ch41_02.htm

the unix power tools

people make different assumptions about how tabs should work. The
ASCII code for TAB, code 9, is intended to move the cursor right to the
next tabstop. But where <EM
CLASS="emphasis"
>is</EM
> that? Moreover, once the cursor has
gone there, how does the kernel move it back if you decided to erase
the tab?</P
><P
CLASS="para"
>Many UNIX kernels can be told to expand tabs. When they do this, they
set the tabstops at every eight characters. This is where they think
tabstops belong. That is, if you print a newline, two ordinary
letters, and a tab, the tab will turn into six spaces. If a <EM
CLASS="emphasis"
>tty</EM
> is in
icanon/cooked mode, and is expanding tabs, it can &quot;unexpand&quot; them
to backspace over the tab. Berkeley kernels will do this, and it
works fairly well. They can get it wrong, however, under certain
conditions. For instance, if you set the <EM
CLASS="emphasis"
>tty</EM
> to pass tabs unmodified,
and if the terminal itself puts tabstops at every ten
characters&nbsp;- this would be the proper setting for dealing with a
DEC-10, for instance&nbsp;- the kernel <EM
CLASS="emphasis"
>tty</EM
> code will put out fewer
backspaces than needed.</P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-45579"
></A
><A
CLASS="indexterm"
NAME="AUTOID-45582"
></A
><A
CLASS="indexterm"
NAME="AUTOID-45585"
></A
>Even if the terminal sets its tabstops at eight, the kernel's <EM
CLASS="emphasis"
>tty</EM
> code
and the terminal can get different ideas of the current cursor column.
Most Berkeley kernels count control codes as &quot;ordinary&quot; output
characters, for instance, even though those characters are likely to
have no effect on the cursor, or might even move it to an arbitrary
position. To help prevent <EM
CLASS="emphasis"
>input</EM
> control characters from goofing up
backspacing, Berkeley kernels can echo them as two-character
sequences. For instance, CTRL-g will normally echo as <CODE
CLASS="literal"
>^G</CODE
>. Erasing
such a control character works properly: the <EM
CLASS="emphasis"
>tty</EM
> code puts out <EM
CLASS="emphasis"
>two</EM
>
backspaces, <EM
CLASS="emphasis"
>two</EM
> spaces, and two more backspaces. Erasing more
characters, possibly including a TAB, then still works. This &quot;control
echo&quot; can be switched on and off individually as well.</P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-45594"
></A
>In addition to carriage return delays, which exist to allow time
for the teletype's print carriage to move left, some UNIX systems
also support tab delays, for more or less the same reason. Like
return-delays, these are pretty much outmoded and useless. The
POSIX standard leaves room for both kinds of delay, but does not
mandate either one. You may see them in <EM
CLASS="emphasis"
>stty</EM
> output, as <EM
CLASS="emphasis"
>cr2</EM
>,
<EM
CLASS="emphasis"
>cr3</EM
>, <EM
CLASS="emphasis"
>tab1</EM
>, and the like, but your system's default is probably
&quot;no delay,&quot; and few people are likely to change this deliberately.</P
><P
CLASS="para"
>Article
<A
CLASS="xref"
HREF="ch41_04.htm"
TITLE="How UNIX Handles TAB Characters "
>41.4</A
>
has some higher-level information about TABs.</P
></DIV
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="UPT-ART-1140-SECT-1.4"
>41.2.4 Flow Control (We Hope) </A
></H3
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-45605"
></A
><A
CLASS="indexterm"
NAME="AUTOID-45607"
></A
><A
CLASS="indexterm"
NAME="AUTOID-45609"
></A
><A
CLASS="indexterm"
NAME="AUTOID-45611"
></A
><A
CLASS="indexterm"
NAME="AUTOID-45613"
></A
><A
CLASS="indexterm"
NAME="AUTOID-45615"
></A
>Finally, flow control&nbsp;- avoiding lost input and output characters&nbsp;- is
perhaps the dirtiest swamp of all. Most of the terminals built in the
1980s support, and at higher speeds require, something called
\h'-1p'<EM
CLASS="emphasis"
>XON/XOFF</EM
> flow control. Here, when the terminal falls behind in
printing characters, it shouts &quot;stop!&quot; by sending an XOFF
character&nbsp;- ASCII code 19, or CTRL-s&nbsp;- to the UNIX system. If the
UNIX machine does not stop soon enough, some text will be lost. When
the terminal is ready for more, it sends a &quot;go&quot; character&nbsp;- an XON,
ASCII code 17, or CTRL-q. These were never intended as a general
flow control mechanism&nbsp;- on some of the original teletypes, they turned
the paper tape punch off and on&nbsp;- but they have that meaning now.
