Terminal I/O
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POSIX termios
Two terminal I/O modes
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Canonical and non-canonical
Getting and setting terminal attributes
Terminal window size: struct winsize
tty
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One of the first terminals was the
TeleTypeWriter from which we get the
abbreviation tty
A terminal is usually interactive and can be
either local or connected via a network
A tty is connected at two “ends”
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For real terminals, one end is connected to
hardware, such as the monitor, and the other end
to software, such as the shell
Pseudo terminals (covered next time) have both
ends connected to software such as telnet etc
Terminal I/O Modes
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Canonical mode (aka cooked mode)
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Default terminal mode
Provides a simple line editor. Input is sent to
application one line at a time
Most UNIX systems implement this in a module
known as the “terminal line discipline”
Non-canonical mode (aka raw mode)
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Input not assembled into lines. Data passed to
application as it is received
Terminal Operations
int isatty(int desc);
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desc is an open file descriptor
Returns 1 if the FD is connected to a tty
device or 0 otherwise
char *ttyname(int fd);
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Returns the pathname of the terminal
device or NULL on error (such as fd is not
associated with a tty device)
Terminal Operations
char *ctermid(char *s);
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Returns the pathname of the controlling
terminal.
If s is NULL, static buffer is used
Pass in a buffer of length L_ctermid to
avoid the static buffer
termios overview
Structure defined in <termios.h> that
contains the terminal device characteristics
that we can change
struct termios {
tcflag_t c_iflag;
/* input flags */
tcflag_t c_oflag;
/* output flags */
tcflag_t c_cflag;
/* control flags */
tcflag_t c_lflag;
/* local flags */
cc_t
c_cc[NCCS]; /* control chars */
};
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termios overview
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c_iflag
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Input flags. Controls the input of
characters by the terminal device driver
(strip 8th bit on input, enable parity, etc)
c_oflag
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Output flags. Controls driver output
(perform output processing, map newline
to CR/LF, etc)
termios overview
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c_cflag
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Control flags. Affect the RS-232 serial lines (ignore
modem status lines, one or two stop bits per
character, etc)
c_lflag
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Local flags. Affect interface between driver and
user (echo on or off, visually erase characters,
enable terminal-generated signals, job control
stop signal for background output, etc)
termios overview
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c_cc
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An array of special characters we can change such
as Ctrl D etc. NCCS is usually 15-20 elements.
POSIX defines 11, the rest are implementation
specific
cc_t is large enough to hold a special character.
Usually an unsigned char
See figs 18.3-18.6 staring on page 635 for list
of flags
Fig 18.7 on page 637 lists functions that
manipulate these flags
termios overview
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tcgetattr - fetch attributes
tcsetattr - set attributes
tcdrain - wait for all output to be
transmitted
tcflush
- flush pending input and/or
output
Special Input Characters
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Fig 18.9 on page 638 lists the special
input characters
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11 are POSIX standard. Of those, we can
not change the value of CR (\r) or NL (\n).
The other 9 can be changed
Getting and Setting Terminal
Attributes
int tcgetattr(int fd, struct termios *termios_p);
int tcsetattr(int fd, int optional_actions, const
struct termios *termios_p);
 termios_p points to a termios structure
 optional_actions specifies when to apply
the new attributes
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TCSANOW - set attributes now
TCSADRAIN – wait for all output to transmit
TCSAFLUSH – flush pending input/output
Terminal Option Flags
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Section 18.5 lists the various flags that
can be used with the termios structure.
See page 645-651 for full list
c_cflg
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CSIZE a mask that specifies the number of
bits per byte (not including parity bit if
used) Values are CS5, CS6, CS7 and CS8
See example on page 644 for usage
Terminal Option Flags
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PARENB if set, generates and checks parity
for outgoing and incoming characters
PARODD If set, parity is odd, otherwise
parity is even
c_lflag
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ECHO if set, input characters are echoed
back to the terminal device. Can be used in
either canonical or non-canonical modes
Terminal Option Flags
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c_iflag
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ICANON if set, canonical mode is in effect
ECHONL if set, and if ICANON is set, the
NL character is echoed, even if ECHO is not
set
ISTRIP when set, valid input bytes are
stripped to 7 bits. Otherwise all 8 bits are
processed
ETC…
stty command
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Shell command to examine and modify
terminal attributes
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stty –a displays all the terminal attributes
Baud Rate Functions
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Baud Rate – bits per second
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B2400, B9600, B19200, B38400, etc
Get I/O Baud Rate
speed_t cfgetispeed(const struct termios *termios_p);
speed_t cfgetospeed(const struct termios *termios_p);
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Set I/O Baud Rate
int cfsetispeed(struct termios *termios_p, speed_t speed);
int cfsetospeed(struct termios *termios_p, speed_t speed);
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Returns 0 if ok, -1 on error
Line Control Functions
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See page 653 for full explanation
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int
int
int
int
tcdrain(int fd);
tcflow(int fd, int action);
tcflush(int fd, int queue_selector);
tcsendbreak(int fd, int duration);
Canonical Mode
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Provides limited line editing. Data sent
to application one line at a time
When we issue a read it returns when
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The requested number of bytes have been
read
A line delimiter is encountered (NL, EOF,
EOL, EOL2 and sometimes CR)
A signal is caught but read not
automatically restarted
Non-Canonical Mode
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Specified by turning off the ICANON
flag in the c_lflag field of the termios
structure
Input data not assembled into lines
Some characters not processed (ERASE,
KILL, EOF, NL, EOL, EOL2, CR,
REPRINT, STATUS and WERASE)
Non-Canonical Mode
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Two variables (MIN and TIME) in the
c_cc array indexed by VMIN and VTIME
MIN - specifies the minimum number
of bytes to read before a read returns
TIME – specifies the number of tenths
of a second to wait for data to arrive
Non-Canonical Mode
MIN > 0
MIN == 0
A: read returns [MIN, nbytes]
before timer expires
TIME > 0 Read returns [1,MIN) if time
expires
(TIME=interbyte timer. Caller
can block indefinitely)
C: read return [1, nbytes]
before timer expires
Read returns 0 if timer
expires
(TIME=read timer)
B: read returns [MIN, nbytes]
when
TIME == 0 available
(Caller can block indefinitely)
D: read returns[0, nbytes]
immediately
Terminal Window Size
Most UNIX systems provide a way to keep track of
the current terminal window size and have the kernel
notify the foreground process group when the size
changes
struct winsize{
unsigned short ws_row; /* rows in characters */
unsigned short ws_col; /* cols in characters */
unsigned short ws_xpixel /* unused */
unsigned short ws_ypixel /* unused */
};
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Terminal Window Size
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Current window size can be fetched by
calling ioctl with TIOCGWINSZ
Can set a new value by calling ioctl with
TIOCSWINSZ. If new value is different
than old value it is stored in the kernel
and a SIGWINCH is sent to the
foreground process group (by default
the signal is ignored)