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List:       linux-newbie
Subject:    Re: modules: aliases and configuration
From:       "Steven Smith" <sos22 () cam ! ac ! uk>
Date:       2003-11-09 12:10:04
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> So what is it with these "formalized" (for want of a better word) alias 
> names?  If I see an alias char-major-<n> (for some number <n>), what is 
> the significance of <n>?
When the kernel wants to load a driver, it usually doesn't know the
name of the driver (so, for instance, if it wants to start a network
interface, and the driver isn't currently loaded, it won't know
whether to load rtl8139 or 3c59x).  The standard names reflect what it
*does* know about the driver i.e. why it needs to be loaded.

On older Linux systems, before the introduction of devfs, devices were
identified by a major number, which specified the class of device
e.g. serial ports, and a minor number, which specified the specific
instance.  ``char-major-<n>'' is supposed to be an alias for the
driver which provides a character-by-character device with major
number <n>.

I'm not aware of any authoritative list of such aliases.  The ones
I've come across are:

char-major-<n> 		-- see above
char-major-<n>-<m> 	-- same, but request the specific minor number <m>
block-major-<n> 	-- block oriented major device <n>
block-major-<n>-<m> 	-- same, but request the specific minor number <m>
tty-ldisc-<n> 		-- tty line discipline number <n> (a tty is an
			   abstraction layer which can get put on top of
			   basically any bi-directional	character device)
scsi_hostadapter 	-- SCSI host adapter for the machine
sound-slot-<n> 		-- Sound card number <n>
sound-service-<n>-<m> 	-- service <m> provided by soundcard <n>
	(Service 0 == mixer, 1 == sequencer, 2 == midi, 3 == dsp)
binfmt-<n> 		-- binary (i.e. executable) format <n>.  The <n> is
			   a magic number from near the start of the file,
			   rather than anything	predictable.
iptable_<s> 		-- the iptables table which is called <s>
ipt_<s> 		-- the iptables target or match rule which is
			   called <s>

In addition, network interfaces are requested directly by name e.g.
the module responsible for eth0 is requested as eth0.

I'm sure there are lots of other entertaining special cases.

> Question 2) Module configuration info:
> If I look in my system's /etc/modules.conf it consists of 2 lines.
> Yet if I do 'modprobe -c' I get a couple of hundred lines.  The man page 
>  for modprobe says that 'modprobe -c' shows the "currently used 
> configuration (default + configuration file)".  So where does this 
> "default" come from?  Presumably it's established somewhere during the 
> kernel build, but where?
I think it's actually chosen when modprobe is built, rather than as
part of the kernel build.  I' haven't got a Linux machine handy to
check, though.

Steven Smith,
sos22@cam.ac.uk.

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