Unfortunately, most terminals also allow users to type CTRL-s and
CTRL-q, but they provide no way to distinguish between the terminal
yelling &quot;stop&quot; and the user pushing CTRL-s. The result is a
constant battle between people who want to use CTRL-s and computer
systems that want to take it for themselves.</P
><P
CLASS="para"
>Other systems, notably HP-based systems, use something called <EM
CLASS="emphasis"
>ENQ/ACK</EM
>
flow control. Here the terminal and the host system must agree up-front on a minimum buffer size. Then either system is allowed to send
that many characters to the other, after which it must stop and wait
for a &quot;go-ahead&quot; signal. Each system requests such a signal by
sending an &quot;enquire&quot;: ASCII code 5, or CTRL-e. When the listening
system encounters the ENQ, and is ready for more, it sends an
acknowledgement: ASCII code 6, or CTRL-f. This system is superior
to the XON/XOFF system in one way, as it never has problems with a
busy system failing to stop immediately on command, but it still does
not prevent users from typing CTRL-e and CTRL-f. Moreover, it
is not implemented on most UNIX systems.</P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-45621"
></A
><A
CLASS="indexterm"
NAME="AUTOID-45623"
></A
><A
CLASS="indexterm"
NAME="AUTOID-45626"
></A
>A third method of flow control, and the most reliable where it is
available, is the so-called <EM
CLASS="emphasis"
>out of band</EM
> approach. &quot;Out of band&quot;
simply means that users cannot accidentally simulate it by typing
control characters. Out of band control can be done in software,
using something similar to HP's ENQ/ACK and some encoding tricks,
but in practice, most UNIX machines that support any kind of out
of band flow control use something called either &quot;hardware flow
control&quot; or &quot;<EM
CLASS="emphasis"
>RTS/CTS</EM
> flow control.&quot; (This can be implemented
with no special hardware at all on many systems, so the latter name
is better.)</P
><P
CLASS="para"
><A
CLASS="indexterm"
NAME="AUTOID-45631"
></A
><A
CLASS="indexterm"
NAME="AUTOID-45633"
></A
>With RTS/CTS flow control, two existing serial-cable wires, RTS and
CTS, are &quot;taken over.&quot; (RTS and CTS-which stand for Request to
Send and Clear to Send respectively&nbsp;- were originally intended for use
with half duplex modems. Since half duplex modems are today merely
museum pieces, this is a sensible approach, but it does violate the
RS232 standard.) RTS at the terminal is cross-connected to CTS at the
host computer, and vice versa. The terminal and the computer both
assert RTS whenever they are ready to receive data, and wait for CTS
before sending. Unfortunately, not enough systems implement this, and
of those that do, many get it wrong.
