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<TITLE>Developer.com - Online Reference Library - 0672311739:RED HAT LINUX 2ND EDITION:Filesystems, Disks, and Other Devices</TITLE>
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<!-- ISBN=0672311739 //-->
<!-- TITLE=RED HAT LINUX 2ND EDITION //-->
<!-- AUTHOR=DAVID PITTS ET AL //-->
<!-- PUBLISHER=MACMILLAN //-->
<!-- IMPRINT=SAMS PUBLISHING //-->
<!-- PUBLICATION DATE=1998 //-->
<!-- CHAPTER=11 //-->
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<a href="0219-0221.html">Previous</A> | <a href="../ewtoc.html">Table of Contents</A> | <a href="0225-0227.html">Next</A>
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<A NAME="PAGENUM-222"><P>Page 222</P></A>
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TIP
</B></TD></TR>
<TR><TD>
<BLOCKQUOTE>
If you ever find that your terminal state has been messed up, then you can usually fix
this with the command stty sane and Ctrl+J. Note that the command is ended with
Ctrl+J, rather than Enter (which is the same as Ctrl+M). This is because the
icrnl option might have been turned off. This is fixed again with
stty sane. GNU bash will always cope with CRs that have not been converted to LF anyway, but some other programs won't.
<BR><BR>
If this still doesn't work, and the screen font appears to have been changed, try typing
echo, pressing Ctrl+V and Esc, typing c, and pressing Ctrl+J. You press Ctrl+V to make
the terminal driver pass the next key through without processing. You can get a similar effect
by typing reset and pressing Ctrl+J, but the program
reset is only available if the ncurses package is installed.
</BLOCKQUOTE></TD></TR>
</TABLE></CENTER>
</P>
<P>Programs can turn off the processing that the line driver does by default; the resulting
behavior (raw mode) allows programs to read unprocessed input from the terminal driver (for
example, CR is not mapped to LF), and control characters don't produce the signals described in
the table earlier in this section. The stty sane command will return things to
normal.
</P>
<H4><A NAME="ch11_ 25">
Serial Communications
</A></H4>
<P>Although the terminal interfaces used most commonly under Linux are the console driver
and the pseudo-terminals driven by programs like
xterm, script, and expect, the original terminal interface involved serial communications. In fact, this still lingers on; a
pseudo-tty associated with an xterm window still has an associated baud rate as shown in the example in the
section "The Terminal Device Driver." Changing this baud rate has no actual effect, though. For
real serial ports, however, the baud rate and many other parameters have a direct relevance.
The device nodes relating to the serial ports are composed of two "teams," with the names
/dev/cua* and /dev/ttyS*. These allow the same serial hardware to be used for both incoming
and outgoing serial connections, as explained later in the section "Using Modems."
</P>
<H4><A NAME="ch11_ 26">
Configuring the Serial Ports
</A></H4>
<P>Serial port configuration is mostly done either with the
stty command or directly by programs using the interface outlined in the
termios manual page. The stty command offers almost
all the configuration possibilities provided by
termios; however, there are configuration issues
for serial hardware that are not addressed by stty. The
setserial command allows the configuration of the correct IRQ settings for each serial port and of extra-fast baud rates that the
standard termios specification doesn't provide. For more detailed information, refer to the
Linux Serial-HOWTO and the manual page for
setserial.
</P>
<H4><A NAME="ch11_ 27">
Using Modems
</A></H4>
<P>Modems (other than "WinModems") are very flexible devices. They allow dial-up
terminal access and wide area networking, and they also often allow the sending and receiving of
faxes
</P>
<A NAME="PAGENUM-223"><P>Page 223</P></A>
<P>or voice-mail. The two "teams" of serial device nodes mentioned earlier are intended
specifically for good modem support, and they look like this:
</P>
<!-- CODE //-->
<PRE>
$ ls -l /dev/ttyS* /dev/cua* /dev/mo*
crw-rw---- 1 root uucp 5, 64 Jan 1 1980 /dev/cua0
crw-rw---- 1 root uucp 5, 65 Jan 1 1980 /dev/cua1
crw-rw---- 1 root uucp 5, 66 Jan 1 1980 /dev/cua2
crw-rw---- 1 root uucp 5, 67 Jan 1 1980 /dev/cua3
lrwxrwxrwx 1 root uucp 4 Jun 7 14:16 /dev/modem -> cua2
lrwxrwxrwx 1 root root 10 Sep 6 1996 /dev/mouse -> /dev/ttyS0
crw-r--r-- 1 root root 4, 64 Nov 1 08:56 /dev/ttyS0
crw-r--r-- 1 root root 4, 65 May 29 1995 /dev/ttyS1
crw-r--r-- 1 root root 4, 66 Jan 1 1980 /dev/ttyS2
crw-r--r-- 1 root root 4, 67 Jan 1 1980 /dev/ttyS3
</PRE>
<!-- END CODE //-->
<P>The /dev/cua* devices are intended for use for dialing out, and the
/dev/ttyS* devices are intended for dialing in. A process such as
getty can be waiting for an open system call to
complete (that is, waiting for the modem to signal that there is an incoming call) on the
/dev/ttyS0 device while you are opening the corresponding device
/dev/cua0, which refers to the same hardware. Because there can't be an incoming call on that line while you're using the
modem, getty continues to wait. Lock files keep everyone from trying to use the same device at
the same time.
