This describes a general terminal line discipline that is supported on tty asynchronous communication
ports.
OpeningaTerminalDeviceFile
When a terminal file is opened, it normally causes the process to wait until a connection is established.
For most hardware, the presence of a connection is indicated by the assertion of the hardware CARRIER
line. If the termios structure associated with the terminal file has the CLOCAL flag set in the cflag,
or if the O_NONBLOCK flag is set in the open(2) call, then the open will succeed even without a
connection being present. In practice, applications seldom open these files; they are opened by special
programs, such as getty(8), and become an application's standard input, output, and error files.
JobControlinaNutshell
Every process is associated with a particular process group and session. The grouping is hierarchical:
every member of a particular process group is a member of the same session. This structuring is used in
managing groups of related processes for purposes of jobcontrol; that is, the ability from the keyboard
(or from program control) to simultaneously stop or restart a complex command (a command composed of one
or more related processes). The grouping into process groups allows delivering of signals that stop or
start the group as a whole, along with arbitrating which process group has access to the single
controlling terminal. The grouping at a higher layer into sessions is to restrict the job control
related signals and system calls to within processes resulting from a particular instance of a “login”.
Typically, a session is created when a user logs in, and the login terminal is setup to be the
controlling terminal; all processes spawned from that login shell are in the same session, and inherit
the controlling terminal.
A job control shell operating interactively (that is, reading commands from a terminal) normally groups
related processes together by placing them into the same process group. A set of processes in the same
process group is collectively referred to as a “job”. When the foreground process group of the terminal
is the same as the process group of a particular job, that job is said to be in the “foreground”. When
the process group of the terminal is different from the process group of a job (but is still the
controlling terminal), that job is said to be in the “background”. Normally the shell reads a command
and starts the job that implements that command. If the command is to be started in the foreground
(typical), it sets the process group of the terminal to the process group of the started job, waits for
the job to complete, and then sets the process group of the terminal back to its own process group (it
puts itself into the foreground). If the job is to be started in the background (as denoted by the shell
operator "&"), it never changes the process group of the terminal and does not wait for the job to
complete (that is, it immediately attempts to read the next command). If the job is started in the
foreground, the user may type a key (usually ‘^Z’) which generates the terminal stop signal (SIGTSTP) and
has the effect of stopping the entire job. The shell will notice that the job stopped, and will resume
running after placing itself in the foreground. The shell also has commands for placing stopped jobs in
the background, and for placing stopped or background jobs into the foreground.
OrphanedProcessGroups
An orphaned process group is a process group that has no process whose parent is in a different process
group, yet is in the same session. Conceptually it means a process group that does not have a parent
that could do anything if it were to be stopped. For example, the initial login shell is typically in an
orphaned process group. Orphaned process groups are immune to keyboard generated stop signals and job
control signals resulting from reads or writes to the controlling terminal.
TheControllingTerminal
A terminal may 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. The controlling terminal for a session is allocated by the session leader by
issuing the TIOCSCTTY ioctl. A controlling terminal is never acquired by merely opening a terminal
device file. When a controlling terminal becomes associated with a session, its foreground process group
is set to the process group of the session leader.
The controlling terminal is inherited by a child process during a fork(2) function call. A process
relinquishes its controlling terminal when it creates a new session with the setsid(2) function; other
processes remaining in the old session that had this terminal as their controlling terminal continue to
have it. A process does not relinquish its controlling terminal simply by closing all of its file
descriptors associated with the controlling terminal if other processes continue to have it open.
When a controlling process terminates, the controlling terminal is disassociated from the current
session, allowing it to be acquired by a new session leader. Subsequent access to the terminal by other
processes in the earlier session will be denied, with attempts to access the terminal treated as if modem
disconnect had been sensed.
TerminalAccessControl
If a process is in the foreground process group of its controlling terminal, read operations are allowed.
Any attempts by a process in a background process group to read from its controlling terminal causes a
SIGTTIN signal to be sent to the process's group unless one of the following special cases apply: if the
reading process is ignoring or blocking the SIGTTIN signal, or if the process group of the reading
process is orphaned, the read(2) returns -1 with errno set to EIO and no signal is sent. The default
action of the SIGTTIN signal is to stop the process to which it is sent.
If a process is in the foreground process group of its controlling terminal, write operations are
allowed. Attempts by a process in a background process group to write to its controlling terminal will
cause the process group to be sent a SIGTTOU signal unless one of the following special cases apply: 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 signal is not sent. If TOSTOP is set, and
the process group of the writing process is orphaned, and the writing process is not ignoring or blocking
SIGTTOU, the write(2) returns -1 with errno set to EIO and no signal is sent.
Certain calls that set terminal parameters are treated in the same fashion as write, except that TOSTOP
is ignored; that is, the effect is identical to that of terminal writes when TOSTOP is set.
