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发信人: georgehill (人生不定式), 信区: Linux
标 题: Unix Programming FAQ (v1.37)-Terminal(转寄)
发信站: BBS 荔园晨风站 (Thu Sep 14 13:02:37 2000), 站内信件
【 以下文字转载自 georgehill 的信箱 】
【 原文由 georgehill.bbs@smth.org 所发表 】
发信人: hhuu (什么是水，就是water啊), 信区: FreeBSD
标 题: Unix Programming FAQ (v1.37)-Terminal(转寄)
发信站: BBS 水木清华站 (Wed Sep 13 20:51:27 2000)
发信人: Luther (天語『至菜※努力』), 信区: Security
标 题: Unix Programming FAQ (v1.37)-Terminal(转寄)
发信站: 武汉白云黄鹤站 (Wed Sep 13 15:47:14 2000), 站内信件
3. Terminal I/O
***************
3.1 How can I make my program not echo input?
=============================================
How can I make my program not echo input, like login does when asking
for your password?
There is an easy way, and a slightly harder way:
The easy way, is to use `getpass()', which is probably found on almost all
Unices. It takes a string to use as a prompt. It will read up to an `EOF'
or newline and returns a pointer to a static area of memory holding the
string typed in.
The harder way is to use `tcgetattr()' and `tcsetattr()', both use a
`struct termios' to manipulate the terminal. The following two routines
should allow echoing, and non-echoing mode.
#include <stdlib.h>
#include <stdio.h>
#include <termios.h>
#include <string.h>
static struct termios stored_settings;
void echo_off(void)
{
struct termios new_settings;
tcgetattr(0,&stored_settings);
new_settings = stored_settings;
new_settings.c_lflag &= (~ECHO);
tcsetattr(0,TCSANOW,&new_settings);
return;
}
void echo_on(void)
{
tcsetattr(0,TCSANOW,&stored_settings);
return;
}
Both routines used, are defined by the POSIX standard.
3.2 How can I read single characters from the terminal?
3.2 How can I read single characters from the terminal?
=======================================================
How can I read single characters from the terminal? My program is
always waiting for the user to press `<RETURN>'.
Terminals are usually in canonical mode, where input is read in lines after
it is edited. You may set this into non-canonical mode, where you set how
many characters should be read before input is given to your program. You
also may set the timer in non-canonical mode terminals to 0, this timer
flushs your buffer at set intervals. By doing this, you can use `getc()' to
grab the key pressed immediately by the user. We use `tcgetattr()' and
`tcsetattr()' both of which are defined by POSIX to manipulate the
`termios' structure.
#include <stdlib.h>
#include <stdio.h>
#include <termios.h>
#include <string.h>
static struct termios stored_settings;
void set_keypress(void)
{
struct termios new_settings;
struct termios new_settings;
tcgetattr(0,&stored_settings);
new_settings = stored_settings;
/* Disable canonical mode, and set buffer size to 1 byte */
new_settings.c_lflag &= (~ICANON);
new_settings.c_cc[VTIME] = 0;
new_settings.c_cc[VMIN] = 1;
tcsetattr(0,TCSANOW,&new_settings);
return;
}
void reset_keypress(void)
{
tcsetattr(0,TCSANOW,&stored_settings);
return;
}
3.3 How can I check and see if a key was pressed?
=================================================
How can I check and see if a key was pressed? On DOS I use the
How can I check and see if a key was pressed? On DOS I use the
`kbhit()' function, but there doesn't seem to be an equivalent?
If you set the terminal to single-character mode (see previous answer),
then (on most systems) you can use `select()' or `poll()' to test for
readability.
3.4 How can I move the cursor around the screen?
================================================
How can I move the cursor around the screen? I want to do full screen
editing without using curses.
Seriously, you probably *don't* want to do this. Curses knows about how to
handle all sorts of oddities that different terminal types exhibit; while
the termcap/terminfo data will tell you whether any given terminal type
possesses any of these oddities, you will probably find that correctly
handling all the combinations is a *huge* job.
However, if you insist on getting your hands dirty (so to speak), look into
the `termcap' functions, particularly `tputs()', `tparm()' and `tgoto()'.
3.5 What are pttys?
===================
Pseudo-teletypes (pttys, ptys, other variant abbreviations) are
pseudo-devices that have two parts: the "master" side, which can be thought
of as the `user', and the "slave" side, which behaves like a standard tty
device.
They exist in order to provide a means to emulate the behaviour of a serial
They exist in order to provide a means to emulate the behaviour of a serial
terminal under the control of a program. For example, `telnet' uses a
pseudo-terminal on the remote system; the remote login shell sees the
behaviour it expects from a tty device, but the master side of the
pseudo-terminal is being controlled by a daemon that forwards all data over
the network. They are also used by programs such as `xterm', `expect',
`script', `screen', `emacs', and many others.
