lfs-book/chapter06/makedev.xml

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<sect1 id="ch-system-MAKEDEV" xreflabel="Make_devices">
<title>Creating devices with Make_devices-&makedev-version;</title>
<?dbhtml filename="makedevices.html"?>

<indexterm zone="ch-system-MAKEDEV"><primary sortas="a-Make_devices">Make_devices</primary></indexterm>

<para>The Make_devices package contains a script for creating device
nodes.</para>

<screen>&buildtime; 1 SBU
&diskspace; 160 KB</screen>

<para>For its installation Make_devices depends on: Bash, Bzip2,
Coreutils.</para>



<sect2>
<title>Making devices</title>

<para>Note that unpacking the
<filename>make_devices-&makedev-version;.bz2</filename> file doesn't
create a directory for you to <command>cd</command> into, as the file
contains only a shell script.</para>

<para>Install the <filename>make_devices</filename> script:</para>

<screen><userinput>bzcat make_devices-&makedev-version;.bz2 &gt; /dev/make_devices
chmod 754 /dev/make_devices</userinput></screen>

<para>Device nodes are special files: things that can generate or receive data.
They usually correspond to physical pieces of hardware. Device nodes can be
created by issuing commands of the form: <command>mknod -m mode name type major
minor</command>. In such a command, <emphasis>mode</emphasis> is the usual
octal read/write/execute permissions triplet, and <emphasis>name</emphasis> is
the name of the device file to be created. It may seem surprising, but the
device name is actually arbitrary, except that most programs rely on devices
such as <filename>/dev/null</filename> having their usual names. The remaining
three parameters tell the kernel what device the node
actually refers to. The <emphasis>type</emphasis> is a letter, either b or c,
indicating whether the device is accessed in blocks (such as a hard disk) or
character by character (such as the console). And <emphasis>major</emphasis>
and <emphasis>minor</emphasis> are numbers, together forming a code that
identifies the device to the kernel. A list of the currently assigned device
numbers for Linux can be found in the file <filename>devices.txt</filename> in
the <filename class="directory">Documentation</filename> subdirectory of the
kernel sources.</para>

<para>Note that the same major/minor combination is usually assigned to both a
block and a character device. These are, however, completely unrelated devices
that cannot be interchanged. A device is identified by the type/major/minor
triple, not just the major/minor pair, so when creating a device node it is
important to choose the correct <emphasis>type</emphasis> of device.</para>

<para>Because looking up the type/major/minor triples and using
<command>mknod</command> manually is tedious and error-prone, the
<filename>make_devices</filename> script has been created. It contains a whole
series of <command>mknod</command> commands, one for each device, complete with
recommended name, permissions and group assignment. It has been set up so that
only a minimal set of commonly used devices is enabled and the other lines are
commented out. You should open <filename>make_devices</filename> in an editor
and customize it to your needs. This takes some time, but is very simple. When
you are satisfied, run the script to create the device files:</para>

<screen><userinput>cd /dev
./make_devices</userinput></screen>

<para>If you had success with mounting the devpts file system earlier in 
<xref linkend="ch-system-proc"/>, you can continue with the next section. If you were
unable to mount devpts, you will have to create a few static ptyXX and ttyXX
device nodes instead. To do this, open <filename>make_devices</filename> in
your editor, go to the section <quote>Pseudo-TTY masters</quote> and enable a
few ptyXX devices -- a handful are enough to enable the test suites to run, but
if you plan to run a kernel without devpts support you will probably need many
more (every xterm, ssh connection, telnet connection, and the like, uses one of
these pseudo terminals). In the immediately following section <quote>Pseudo-TTY
slaves</quote>, enable the corresponding ttyXX devices. When you are done, rerun
<command>./make_devices</command> from inside <filename>/dev</filename> to
have it create the new devices.</para>

</sect2>


<sect2 id="contents-makedev"><title>Contents of Make_devices</title>

<para><emphasis>Installed script</emphasis>: make_devices</para>

</sect2>

<sect2><title>Short description</title>

<indexterm zone="ch-system-MAKEDEV make_devices"><primary sortas="d-make_devices">make_devices</primary></indexterm>
<para id="make_devices"><command>make_devices</command> is a script for creating a basic set of
static device nodes, usually residing in the <filename class="directory">/dev</filename> directory.</para>

</sect2>



</sect1>