termios.4

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.TH termios 4
.SH Name
termios \- POSIX terminal interface
.SH Description
.NXR "terminal interface"
.SH Interface Characteristics
The POSIX terminal interface is provided to
control asynchronous communications ports, pseudoterminals, and the special
file,
.PN /dev/tty .
The ULTRIX operating system also provides a SVID termio
terminal interface as defined in 
.MS termio 4
and a Berkeley terminal interface as defined in 
.MS tty 4 .
The following sections describe the general terminal interface as defined
by the POSIX operating system specification.
For a general overview of the various
terminal interfaces, refer to the subsection entitled Terminal 
interface definitions in 
.MS tty 4 .
.PP
The POSIX termios specification defines a set of terminal-related
functions and attributes 
that facilitate the development of portable programs.  The specification
allows for local extensions to the terminal interface.  Throughout this 
description, all local extensions to the termios interface are noted.
Programs that are written to be highly portable should avoid the usage of
local extensions.
.SH Opening a Terminal Device File
When a terminal file is opened, it normally causes the process to wait until
a connection is established.  In practice, application programs seldom open these 
files; they are opened by special programs such as 
.MS getty 8
and become a user's standard input, output, and error files.
.PP
As described in 
.MS open 2 ,
opening a terminal device file with the O_NONBLOCK
flag clear causes the process to block until the terminal device is ready
and available.
If CLOCAL mode is not set, blocking occurs until a connection is 
established.
If CLOCAL mode is set, or the O_NONBLOCK flag is specified in the
.MS open 2 ,
the
.MS open 2
returns a file descriptor, without waiting for a connection to be 
established.
.SH Process Groups
A terminal may have a foreground process group associated with it.  This
foreground process group plays a special role in handling signal-generating
input characters, as discussed in the Special Characters description.
.PP
A command interpreter that is capable of
supporting job control,
.MS csh 1
for example,
can allocate the terminal to different jobs
or process groups by placing related processes in a single process group and
associating this process group with the terminal.
A terminal's foreground
process group can be set or examined by a process with sufficient privileges.
The terminal interface aids in this allocation by restricting access to the
terminal by processes that are not in the foreground process group.  See the Job
Access Control description for more information.
.SH The Controlling Terminal
A terminal can belong to a process as its controlling terminal.  
Each process of a session that has a controlling terminal has the same
controlling terminal.  A terminal may be the controlling terminal for at most
one session.  If a session leader has no controlling terminal and opens a 
terminal device file that is not already associated with a session, without 
using the O_NOCTTY 
.MS open 2
flag, the terminal then becomes the controlling terminal of the session leader.
If a process that is not a session leader opens a terminal file, or the
O_NOCTTY option is used with 
.MS open 2 ,
that terminal cannot become the controlling terminal of the calling
process.  When a controlling terminal becomes associated with a session, its
foreground process group is set to the process group of the session 
leader.
.PP
The controlling terminal is inherited by a child process during a 
.MS fork 2
function.  A process relinquishes its controlling terminal when it 
creates a new session with the 
.MS setsid 2
function, or when all file descriptors associated with the controlling terminal
have been closed.
.PP
When a controlling process
terminates, the controlling terminal
is disassociated from the current session,
allowing
the terminal to be acquired as a controlling terminal by a new session 
process group leader.
Subsequent access to the terminal by other processes in the earlier session
may be denied, with attempts to access the terminal treated as if a modem 
disconnect had been sensed.
.SH Closing a Terminal Device File
When the last process that has the terminal line open closes the terminal
line, the process waits for all output to clear and any input 
is discarded.  The wait does not 
exceed four minutes, preventing the line from becoming hung if a progress is
not made in the final output.  After the final output has been transmitted, any
pending input is flushed.  If HUPCL is set in the control modes, and the
communications port supports a disconnect function, the terminal device 
performs a disconnect.
