--the hardware may insist on using C-s
regardless of what you specify.
These special characters exist only in BSD systems and the GNU system.
Macro: int VLNEXT
This is the subscript for the LNEXT character in the special control
character array. termios.c_cc[VLNEXT] holds the character
itself.
The LNEXT character is recognized only when IEXTEN is set, but in
both canonical and noncanonical mode. It disables any special
significance of the next character the user types. Even if the
character would normally perform some editing function or generate a
signal, it is read as a plain character. This is the analogue of the
C-q command in Emacs. "LNEXT" stands for "literal next."
The LNEXT character is usually C-v.
Macro: int VDISCARD
This is the subscript for the DISCARD character in the special control
character array. termios.c_cc[VDISCARD] holds the character
itself.
The DISCARD character is recognized only when IEXTEN is set, but
in both canonical and noncanonical mode. Its effect is to toggle the
discard-output flag. When this flag is set, all program output is
discarded. Setting the flag also discards all output currently in the
output buffer. Typing any other character resets the flag.
Macro: int VSTATUS
This is the subscript for the STATUS character in the special control
character array. termios.c_cc[VSTATUS] holds the character
itself.
The STATUS character's effect is to print out a status message about how
the current process is running.
The STATUS character is recognized only in canonical mode, and only if
NOKERNINFO is not set.
In noncanonical input mode, the special editing characters such as
ERASE and KILL are ignored. The system facilities for the user to edit
input are disabled in noncanonical mode, so that all input characters
(unless they are special for signal or flow-control purposes) are passed
to the application program exactly as typed. It is up to the
application program to give the user ways to edit the input, if
appropriate.
Noncanonical mode offers special parameters called MIN and TIME for
controlling whether and how long to wait for input to be available. You
can even use them to avoid ever waiting--to return immediately with
whatever input is available, or with no input.
The MIN and TIME are stored in elements of the c_cc array, which
is a member of the struct termios structure. Each element of
this array has a particular role, and each element has a symbolic
constant that stands for the index of that element. VMIN and
VMAX are the names for the indices in the array of the MIN and
TIME slots.
Macro: int VMIN
This is the subscript for the MIN slot in the c_cc array. Thus,
termios.c_cc[VMIN] is the value itself.
The MIN slot is only meaningful in noncanonical input mode; it
specifies the minimum number of bytes that must be available in the
input queue in order for read to return.
Macro: int VTIME
This is the subscript for the TIME slot in the c_cc array. Thus,
termios.c_cc[VTIME] is the value itself.
The TIME slot is only meaningful in noncanonical input mode; it
specifies how long to wait for input before returning, in units of 0.1
seconds.
The MIN and TIME values interact to determine the criterion for when
read should return; their precise meanings depend on which of
them are nonzero. There are four possible cases:
Both TIME and MIN are nonzero.
In this case, TIME specifies how long to wait after each input character
to see if more input arrives. After the first character received,
read keeps waiting until either MIN bytes have arrived in all, or
TIME elapses with no further input.
read always blocks until the first character arrives, even if
TIME elapses first. read can return more than MIN characters if
more than MIN happen to be in the queue.
Both MIN and TIME are zero.
In this case, read always returns immediately with as many
characters as are available in the queue, up to the number requested.
If no input is immediately available, read returns a value of
zero.
MIN is zero but TIME has a nonzero value.
In this case, read waits for time TIME for input to become
available; the availability of a single byte is enough to satisfy the
read request and cause read to return. When it returns, it
returns as many characters as are available, up to the number requested.
If no input is available before the timer expires, read returns a
value of zero.
TIME is zero but MIN has a nonzero value.
In this case, read waits until at least MIN bytes are available
in the queue. At that time, read returns as many characters as
are available, up to the number requested. read can return more
than MIN characters if more than MIN happen to be in the queue.
What happens if MIN is 50 and you ask to read just 10 bytes?
Normally, read waits until there are 50 bytes in the buffer (or,
more generally, the wait condition described above is satisfied), and
then reads 10 of them, leaving the other 40 buffered in the operating
system for a subsequent call to read.