3.6 How to handle a serial port or modem?
=========================================
The handling of serial devices under Unix is heavily influenced by the
traditional use of serial terminals. Historically, various combinations of
ioctls and other hacks were necessary to control the precise behaviour of a
serial device, but fortunately this is one of the areas that POSIX made
some efforts to standardise.
If you're using a system that doesn't understand `<termios.h>',
`tcsetattr()' and related functions, then you'll have to go elsewhere for
information (or upgrade your system to something less archaeological).
There are still significant differences between systems, however, mainly in
the area of device names, handling of hardware flow control, and modem
signalling. (Whenever possible, leave the device driver to do all the
handshaking work, and don't attempt to manipulate handshaking signals
directly.)
The basic steps for opening and initialising a serial device are:
The basic steps for opening and initialising a serial device are:
* `open()' the device; this may require the use of certain flags:
`O_NONBLOCK'
Opening a dial-in or modem-controlled device will block until
carrier is present, unless this flag is used. A nonblocking open
gives you the opportunity to disable the modem controls (see
CLOCAL below) if necessary.
`O_NOCTTY'
On 4.4BSD-derived systems this is redundant, but on other systems
it controls whether the serial device can become a control
terminal for the session. In most cases you probably *don't* want
to acquire a control terminal, and should therefore specify this
flag, but there are exceptions.
* Use `tcgetattr()' to retrieve the current device modes. While one will
often ignore most or all of the initial settings thus obtained, it's
still a convenient way of initialising a `struct termios'.
* Set suitable values for `c_iflag', `c_oflag', `c_cflag', `c_lflag',
and `c_cc' in the termios structure. (See below.)
* Use `cfsetispeed()' and `cfsetospeed()' to set the desired baud rate.
Very few systems allow you to set differing input and output speeds,
so as a general rule you should set both to your desired speed.
* Use `tcsetattr()' to set the device modes.
* You may wish, if you used `O_NONBLOCK' when opening the port, to use
* You may wish, if you used `O_NONBLOCK' when opening the port, to use
`fcntl()' to ensure that `O_NONBLOCK' is turned off again. Systems
seem to differ as to whether a nonblocking open on a tty will affect
subsequent `read()' calls; better to be explicit.
Once you have opened and set up the port, you can then use `read()' and
`write()' normally. Note that the behaviour of `read()' will be controlled
by the flag settings you gave to `tcsetattr()'.
`tcflush()', `tcdrain()', `tcsendbreak()' and `tcflow()' are additional
useful functions that you should be aware of.
When you're done with the port, and want to close it, be aware of a very
nasty little hazard on some systems; if there's any pending output waiting
to be written to the device (e.g. if output flow is stopped by hardware or
software handshaking), your process can hang *unkillably* in the `close()'
call until the output drains. Calling `tcflush()' to discard any pending
output is probably a wise move.
(Blocked output on tty devices is by far the most common cause of
"unkillable" processes in my experience.)
3.6.1 Serial device names and types
-----------------------------------
The device names used for serial port devices vary quite widely between
systems. Some examples from different systems are:
* `/dev/tty[0-9][a-z]' for direct access devices, and
`/dev/tty[0-9][A-Z]' for modem control devices (e.g. SCO Unix)
`/dev/tty[0-9][A-Z]' for modem control devices (e.g. SCO Unix)
* `/dev/cua[0-9]p[0-9]' for direct access devices, `/dev/cul[0-9]p[0-9]'
for dial-out devices and `/dev/ttyd[0-9]p[0-9]' for dial-in devices
(e.g. HP-UX)
* `/dev/cua[a-z][0-9]' for dial-out devices and `/dev/tty[a-z][0-9]' for
dial-in devices (e.g. FreeBSD)
The precise interaction between the device name used, and the effect on any
hardware handshake lines is system-, configuration- and hardware-dependant,
but will usually follow approximately these rules (assuming that the
hardware is RS-232 DTE):
- A successful open of any device should assert DTR and RTS
- A blocking open of a modem-control or dial-in device will wait for DCD
(and possibly also DSR and/or CTS) to be raised, usually after
asserting DTR/RTS.
- An open of a dial-out device while an open call to the corresponding
dial-in device is blocked waiting for carrier *may or may not* cause
the open of the dial-in port to complete. Some systems implement a
simple sharing scheme for dial-in and dial-out ports whereby the
dial-in port is effectively "put to sleep" while the dial-out port is
in use; other systems do not do this, and sharing the port between
dial-in and dial-out on such systems requires external cooperation
(e.g. use of UUCP lockfiles) to avoid contention problems.