[7]
Thus, while RTS/CTS flow
control offers the possibility of working perfectly, you cannot count
on it. Still, it is worth looking for an <EM
CLASS="emphasis"
>rts/cts</EM
> option in your
UNIX's <EM
CLASS="emphasis"
>stty</EM
>.</P
><BLOCKQUOTE
CLASS="footnote"
><P
CLASS="para"
>[7] For instance, on Sun workstations, RTS/CTS is supported in
hardware, but the particular Zilog chip that does this also uses
the DCD (Data Carrier Detect) line to control the receiver. Thus,
if you set <EM
CLASS="emphasis"
>stty crtscts</EM
>, you cannot tell a modem to dial out,
because DCD is off. It is possible to work around this, but only
with control over both the hardware and the UNIX kernel.</P
></BLOCKQUOTE
></DIV
><DIV
CLASS="sect2"
><H3
CLASS="sect2"
><A
CLASS="title"
NAME="UPT-ART-1140-SECT-1.5"
>41.2.5 Then What? </A
></H3
><P
CLASS="para"
>If you think this is complicated, just hope you never have to deal with
synchronous transmission, RS422, DIN connectors, lightning strike
protection, and many of the other hardware and electrical aspects that
surround computer communications. Getting two arbitrary computers to
talk to each other can be excessively difficult. Here again, standards
come to the rescue. If everything you have is proper RS232-
<SPAN
CLASS="link"
>modulo (<A
CLASS="linkend"
HREF="glossary.htm#UPT-ART-1010"
TITLE="Glossary"
>52.9</A
>)</SPAN
>,
perhaps, RTS/CTS flow control&nbsp;- and POSIX, things should usually go
smoothly.</P
></DIV
><DIV
CLASS="sect1info"
><P
CLASS="SECT1INFO"
>- <SPAN
CLASS="authorinitials"
>CT</SPAN
></P
></DIV
></DIV
><DIV
CLASS="htmlnav"
><P
></P
><HR
ALIGN="LEFT"
WIDTH="515"
TITLE="footer"><TABLE
WIDTH="515"
BORDER="0"
CELLSPACING="0"
CELLPADDING="0"
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="172"
><A
CLASS="SECT1"
HREF="ch41_01.htm"
TITLE="41.1 Delving a Little Deeper "
><IMG
SRC="gifs/txtpreva.gif"
SRC="gifs/txtpreva.gif"
ALT="Previous: 41.1 Delving a Little Deeper "
BORDER="0"></A
></TD
><TD
ALIGN="CENTER"
VALIGN="TOP"
WIDTH="171"
><A
CLASS="book"
HREF="index.htm"
TITLE="UNIX Power Tools"
><IMG
SRC="gifs/txthome.gif"
SRC="gifs/txthome.gif"
ALT="UNIX Power Tools"
BORDER="0"></A
></TD
><TD
ALIGN="RIGHT"
VALIGN="TOP"
WIDTH="172"
><A
CLASS="SECT1"
HREF="ch41_03.htm"
TITLE="41.3 Find Out Terminal Settings with stty "
><IMG
SRC="gifs/txtnexta.gif"
SRC="gifs/txtnexta.gif"
ALT="Next: 41.3 Find Out Terminal Settings with stty "
BORDER="0"></A
></TD
></TR
><TR
><TD
ALIGN="LEFT"
VALIGN="TOP"
WIDTH="172"
>41.1 Delving a Little Deeper </TD
><TD
ALIGN="CENTER"
VALIGN="TOP"
WIDTH="171"
><A
CLASS="index"
HREF="index/idx_0.htm"
TITLE="Book Index"
><IMG
SRC="gifs/index.gif"
SRC="gifs/index.gif"
ALT="Book Index"
BORDER="0"></A
></TD
><TD
ALIGN="RIGHT"
VALIGN="TOP"
WIDTH="172"
>41.3 Find Out Terminal Settings with stty </TD
></TR
></TABLE
><HR
ALIGN="LEFT"
WIDTH="515"
TITLE="footer"><IMG
SRC="gifs/smnavbar.gif"
SRC="gifs/smnavbar.gif"
USEMAP="#map"
BORDER="0"
ALT="The UNIX CD Bookshelf Navigation"><MAP
NAME="map"
><AREA
SHAPE="RECT"
COORDS="0,0,73,21"
HREF="../index.htm"
ALT="The UNIX CD Bookshelf"><AREA
SHAPE="RECT"
COORDS="74,0,163,21"
HREF="index.htm"
ALT="UNIX Power Tools"><AREA
SHAPE="RECT"
COORDS="164,0,257,21"
HREF="../unixnut/index.htm"
ALT="UNIX in a Nutshell"><AREA
SHAPE="RECT"
COORDS="258,0,321,21"
HREF="../vi/index.htm"
ALT="Learning the vi Editor"><AREA
SHAPE="RECT"
COORDS="322,0,378,21"
HREF="../sedawk/index.htm"
ALT="sed &amp; awk"><AREA
SHAPE="RECT"
COORDS="379,0,438,21"
HREF="../ksh/index.htm"
ALT="Learning the Korn Shell"><AREA
SHAPE="RECT"
COORDS="439,0,514,21"
HREF="../lrnunix/index.htm"
ALT="Learning the UNIX Operating System"></MAP
></DIV
></BODY
></HTML
>