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 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'.
1 if filedes is a file descriptor
associated with an open terminal device, and @math{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 section Identifying the Controlling Terminal.
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.
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 @math{0}. Otherwise an
error number is returned to indicate the error. The following
errno error conditions are defined for this function:
EBADF
ENOTTY
ERANGE
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 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 section 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 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 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 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 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 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 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 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 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 section 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.
tcflag_t c_iflag
tcflag_t c_oflag
tcflag_t c_cflag
tcflag_t c_lflag
cc_t c_cc[NCCS]
The struct termios structure also contains members which
encode input and output transmission speeds, but the representation is
not specified. See section 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.
c_cc
array.
If successful, tcgetattr returns @math{0}. A return value of @math{-1}
indicates an error. The following errno error conditions are
defined for this function:
EBADF
ENOTTY
The when argument specifies how to deal with input and output already queued. It can be one of the following values:
TCSANOW
TCSADRAIN
TCSAFLUSH
TCSADRAIN, but also discards any queued input.
TCSASOFT
TCSASOFT is exactly the same as setting the CIGNORE
bit in the c_cflag member of the structure termios-p points
to. See section 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 Job Control.
If successful, tcsetattr returns @math{0}. A return value of
@math{-1} indicates an error. The following errno error
conditions are defined for this function:
EBADF
ENOTTY
EINVAL
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 Setting Terminal Modes Properly).
Parity checking on input processing is independent of whether parity
detection and generation on the underlying terminal hardware is enabled;
see section 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.
INPCK is also set.
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, 0377 0377, in this case.
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.
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'.
'\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.
IGNCR is not set, carriage return characters
('\r') received as input are passed to the application as newline
characters ('\n').
'\n') received as input
are passed to the application as carriage return characters ('\r').
This is a BSD extension; it exists only on BSD systems and the GNU system.
007) to the terminal to ring the bell.
This is a BSD extension.
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 Setting Terminal Modes Properly).
'\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.
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 Setting Terminal Modes Properly).
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 section 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.
If this bit is not set, no parity bit is added to output characters, and input characters are not checked for correct parity.
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.
The following four bits are BSD extensions; this exist only on BSD systems and the GNU system.
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 section 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 Setting Terminal Modes Properly).
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.
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.
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.
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.
ICANON bit is also set, then the
newline ('\n') character is echoed even if the ECHO bit
is not set.
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.
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.
See section Characters that Cause Signals.
IEXTEN implementation-defined meaning,
so you cannot rely on this interpretation on all systems.
On BSD systems and the GNU system, it enables the LNEXT and DISCARD characters. See section Other Special Characters.
SIGTTOU signals are generated by background processes that
attempt to write to the terminal. See section Access to the Controlling Terminal.
The following bits are BSD extensions; they exist only in BSD systems and the GNU system.
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.
See section Characters for Input Editing, for more information about the WERASE character.
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:
*termios-p.
*termios-p.
*termios-p as the output
speed. The normal return value is @math{0}; a value of @math{-1}
indicates an error. If speed is not a speed, cfsetospeed
returns @math{-1}.
*termios-p as the input
speed. The normal return value is @math{0}; a value of @math{-1}
indicates an error. If speed is not a speed, cfsetospeed
returns @math{-1}.
*termios-p as both the
input and output speeds. The normal return value is @math{0}; a value
of @math{-1} indicates an error. If speed is not a speed,
cfsetspeed returns @math{-1}. This function is an extension in
4.4 BSD.
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 @math{-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.
B0 B50 B75 B110 B134 B150 B200 B300 B600 B1200 B1800 B2400 B4800 B9600 B19200 B38400 B57600 B115200 B230400 B460800
BSD defines two additional speed symbols as aliases: EXTA is an
alias for B19200 and EXTB is an alias for B38400.
These aliases are obsolete.
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 Optional Features in File Support, for more information about how to tell whether the operating
system you are using supports _POSIX_VDISABLE.
These special characters are active only in canonical input mode. See section Two Styles of Input: Canonical or Not.
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.
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.
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.
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.
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 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.
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.
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.
The REPRINT character is usually C-r.
This is a BSD extension.
These special characters may be active in either canonical or noncanonical
input mode, but only when the ISIG flag is set (see section Local Modes).
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 Signal Handling, for more
information about signals.
Typically, the INTR character is C-c.
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 Signal Handling, for more information
about signals.
Typically, the QUIT character is C-\.
termios.c_cc[VSUSP] holds the character
itself.
The SUSP (suspend) character is recognized only if the implementation
supports job control (see section 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 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 Signaling Another Process.
termios.c_cc[VDSUSP] holds the character
itself.
The DSUSP (suspend) character is recognized only if the implementation
supports job control (see section 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).
See section Signal Handling, for more information about signals.
Typically, the DSUSP character is C-y.
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 Input Modes).
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.
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.
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.
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.
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.
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.
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:
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.
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.
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.
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.
*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.
It does exactly this:
termios-p->c_iflag &= ~(IGNBRK|BRKINT|PARMRK|ISTRIP
|INLCR|IGNCR|ICRNL|IXON);
termios-p->c_oflag &= ~OPOST;
termios-p->c_lflag &= ~(ECHO|ECHONL|ICANON|ISIG|IEXTEN);
termios-p->c_cflag &= ~(CSIZE|PARENB);
termios-p->c_cflag |= CS8;
The usual way to get and set terminal modes is with the functions described
in section 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'.
gtty and
stty.
char sg_ispeed
char sg_ospeed
char sg_erase
char sg_kill
int sg_flags
gtty sets *attributes to describe the terminal attributes
of the terminal which is open with file descriptor filedes.
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 Job Control.
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 @math{-1} is returned. The following errno error conditions
are defined for this function:
EBADF
ENOTTY
tcdrain function waits until all queued
output to the terminal filedes has been transmitted.
This function is a cancelation 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 cancelation handlers.
The return value is normally zero. In the event of an error, a value
of @math{-1} is returned. The following errno error conditions
are defined for this function:
EBADF
ENOTTY
EINTR
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
TCOFLUSH
TCIOFLUSH
The return value is normally zero. In the event of an error, a value
of @math{-1} is returned. The following errno error conditions
are defined for this function:
EBADF
ENOTTY
EINVAL
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.
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:
TCOOFF
TCOON
TCIOFF
TCION
For more information about the STOP and START characters, see section Special Characters.
The return value is normally zero. In the event of an error, a value
of @math{-1} is returned. The following errno error conditions
are defined for this function:
EBADF
ENOTTY
EINVAL
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 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 section 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'.
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 @math{-1} is returned instead. The following
errno conditions are defined for this function:
ENOENT
This function is a GNU extension.
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 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 How an Application Can Change Persona). As a
consequence, installing a signal handler for the SIGCHLD signal
(see section Job Control Signals) may interfere with a call to
grantpt.
The normal return value from grantpt is @math{0}; a value of
@math{-1} is returned in case of failure. The following errno
error conditions are defined for this function:
EBADF
ENINVAL
EACCESS
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 @math{0}; a value of
@math{-1} is returned in case of failure. The following errno
error conditions are defined for this function:
EBADF
EINVAL
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.
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'.
*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 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 @math{0}; a value of
@math{-1} is returned in case of failure. The following errno
conditions are defined for this function:
ENOENT
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.
openpty function, but in
addition, forks a new process (see section Creating a Process) and makes the
newly opened slave pseudo-terminal device the controlling terminal
(see section 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 @math{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 @math{-1} in the parent
process.
Warning: The forkpty function has the same problems with
respect to the name argument as openpty.
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