Portability note: On some systems, the MIN and TIME slots are
actually the same as the EOF and EOL slots. This causes no serious
problem because the MIN and TIME slots are used only in noncanonical
input and the EOF and EOL slots are used only in canonical input, but it
isn't very clean. The GNU library allocates separate slots for these
uses.
Function: int cfmakeraw(struct termios *termios-p)
This function provides an easy way to set up *termios-p for
what has traditionally been called "raw mode" in BSD. This uses
noncanonical input, and turns off most processing to give an unmodified
channel to the terminal.
The usual way to get and set terminal modes is with the functions described
in 17.4 Terminal Modes. However, on some systems you can use the
BSD-derived functions in this section to do some of the same thing. On
many systems, these functions do not exist. Even with the GNU C library,
the functions simply fail with errno = ENOSYS with many
kernels, including Linux.
The symbols used in this section are declared in `sgtty.h'.
Data Type:struct sgttyb
This structure is an input or output parameter list for gtty and
stty.
char sg_ispeed
Line speed for input
char sg_ospeed
Line speed for output
char sg_erase
Erase character
char sg_kill
Kill character
int sg_flags
Various flags
Function: int gtty(int filedes, struct sgttyb *attributes)
This function gets the attributes of a terminal.
gtty sets *attributes to describe the terminal attributes
of the terminal which is open with file descriptor filedes.
Function: int stty(int filedes, struct sgttyb * attributes)
This function sets the attributes of a terminal.
stty sets the terminal attributes of the terminal which is open with
file descriptor filedes to those described by *filedes.
These functions perform miscellaneous control actions on terminal
devices. As regards terminal access, they are treated like doing
output: if any of these functions is used by a background process on its
controlling terminal, normally all processes in the process group are
sent a SIGTTOU signal. The exception is if the calling process
itself is ignoring or blocking SIGTTOU signals, in which case the
operation is performed and no signal is sent. See section 27. Job Control.
Function: int tcsendbreak(int filedes, int duration)
This function generates a break condition by transmitting a stream of
zero bits on the terminal associated with the file descriptor
filedes. The duration of the break is controlled by the
duration argument. If zero, the duration is between 0.25 and 0.5
seconds. The meaning of a nonzero value depends on the operating system.
This function does nothing if the terminal is not an asynchronous serial
data port.
The return value is normally zero. In the event of an error, a value
of -1 is returned. The following errno error conditions
are defined for this function:
EBADF
The filedes is not a valid file descriptor.
ENOTTY
The filedes is not associated with a terminal device.
Function: int tcdrain(int filedes)
The tcdrain function waits until all queued
output to the terminal filedes has been transmitted.
This function is a cancellation point in multi-threaded programs. This
is a problem if the thread allocates some resources (like memory, file
descriptors, semaphores or whatever) at the time tcdrain is
called. If the thread gets canceled these resources stay allocated
until the program ends. To avoid this calls to tcdrain should be
protected using cancellation handlers.
The return value is normally zero. In the event of an error, a value
of -1 is returned. The following errno error conditions
are defined for this function:
EBADF
The filedes is not a valid file descriptor.
ENOTTY
The filedes is not associated with a terminal device.
The tcflush function is used to clear the input and/or output
queues associated with the terminal file filedes. The queue
argument specifies which queue(s) to clear, and can be one of the
following values:
TCIFLUSH
Clear any input data received, but not yet read.
TCOFLUSH
Clear any output data written, but not yet transmitted.
TCIOFLUSH
Clear both queued input and output.
The return value is normally zero. In the event of an error, a value
of -1 is returned. The following errno error conditions
are defined for this function:
EBADF
The filedes is not a valid file descriptor.
ENOTTY
The filedes is not associated with a terminal device.
EINVAL
A bad value was supplied as the queue argument.