InputProcessingandReadingData
A terminal device associated with a terminal device file may operate in full-duplex mode, so that data
may arrive even while output is occurring. Each terminal device file has associated with it an input
queue, into which incoming data is stored by the system before being read by a process. The system
imposes a limit, {MAX_INPUT}, on the number of bytes that may be stored in the input queue. The behavior
of the system when this limit is exceeded depends on the setting of the IMAXBEL flag in the termios
c_iflag. If this flag is set, the terminal is sent an ASCII BEL character each time a character is
received while the input queue is full. Otherwise, the input queue is flushed upon receiving the
character.
Two general kinds of input processing are available, determined by whether the terminal device file is in
canonical mode or noncanonical mode. Additionally, input characters are processed according to the
c_iflag and c_lflag fields. 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 data is provided to a process reading from a terminal device file is dependent on
whether the terminal device file is in canonical or noncanonical mode.
Another dependency is whether the O_NONBLOCK flag is set by open(2) or 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 is completed, without blocking, in one of three ways:
1. If there is enough data available to satisfy the entire request, and the read completes
successfully the number of bytes read is returned.
2. If there is not enough data available to satisfy the entire request, and the read completes
successfully, having read as much data as possible, the number of bytes read is returned.
3. If there is no data available, the read returns -1, with errno set to EAGAIN.
When data is available depends on whether the input processing mode is canonical or noncanonical.
CanonicalModeInputProcessing
In canonical mode input processing, terminal input is processed in units of lines. A line is delimited
by a newline ‘\n’ character, an end-of-file (EOF) character, or an end-of-line (EOL) character. See the
“Special Characters” section for more information on EOF and EOL. This means that a read request will
not return until an entire line has been typed, or a signal has been received. Also, no matter how many
bytes are requested in the read call, at most one line is returned. It is not, however, necessary to
read a whole line at once; any number of bytes, even one, may be requested in a read without losing
information.
{MAX_CANON} is a limit on the number of bytes in a line. The behavior of the system when this limit is
exceeded is the same as when the input queue limit {MAX_INPUT}, is exceeded.
Erase and kill processing occur when either of two special characters, the ERASE and KILL characters (see
the “Special Characters” section), is received. This processing affects data in the input queue that has
not yet been delimited by a newline NL, EOF, or EOL character. This un-delimited data makes up 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.
NoncanonicalModeInputProcessing
In noncanonical mode input processing, input bytes are not assembled into lines, and erase and kill
processing does not occur. The values of the VMIN and VTIME members of the c_cc array are used to
determine how to process the bytes received.
MIN represents the minimum number of bytes that should be received when the read(2) function successfully
returns. 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}, the response to the request is undefined. The four
possible values for MIN and TIME and their interactions are described below.
CaseA:MIN>0,TIME>0
In this case TIME serves as an inter-byte timer and is activated after the first byte is received. Since
it is an inter-byte timer, it is reset after a byte is received. The interaction between MIN and TIME is
as follows: as soon as one byte is received, the inter-byte timer is started. If MIN bytes are received
before the inter-byte timer expires (remember that the timer is reset upon receipt of each byte), the
read is satisfied. If the timer expires before MIN bytes are received, the characters received to that
point are returned to the user. Note that if TIME expires at least one byte is returned because the
timer would not have been enabled unless a byte was received. In this case (MIN > 0, TIME > 0) the read
blocks until the MIN and TIME mechanisms are activated by the receipt of the first byte, or a signal is
received. If data is in the buffer at the time of the read(), the result is as if data had been received
immediately after the read().
CaseB:MIN>0,TIME=0
In this case, since the value of TIME is zero, the timer plays no role and only MIN is significant. A
pending read is not satisfied until MIN bytes are received (i.e., the pending read blocks until MIN bytes
are received), or a signal is received. A program that uses this case to read record-based terminal I/O
may block indefinitely in the read operation.
CaseC:MIN=0,TIME>0
In this case, since MIN = 0, TIME no longer represents an inter-byte timer. It now serves as a read
timer that is activated as soon as the read function is processed. A read is satisfied as soon as a
single byte is received or the read timer expires. Note that in this case if the timer expires, no bytes
are returned. If the timer does not expire, the only way the read can be satisfied is if a byte is
received. In this case the read will not block indefinitely waiting for a byte; if no byte is received
within TIME*0.1 seconds after the read is initiated, the read returns a value of zero, having read no
data. If data is in the buffer at the time of the read, the timer is started as if data had been
received immediately after the read.
CaseD:MIN=0,TIME=0
The minimum of either the number of bytes requested or the number of bytes currently available is
returned without waiting for more bytes to be input. If no characters are available, read returns a
value of zero, having read no data.
WritingDataandOutputProcessing
When a process writes one or more bytes to a terminal device file, they are processed according to the
c_oflag field (see the “Output Modes” section). The implementation may provide a buffering mechanism; as
such, when a call to write() completes, all of the bytes written have been scheduled for transmission to
the device, but the transmission will not necessarily have been completed.