</P>
<P>These days, internetworking is the most common use for modems by far. There are many
resources that will help you set up your machine for Internet access via a modem. See the
section "Connecting to the Net with PPP" in Chapter 13. Other resources include the Red Hat
PPP-Tips document, which can be found in the support section of Red Hat's Web site, the
EzPPP package, and the ISP-HOOKUP and PPP HOWTOS. There are also a large number of
useful mini-HOWTOs that each help with more specific things; these include the
Diald, Dip+SLiRP+CSLIP, Dynamic-IP-Hacks, IP-Masquerade, PPP-over-ISDN,
PPP-over-minicom, SLIP+proxyARP, and Tiny-News Linux mini-HOWTOs. If, on the other
hand, you send and receive e-mail by UUCP rather than Internet protocols, you should start by
reading the Linux UUCP HOWTO and the Linux Sendmail+UUCP
mini-HOWTO.
</P>
<H4><A NAME="ch11_ 28">
Generic SCSI Devices
</A></H4>
<P>Not all SCSI devices are hard disks, CD-ROM drives, or tape drives. Some are optical
scanners, CD-ROM recorders, or even electron microscopes. The kernel can't possibly abstract
the interfaces for all possible SCSI devices, so it gives user programs direct access to SCSI
hardware via the generic SCSI devices. These enable programs to send arbitrary SCSI commands to
hardware. Although this offers the opportunity of wreaking havoc by mistake, it also offers the
capability of driving all sorts of interesting hardware, of which the principal examples are
CD-ROM recorders. The SCSI device nodes all have names starting with
/dev/sg. SCSI commands are sent to the devices by writing data to the device, and the results are read back
by reading from the device.
</P>
<A NAME="PAGENUM-224"><P>Page 224</P></A>
<H3><A NAME="ch11_ 29">
CD-ROM Recorders
</A></H3>
<P>CD-ROM recorders are devices for recording data on special media that can be read in
ordinary CD-ROM drives. There are two stages in the writing of a CD: the generation of the
CD image and the writing of that image to the media.
</P>
<P>The surface of a CD-R (recordable CD) is only writable once, so if
mkisofs worked like the other mkfs tools it would always generate image files representing empty CDs. For this
reason, mkisofs populates the filesystem with files as it generates the image file.
</P>
<P>The CD image file is produced by the mkisofs program, which generates the structures for
an ISO9660 filesystem and populates it with the files from a directory tree. CDs are not
writable in the same sense as block devices; this is why they are not
actually block devices. The image file must be written to the CD-R with a specialized program,
cdwrite, which understands all the various proprietary schemes used for driving CD writers. All the CD writers supported
by Linux (as of version 2.0.30 of the kernel) are SCSI devices, so the kernel accommodates this
by providing access to the generic SCSI device interface that enables a program to send SCSI
commands to these devices.
</P>
<P>While burning (writing) a CD, it is usually important that the flow of data to the writer
keeps up with the speed at which the writer is going; otherwise, if the writer runs out of data to
write, the CD-R is ruined. For this reason, it is usual to use
mkisofs to generate an image file and then separately use
cdwrite to write this image file to the CD writer.
</P>
<P>It is possible to use a pipe to send the data from
mkisofs directly to cdwrite. This often works either because a fast machine can ensure that
mkisofs supplies the data fast enough to keep the CD writer busy or because the CD writer is not sensitive to data underruns. (Some of the
more expensive ones have internal hard disks to which the data is written as an intermediate
stage.) This technique is not recommended, however, because the generation of the intermediate
image file has other benefits; it enables you to test your CD image before the final writing of the
data takes place.
</P>
<H4><A NAME="ch11_ 30">
Testing CD Images
</A></H4>
<P>Just as you can use mkfs to create a filesystem inside an ordinary file, you can mount
filesystems contained in ordinary files by using the loopback device driver described previously. The
first example of mounting a loopback filesystem is a demonstration of how you can test a CD
image.
</P>
<H4><A NAME="ch11_ 31">
Other Character Devices
</A></H4>
<P>There are several other varieties of character devices, some which are used frequently. For
example, /dev/null is used very frequently.
</P>
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