It is unfortunate that this function is named tcflush, because
the term "flush" is normally used for quite another operation--waiting
until all output is transmitted--and using it for discarding input or
output would be confusing. Unfortunately, the name tcflush comes
from POSIX and we cannot change it.
Function: int tcflow(int filedes, int action)
The tcflow function is used to perform operations relating to
XON/XOFF flow control on the terminal file specified by filedes.
The action argument specifies what operation to perform, and can
be one of the following values:
The return value is normally zero. In the event of an error, a value
of -1 is returned. The following errno error conditions
are defined for this function:
EBADF
The filedes is not a valid file descriptor.
ENOTTY
The filedes is not associated with a terminal device.
Here is an example program that shows how you can set up a terminal
device to read single characters in noncanonical input mode, without
echo.
#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <termios.h>
/* Use this variable to remember original terminal attributes. */
struct termios saved_attributes;
void
reset_input_mode (void)
{
tcsetattr (STDIN_FILENO, TCSANOW, &saved_attributes);
}
void
set_input_mode (void)
{
struct termios tattr;
char *name;
/* Make sure stdin is a terminal. */
if (!isatty (STDIN_FILENO))
{
fprintf (stderr, "Not a terminal.\n");
exit (EXIT_FAILURE);
}
/* Save the terminal attributes so we can restore them later. */
tcgetattr (STDIN_FILENO, &saved_attributes);
atexit (reset_input_mode);
/* Set the funny terminal modes. */
tcgetattr (STDIN_FILENO, &tattr);
tattr.c_lflag &= ~(ICANON|ECHO); /* Clear ICANON and ECHO. */
tattr.c_cc[VMIN] = 1;
tattr.c_cc[VTIME] = 0;
tcsetattr (STDIN_FILENO, TCSAFLUSH, &tattr);
}
int
main (void)
{
char c;
set_input_mode ();
while (1)
{
read (STDIN_FILENO, &c, 1);
if (c == '\004') /* C-d */
break;
else
putchar (c);
}
return EXIT_SUCCESS;
}
This program is careful to restore the original terminal modes before
exiting or terminating with a signal. It uses the atexit
function (see section 25.6.3 Cleanups on Exit) to make sure this is done
by exit.
The shell is supposed to take care of resetting the terminal modes when
a process is stopped or continued; see 27. Job Control. But some
existing shells do not actually do this, so you may wish to establish
handlers for job control signals that reset terminal modes. The above
example does so.
A pseudo-terminal is a special interprocess communication channel
that acts like a terminal. One end of the channel is called the
master side or master pseudo-terminal device, the other side
is called the slave side. Data written to the master side is
received by the slave side as if it was the result of a user typing at
an ordinary terminal, and data written to the slave side is sent to the
master side as if it was written on an ordinary terminal.
Pseudo terminals are the way programs like xterm and emacs
implement their terminal emulation functionality.
This subsection describes functions for allocating a pseudo-terminal,
and for making this pseudo-terminal available for actual use. These
functions are declared in the header file `stdlib.h'.
Function: int getpt(void)
The getpt function returns a new file descriptor for the next
available master pseudo-terminal. The normal return value from
getpt is a non-negative integer file descriptor. In the case of
an error, a value of -1 is returned instead. The following
errno conditions are defined for this function:
ENOENT
There are no free master pseudo-terminals available.
This function is a GNU extension.
Function: int grantpt(int filedes)
The grantpt function changes the ownership and access permission
of the slave pseudo-terminal device corresponding to the master
pseudo-terminal device associated with the file descriptor
filedes. The owner is set from the real user ID of the calling
process (see section 29.2 The Persona of a Process), and the group is set to a special
group (typically tty) or from the real group ID of the calling
process. The access permission is set such that the file is both
readable and writable by the owner and only writable by the group.
On some systems this function is implemented by invoking a special
setuid root program (see section 29.4 How an Application Can Change Persona). As a
consequence, installing a signal handler for the SIGCHLD signal
(see section 24.2.5 Job Control Signals) may interfere with a call to
grantpt.