3.6.2 Setting up termios flags
3.6.2 Setting up termios flags
------------------------------
Some hints on setting up the termios flags when using a serial device that
you've opened yourself (as opposed to using your existing control tty):
3.6.2.1 c_iflag
...............
You probably want to set *all* the bits in `c_iflag' to 0, unless you want
to use software flow control (ick) in which case you set `IXON' and `IXOFF'.
3.6.2.2 c_oflag
...............
Most of the bits of `c_oflag' are hacks of one sort or another to make
output to slow terminals work, and as such some newer systems have dropped
almost all of them as obsolete (especially all the gory output-padding
options). As with `c_iflag', setting everything to 0 is reasonable for most
applications.
3.6.2.3 c_cflag
...............
When setting the character size, remember to mask using `CSIZE' first; e.g.
to set 8-bit characters, use:
attr.c_cflag &= ~CSIZE;
attr.c_cflag |= CS8;
Other important flags found in `c_cflag' that you probably want to turn
*on* and `CREAD' and `HUPCL'.
*on* and `CREAD' and `HUPCL'.
If you need to generate even parity, then set `PARENB' and clear `PARODD';
if you need to generate odd parity then set both `PARENB' and `PARODD'. If
you don't want parity at all, then make sure `PARENB' is clear.
Clear `CSTOPB' unless you actually need to generate two stop bits.
Flags for enabling hardware flow control may also be found in `c_cflag',
but they aren't standardised (pity).
3.6.2.4 c_lflag
...............
Most applications will probably want to turn off `ICANON' (canonical, i.e.
line-based, input processing), `ECHO', and `ISIG'.
`IEXTEN' is a more complex issue. If you don't turn it off, the
implementation is allowed to do nonstandard things (like define additional
control characters in `c_cc') that might cause unexpected results, but you
might need to leave `IEXTEN' enabled on some systems to get useful features
like hardware flow control.
3.6.2.5 c_cc
............
This is an array of characters that have special meanings on input. These
characters are given names like `VINTR', `VSTOP' etc.; the names are
indexes into the array.
(Two of these "characters" are not really characters at all, but control
the behaviour of `read()' when `ICANON' is disabled; these are `VMIN' and
the behaviour of `read()' when `ICANON' is disabled; these are `VMIN' and
`VTIME'.)
The indexes are often referred to as though they were actual variables,
e.g. "set VMIN to 1" actually means "set c_cc[VMIN] to 1". The shorthand is
useful and only occasionally confusing.
Many of the slots in `c_cc' are only used if some other combination of
flags is set:
Used only if `ICANON' is set
`VEOF', `VEOL', `VERASE', `VKILL' (and also `VEOL2', `VSTATUS' and
`VWERASE' if defined and `IEXTEN' is set)
Used only if `ISIG' is set
`VINTR', `VQUIT', `VSUSP' (and also `VDSUSP' if defined and `IEXTEN'
is set)
Used only if `IXON' or `IXOFF' is set
`VSTOP', `VSTART'
Used only if `ICANON' is *not* set
`VMIN', `VTIME'
Implementations may define additional entries in `c_cc'. It may be prudent
to initialise all the entries to `_POSIX_VDISABLE' (the constant `NCCS'
gives the array size) before setting the specific values you wish to use.
`VMIN' and `VTIME' (which may share slots with `VEOF' and `VEOL'
respectively, depending on the implementation) have the following meaning.
The value of `VTIME' is (if not 0) always interpreted as a timer in tenths
The value of `VTIME' is (if not 0) always interpreted as a timer in tenths
of seconds.
`c_cc[VMIN] > 0, c_cc[VTIME] > 0'
`read()' will return when either VMIN bytes of input are available, or
if at least one character has been read and VTIME has expired between
characters, or if interrupted by a signal.
`c_cc[VMIN] > 0, c_cc[VTIME] == 0'
`read()' will return when VMIN bytes of input are available, or if
interrupted. Otherwise it will wait indefinitely.
`c_cc[VMIN] == 0, c_cc[VTIME] > 0'
`read()' will return as soon as any input is available; if VTIME
expires with no data arriving, it will return with no characters read.
(This conflicts slightly with the end-of-file indication received in
the event of modem hangup; using 1 for VMIN and either `alarm()' or
`select()' for a timeout avoids this particular problem.)
`c_cc[VMIN] == 0, c_cc[VTIME] == 0'
`read()' will always return immediately; if no data is available it
will return with no characters read (with the same problem as above).
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of seconds.
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