.SH Modem Disconnect
When a modem disconnect is detected by the terminal interface
for a controlling terminal, 
and the CLOCAL bit is not set in the control flag, the SIGHUP signal
is sent to all the controlling processes associated with the
terminal.  Unless other arrangements have been made, this signal causes the
controlling processes to terminate.  
If SIGHUP is ignored or caught, any subsequent read
returns with an end-of-file indication.  Thus, programs that read a terminal 
file and test for end-of-file can terminate appropriately after a disconnect.
Any subsequent 
.MS write 2
to the terminal device returns \-1 with 
.I errno 
set to [EIO], until the device
is closed.
.SH Terminal Access Control
If a process is in the foreground process group (nonzero) of its
controlling terminal (that is, if the process is a foreground process),
.MS read 2
operations are allowed as described in the Input Processing
and Reading Characters description. 
Any attempts by a process in a background process group to read from its
controlling terminal causes its process group to receive a SIGTTIN
signal, unless one of the following is true:  
.IP \(bu 5
If the reading process is ignoring or blocking the SIGTTIN signal.
.IP \(bu 5
If the process group of the reading process is orphaned, the
.MS read 2
returns \-1, with 
.I errno 
set to [EIO], and no signal is sent.
.PP
Attempts by a process in a background process group to write to its 
controlling terminal causes the process group to receive 
a SIGTTOU
signal, unless one of the following is true:
.IP \(bu 5
If TOSTOP is not set, or if TOSTOP is set and the process is ignoring or 
blocking the SIGTTOU signal, the process is allowed to write to the 
terminal  and the SIGTTOU is not sent.  
.IP \(bu 5
If TOSTOP is set, and the process
group of the writing process is orphaned, and the writing process is not
blocking SIGTTOU, the
.MS write 2
returns \-1, with 
.I errno 
set to [EIO], and no signal is sent.
.PP
Certain calls that set terminal parameters are treated in the same fashion
as
.MS write 2 ,
except that TOSTOP is ignored. That is, the effect is identical to that of
terminal writes when TOSTOP is set.
See the Control Functions description
for more information.
.SH Input Processing and Reading Characters
A terminal device associated with a terminal device file operates in 
full-duplex mode, so that characters can arrive while output is occurring.
Each terminal device file has associated with it an input queue, in which
incoming characters are stored by the system before being read by a process.
The system imposes a limit (MAX_INPUT), on the number of bytes that can be
stored in the input queue.  MAX_INPUT is limited to 256 characters.
When ICANON is not set and
the system's character input buffers become full, the input buffer is
flushed without notice. This causes all the characters in the input queue
to be lost.  If ICANON is set and the system's character input buffers become 
full, the driver discards additional characters and echos a bell
(ASCII BEL) to notify the user of the full condition.
.PP
Depending on whether or not the terminal device is in canonical mode or 
noncanonical mode, two general types of input processing are available.
See the Canonical Mode Input Processing and Noncanonical
Mode Input Processing descriptions for more information.
Additionally, input characters are processed
according to the c_iflag and c_lflag fields. See the Input Modes and Local
Modes descriptions.
Such processing can include echoing, which, in general, means transmitting input
characters immediately back to the terminal when they are received from the
terminal.  This is useful for terminals that can operate in full-duplex mode.
The manner in which characters are provided to a process reading from a 
terminal device is dependent on whether the terminal file is in canonical
or noncanonical mode.
.PP
Another dependency is whether or not the O_NONBLOCK is set by 
.MS open 2 
or 
.MS fcntl 2 .  
If the O_NONBLOCK flag is clear, then the read request
is blocked until data is available or a signal has been received.  If 
the O_NONBLOCK flag is set, then the read request completes, without 
blocking, in one of the following ways:
.IP \(bu 5
If there is enough data available to satisfy the entire request,
the read completes successfully and returns the number of bytes read.
.IP \(bu 5
If there is not enough data available to satisfy the entire request,
the read completes successfully, having read as much data as
possible and returns the number of bytes it was able to read.