The normal return value from grantpt is 0; a value of
-1 is returned in case of failure. The following errno
error conditions are defined for this function:
EBADF
The filedes argument is not a valid file descriptor.
EINVAL
The filedes argument is not associated with a master pseudo-terminal
device.
EACCES
The slave pseudo-terminal device corresponding to the master associated
with filedes could not be accessed.
Function: int unlockpt(int filedes)
The unlockpt function unlocks the slave pseudo-terminal device
corresponding to the master pseudo-terminal device associated with the
file descriptor filedes. On many systems, the slave can only be
opened after unlocking, so portable applications should always call
unlockpt before trying to open the slave.
The normal return value from unlockpt is 0; a value of
-1 is returned in case of failure. The following errno
error conditions are defined for this function:
EBADF
The filedes argument is not a valid file descriptor.
EINVAL
The filedes argument is not associated with a master pseudo-terminal
device.
Function: char * ptsname(int filedes)
If the file descriptor filedes is associated with a
master pseudo-terminal device, the ptsname function returns a
pointer to a statically-allocated, null-terminated string containing the
file name of the associated slave pseudo-terminal file. This string
might be overwritten by subsequent calls to ptsname.
Function: int ptsname_r(int filedes, char *buf, size_t len)
The ptsname_r function is similar to the ptsname function
except that it places its result into the user-specified buffer starting
at buf with length len.
This function is a GNU extension.
Portability Note: On System V derived systems, the file
returned by the ptsname and ptsname_r functions may be
STREAMS-based, and therefore require additional processing after opening
before it actually behaves as a pseudo terminal.
Typical usage of these functions is illustrated by the following example:
int
open_pty_pair (int *amaster, int *aslave)
{
int master, slave;
char *name;
master = getpt ();
if (master < 0)
return 0;
if (grantpt (master) < 0 || unlockpt (master) < 0)
goto close_master;
name = ptsname (master);
if (name == NULL)
goto close_master;
slave = open (name, O_RDWR);
if (slave == -1)
goto close_master;
if (isastream (slave))
{
if (ioctl (slave, I_PUSH, "ptem") < 0
|| ioctl (slave, I_PUSH, "ldterm") < 0)
goto close_slave;
}
*amaster = master;
*aslave = slave;
return 1;
close_slave:
close (slave);
close_master:
close (master);
return 0;
}
These functions, derived from BSD, are available in the separate
`libutil' library, and declared in `pty.h'.
Function: int openpty(int *amaster, int *aslave, char *name, struct termios *termp, struct winsize *winp)
This function allocates and opens a pseudo-terminal pair, returning the
file descriptor for the master in *amaster, and the file
descriptor for the slave in *aslave. If the argument name
is not a null pointer, the file name of the slave pseudo-terminal
device is stored in *name. If termp is not a null pointer,
the terminal attributes of the slave are set to the ones specified in
the structure that termp points to (see section 17.4 Terminal Modes).
Likewise, if the winp is not a null pointer, the screen size of
the slave is set to the values specified in the structure that
winp points to.
The normal return value from openpty is 0; a value of
-1 is returned in case of failure. The following errno
conditions are defined for this function:
ENOENT
There are no free pseudo-terminal pairs available.
Warning: Using the openpty function with name not
set to NULL is very dangerous because it provides no
protection against overflowing the string name. You should use
the ttyname function on the file descriptor returned in
*slave to find out the file name of the slave pseudo-terminal
device instead.
This function is similar to the openpty function, but in
addition, forks a new process (see section 26.4 Creating a Process) and makes the
newly opened slave pseudo-terminal device the controlling terminal
(see section 27.3 Controlling Terminal of a Process) for the child process.
If the operation is successful, there are then both parent and child
processes and both see forkpty return, but with different values:
it returns a value of 0 in the child process and returns the child's
process ID in the parent process.
If the allocation of a pseudo-terminal pair or the process creation
failed, forkpty returns a value of -1 in the parent
process.
Warning: The forkpty function has the same problems with
respect to the name argument as openpty.