SpecialCharacters
Certain characters have special functions on input or output or both. These functions are summarized as
follows:
INTR Special character on input and is recognized if the ISIG flag (see the “Local Modes” section) is
enabled. Generates a SIGINT signal which is sent to all processes in the foreground process
group for which the terminal is the controlling terminal. If ISIG is set, the INTR character is
discarded when processed.
QUIT Special character on input and is recognized if the ISIG flag is enabled. Generates a SIGQUIT
signal which is sent to all processes in the foreground process group for which the terminal is
the controlling terminal. If ISIG is set, the QUIT character is discarded when processed.
ERASE Special character on input and is recognized if the ICANON flag is set. Erases the last
character in the current line; see “Canonical Mode Input Processing”. It does not 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.
KILL Special character on input and is recognized if the ICANON flag is set. Deletes the entire line,
as delimited by a NL, EOF, or EOL character. If ICANON is set, the KILL character is discarded
when processed.
EOF Special character on input and is recognized if the ICANON flag is set. When received, all the
bytes waiting to be read are immediately passed to the process, without waiting for a newline,
and the EOF is discarded. Thus, if there are no bytes waiting (that is, the EOF occurred at the
beginning of a line), a byte count of zero is returned from the read(), representing an end-of-
file indication. If ICANON is set, the EOF character is discarded when processed.
NL Special character on input and is recognized if the ICANON flag is set. It is the line delimiter
‘\n’.
EOL Special character on input and is recognized if the ICANON flag is set. Is an additional line
delimiter, like NL.
SUSP If the ISIG flag is enabled, receipt of the SUSP character causes a SIGTSTP signal to be sent to
all processes in the foreground process group for which the terminal is the controlling terminal,
and the SUSP character is discarded when processed.
STOP Special character on both input and output and is recognized if the IXON (output control) or
IXOFF (input control) flag is set. Can be used to temporarily suspend output. It is useful with
fast terminals to prevent output from disappearing before it can be read. If IXON is set, the
STOP character is discarded when processed.
START Special character on both input and output and is recognized if the IXON (output control) or
IXOFF (input control) flag is set. Can be used to resume output that has been suspended by a
STOP character. If IXON is set, the START character is discarded when processed.
CR Special character on input and is recognized if the ICANON flag is set; it is the ‘\r’, as
denoted in the C Standard {2}. When ICANON and ICRNL are set and IGNCR is not set, this
character is translated into a NL, and has the same effect as a NL character.
The following special characters are extensions defined by this system and are not a part of IEEE Std
1003.1 (“POSIX.1”) termios.
EOL2 Secondary EOL character. Same function as EOL.
WERASE Special character on input and is recognized if the ICANON flag is set. Erases the last word in
the current line according to one of two algorithms. If the ALTWERASE flag is not set, first any
preceding whitespace is erased, and then the maximal sequence of non-whitespace characters. If
ALTWERASE is set, first any preceding whitespace is erased, and then the maximal sequence of
alphabetic/underscores or non alphabetic/underscores. As a special case in this second
algorithm, the first previous non-whitespace character is skipped in determining whether the
preceding word is a sequence of alphabetic/underscores. This sounds confusing but turns out to
be quite practical.
REPRINT
Special character on input and is recognized if the ICANON flag is set. Causes the current input
edit line to be retyped.
DSUSP Has similar actions to the SUSP character, except that the SIGTSTP signal is delivered when one
of the processes in the foreground process group issues a read() to the controlling terminal.
LNEXT Special character on input and is recognized if the IEXTEN flag is set. Receipt of this
character causes the next character to be taken literally.
DISCARD
Special character on input and is recognized if the IEXTEN flag is set. Receipt of this
character toggles the flushing of terminal output.
STATUS Special character on input and is recognized if the ICANON flag is set. Receipt of this
character causes a SIGINFO signal to be sent to the foreground process group of the terminal.
Also, if the NOKERNINFO flag is not set, it causes the kernel to write a status message to the
terminal that displays the current load average, the name of the command in the foreground, its
process ID, the symbolic wait channel, the number of user and system seconds used, the percentage
of cpu the process is getting, and the resident set size of the process.
The NL and CR characters cannot be changed. The values for all the remaining characters can be set and
are described later in the document under Special Control Characters.
Special character functions associated with changeable special control characters can be disabled
individually by setting their value to {_POSIX_VDISABLE}; see “Special Control Characters”.
If two or more special characters have the same value, the function performed when that character is
received is undefined.
ModemDisconnect
If a modem disconnect is detected by the terminal interface for a controlling terminal, and if CLOCAL is
not set in the c_cflag field for the terminal, the SIGHUP signal is sent to the controlling process
associated with the terminal. Unless other arrangements have been made, this causes the controlling
process to terminate. Any subsequent call to the read() function returns the value zero, indicating end
of file. Thus, processes that read a terminal file and test for end-of-file can terminate appropriately
after a disconnect. Any subsequent write() to the terminal device returns -1, with errno set to EIO,
until the device is closed.
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