TERMIOS(4) BSD Kernel Interfaces Manual TERMIOS(4)
NAME
termios -- general terminal line discipline
SYNOPSIS
#include <termios.h>
DESCRIPTION
This describes a general terminal line discipline that is supported on tty asynchronous communication ports.
Opening a Terminal Device File
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(2) or rlogind(2), and
become an application's standard input, output, and error files.
Job Control in a Nutshell
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 job control; 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 partic-
ular job, that job is said to be in the "foreground". When the process group of the terminal is different than 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 doesn't wait for the job to complete (that is, it immediately attempts to read the next com-
mand). 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 affect 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.
Orphaned Process Groups
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 doesn't 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.
The Controlling Terminal
A terminal may belong to a process as its controlling terminal. Each process of a session that has a controlling terminal has the same con-
trolling terminal. A terminal may be the controlling terminal for at most one session. 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 con-
trolling 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.
Terminal Access Control
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 back-
ground process group to write to its controlling terminal will cause the process group to be sent a SIGTTOU signal unless one of the follow-
ing 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 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.
Input Processing and Reading Data
A terminal device associated with a terminal device file may operate in full-duplex mode, so that data may arrive even while output is occur-
ring. 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 sys-
tem 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() or fcntl(). 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.
Canonical Mode Input Processing
In canonical mode input processing, terminal input is processed in units of lines. A line is delimited by a newline '
' 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.
Noncanonical Mode Input Processing
In noncanonical mode input processing, input bytes are not assembled into lines, and erase and kill processing does not occur. The values of
the MIN and TIME 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 function successfully returns. TIME is a timer of 0.1 sec-
ond 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.
Case A: 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().
Case B: 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.
Case C: 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 initi-
ated, 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.
Case D: 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.
Writing Data and Output Processing
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.
Special Characters
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 immedi-
ately 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 indica-
tion. 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 '
'.
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 pro-
cessed. CR Special character on input and is recognized if the ICANON flag is set; it is the '
', 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 charac-
ter.
The following special characters are extensions defined by this system and are not a part of 1003.1 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 alpha-
betic/underscores or non alphabetic/underscores. As a special case in this second algorithm, the first previous non-whitespace char-
acter is skipped in determining whether the preceding word is a sequence of alphabetic/undercores. 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 sym-
bolic 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.
Modem Disconnect
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.
General Terminal Interface
Closing a Terminal Device File
The last process to close a terminal device file causes any output to be sent to the device and any input to be discarded. Then, if HUPCL is
set in the control modes, and the communications port supports a disconnect function, the terminal device performs a disconnect.
Parameters That Can Be Set
Routines that need to control certain terminal I/O characteristics do so by using the termios structure as defined in the header <termios.h>.
This structure contains minimally four scalar elements of bit flags and one array of special characters. The scalar flag elements are named:
c_iflag, c_oflag, c_cflag, and c_lflag. The character array is named c_cc, and its maximum index is NCCS.
Input Modes
Values of the c_iflag field describe the basic terminal input control, and are composed of following masks:
IGNBRK /* ignore BREAK condition */
BRKINT /* map BREAK to SIGINTR */
IGNPAR /* ignore (discard) parity errors */
PARMRK /* mark parity and framing errors */
INPCK /* enable checking of parity errors */
ISTRIP /* strip 8th bit off chars */
INLCR /* map NL into CR */
IGNCR /* ignore CR */
ICRNL /* map CR to NL (ala CRMOD) */
IXON /* enable output flow control */
IXOFF /* enable input flow control */
IXANY /* any char will restart after stop */
IMAXBEL /* ring bell on input queue full */
IUCLC /* translate upper case to lower case */
In the context of asynchronous serial data transmission, a break condition is defined as a sequence of zero-valued bits that continues for
more than the time to send one byte. The entire sequence of zero-valued bits is interpreted as a single break condition, even if it contin-
ues for a time equivalent to more than one byte. In contexts other than asynchronous serial data transmission the definition of a break con-
dition is implementation defined.
If IGNBRK is set, a break condition detected on input is ignored, that is, not put on the input queue and therefore not read by any process.
If IGNBRK is not set and BRKINT is set, the break condition flushes the input and output queues and if the terminal is the controlling termi-
nal of a foreground process group, the break condition generates a single SIGINT signal to that foreground process group. If neither IGNBRK
nor BRKINT is set, a break condition is read as a single '