.IP \(bu 5
If data is not available, the read returns a \-1, with
.I errno 
set to EAGAIN.
.PP
As stated previously, the availability of data is dependent on
the input processing mode.  The input processing mode can be either
canonical or non-canonical.  The following sections discuss
these modes in detail.
.SH Canonical Mode Input Processing
In canonical mode input processing, terminal input is processed in units of
lines.  A line is delimited by a new-line (\\n) character (ASCII LF),
an end-of-file character (ASCII EOF), or a user-defined 
end-of-line character (EOL) .
See the description of Special Characters for more information 
on EOF and EOL.
.PP
A read request cannot be satisfied until an
entire line has been typed or a signal has been received.  Regardless of
the number of characters requested in the read call, at most one line is
returned.  However, it is not necessary to read a whole line at once; one
or more characters can be requested in a read without losing information.
.PP
MAX_CANON (256) defines the maximum number of input characters the system
can buffer in canonical mode.  When this limit is exceeded, the system 
discards additional input.
.PP
Erase and kill processing occur when either the ERASE or KILL character
is received.  This processing affects data in the
input queue that has not been delimited by a new-line (NL), end-of-file
(EOF), or end-of-line (EOL) character.  The data that is not delimited 
creates the current line.  The ERASE character deletes the last 
character in the current line, if there is any.
The KILL character deletes all data in the current line, if there is any.
The ERASE and KILL characters have no effect if there is no data in the 
current line.
The ERASE and KILL characters themselves are not placed in the input queue.
.PP
The reprint (RPRNT) character retypes pending input beginning on a new
line.  Retyping is automatic, if characters that would normally
be erased from the screen are interspersed with program output.
.SH Noncanonical Mode Input Processing
In noncanonical mode input processing, input characters are not assembled into
lines, and erase and kill processing do not occur.  The values of the special
characters MIN and TIME are used to determine how to process the characters
received.  MIN and TIME are defined by the c_cc array of special control
characters.
.PP
MIN represents the minimum number of characters that should be received when
the read is satisfied (for example, the characters are returned to the user).
TIME is a timer of 0.1 second granularity that is used to time out bursty 
and short term data transmissions.  If MIN is greater than MAX_INPUT (256), the 
value of MIN is truncated to be MAX_INPUT. The four possible values for MIN 
and TIME and examples of their interactions are as follows:
.PP
1.
MIN > 0, TIME > 0.
.br
In this case, TIME serves as an intercharacter timer and is 
activated after the first character is received.  Because it is an 
intercharacter timer, it is reset after a character is received.  The 
interaction between MIN and TIME provokes the intercharacter time to start 
as soon as one character is received. If MIN characters are received before 
the intercharacter timer expires, the read is satisfied. 
.PP
If the timer expires before 
MIN characters are received, the characters received
to that point are returned to the user.  If TIME expires, at least 
one character is returned, because the timer would not have been enabled unless
a character was received.  The read blocks until the MIN and TIME 
mechanisms are activated by the receipt of the first character, 
or a signal is received.
.PP
2.
MIN > 0, TIME = 0.
.br
In this case, the value of TIME is zero, the timer is inactive. However,
MIN is significant.  A pending read is not satisfied until MIN 
characters or a signal are received.  
That is, the pending read sleeps until MIN characters
are received.  A program that uses this example to read record-based 
terminal I/O can indefinitely block the read operation.
.PP
3.
MIN = 0, TIME > 0.
.br
In this case, because MIN = 0, TIME does not represent an intercharacter
timer. Instead, it serves as a read timer that is activated as soon as the 
.MS read 2
function is processed.
A read is satisfied as soon as a single character is received
or the read timer expires.  Note, if the timer expires, a
character is not returned.  If the timer does not expire, a read is only
satisfied if a character is received.  For example, the read
cannot block indefinitely waiting for a character. If a character is not
received within TIME*0.1 seconds after the read is initiated, the read 
returns a value of zero, having read no data.
.PP
4.
MIN = 0, TIME = 0.
.br
In this case, only the  minimum number of characters requested or the 
number of characters currently available are returned without waiting 
for more characters to be input.  In this example, the return is 
immediate.
.PP
The following list summarizes the previous examples:
.IP \(bu 5
The interactions of MIN
and TIME are not symmetric.  For example, when MIN > 0 and TIME = 0,
TIME has no effect.  However, if MIN = 0 and 
TIME > 0, MIN is activated by the receipt of a single character.
.IP \(bu 5
When MIN > 0 and TIME > 0, TIME represents an intercharacter
timer; when MIN = 0, TIME > 0, TIME represents a read timer.
.PP
The previous summary highlights the dual purpose of the MIN and TIME feature.  
Cases 1 and 2 handle burst mode activity (such
as file transfer programs) where a program would like to process at 
least MIN characters at a
time.  In case 1, the intercharacter timer is activated by a user 
as a safety precaution. However, in case 2, it is turned off. 
.PP
Cases 3 and 4 exist to handle single character timed transfers.  These 
examples are readily adaptable to screen-based applications that need 
to know if a character is present in the input queue before refreshing 
the screen.  In case 3, the read is timed. However, in case 4, it is not.
.PP
Note that MIN is always a minimum.  It does not denote
a record length.  That is, if a program performs a read of 20 bytes, MIN
has a value of 10, and there are 25 characters present, 20 characters
are returned to the user.
.SH Writing Characters and Output Processing
When a process writes one or more characters to a terminal device file, 
they are processed according to the c_oflag (see Output Modes).
The terminal interface provides a buffering mechanism.  For example,
when a call to 
.MS write 2
completes, all
of the characters written have been scheduled for transmission to the device,
but the transmission is not necessarily complete.  The characters are
transmitted to the terminal as soon as previously written characters
have output successfully.
.SH Signal Handling
Signals caught during a 
.MS read 2 ,
.MS write 2 ,
or other operation on the file descriptor
associated with the terminal file are handled appropriately, as described in
.MS signal 3 .
.SH Special Characters
Certain characters have special functions on input or output.
These functions are:
.IP "\fBINTR\fR" 11
Special character on input that is recognized if the ISIG flag
is enabled.  Generates a SIGINT signal that is sent to all 
processes in the foreground process group associated with
the terminal. 
If ISIG is set, the INTR character is discarded when processed.
The default value is octal 0177.
.IP "\fBQUIT\fR" 
Special character on input that is recognized if the ISIG flag
is enabled.  Generates a SIGQUIT signal that is sent to all
processes in the foreground process group associated with
the terminal. 
If ISIG is set, the QUIT character is discarded when processed.
The default value is CTRL/| (ASCII FS).
.IP "\fBERASE\fR" 
Special character on input that is recognized if the ICANON 
flag is set.  Erases the last character in the current line.  It cannot
erase beyond the start of a line, as delimited by an NL, EOF,
or EOL character.  
If ICANON is set, the ERASE character is discarded when processed.
The default value is the number sign (#).
.IP "\fBKILL\fR" 
Special character on input that is recognized if the ICANON flag
is set.  Deletes the entire line, as delimited by an NL, EOF, or
EOL character. 
If ICANON is set, the KILL character is discarded when processed.
The default value is the at sign (@).
.IP "\fBEOF\fR" 
Special character on input that is recognized if the ICANON flag
is set. This character is used to generate an end of file (EOF)
from the terminal. 
When received, all the characters waiting to be read are
immediately passed to the program, without waiting for a newline,
and the EOF is discarded.  Thus, if there are no characters
waiting (that is, if the EOF occurred at the beginning of a line),
a byte count of zero is returned from 
.MS read 2 ,
representing an end-of-file indication.  
If ICANON is set, the EOF character is discarded when processed.
The default value is CTRL/D (ASCII EOT).
.IP "\fBNL\fR" 
Special character on input that is recognized if the ICANON flag
is set.  It is the assigned line delimiter (\\n).  It cannot be