Re: 2.0.33 is no good for lic6-dev ?
Hi,
Please read /usr/doc/kernel-headers-2.0.32/debian.README.gz. I
have included a copy below for anyone who thinks reading stuff out of
/usr/doc is way square and uncool.
manoj
--
Sink or swim, live or die, survive or perish, I give my heart and my
hand to this vote. -- Daniel Webster
Manoj Srivastava <srivasta@acm.org> <http://www.datasync.com/%7Esrivasta/>
Key C7261095 fingerprint = CB D9 F4 12 68 07 E4 05 CC 2D 27 12 1D F5 E8 6E
$Id: README.headers,v 1.4 1998/01/30 01:26:20 srivasta Exp $
This is the Debian GNU/Linux prepackaged version of the Linux kernel
headers. Linux was written by Linus Torvalds
<Linus.Torvalds@cs.Helsinki.FI> and others.
This package was put together by Simon Shapiro <Shimon@i-Connect.Net>, from
sources retrieved from directories under
ftp.cs.helsinki.fi:/pub/Software/Linux/Kernel/
This package contains the Linux kernel header files.
Kernel Headers and libc6-dev package
____________________________________
====================================
Need for kernel include files
==== === ====== ======= =====
Even though GNU libc 2.0 (a.k.a. libc6) provides an uniform
interface to C programmers, one should realize that it needs
different underpinnings on different architectures and operating
systems (remember, glibc2 is multi-OS).
glibc provides all the standard files that the C standard and
POSIX require, and those in turn call in OS and platform specific
headers as required transparently to the user. There is an a complete
divorce of the kernel-level interface from the user-level interface:
the application programmer does not need to know kernel level details
at all.
But this has been taken by some to mean that
/usr/include/{linux,asm} would be superfluous, which is a technical
impossibility given that glibc2 is not an architecture and OS
specific library.
I do not believe it is easy for glibc to present an interface
that does not match the underlying OS, and quite possibly people just
punted. If there is a mismatch between the user level structures and
the kernel level structures, then libc6 library shall have to install
translating wrappers around system calls (not such a great idea for
high performance systems). I can foresee cases where it would not be
possible to implement these wrappers, given a sufficiently large set
of architectures and OS's.
In the case of Linux, the kernel header files are the
underpinnings of the architecture independent interface.
Take a simple general ANSI C include file like <errno.h>. This
in turn includes /usr/include/errnos.h, which includes
/usr/include/linux/errno.h, which in turn includes
/usr/include/asm/errno.h. See? A simple, standard include file like
<errno.h>, and one needs kernel include files for that.
Traditional two symlink approach
=========== === ======= ========
Under libc5, it was standard for part of the user interface to
libc to be exported from the kernel includes, via /usr/include/linux
and /usr/include/asm. Traditionally, this was done by linking those
two directories to the appropriate directories in
/usr/src/linux/include. This is the method documented in the install
instructions for the kernel sources, even today.
Why that is bad
=== ==== == ===
Kernel headers no longer make sense exporting to user space
(in early days of Linux, that was not true). It is beginning to get
harder to synchronize the libc and the kernel headers as in the old
days; now linking with the latest kernel headers may subtly break new
code since the headers linked with are different from the compiled
library. In addition, the specter of programs breaking with new
kernel headers was preventing needed new features from being added to
the kernel (and damping innovative experimentation in kernel
development) (see appendix A for details).
Besides, the kernel itself no longer needs /usr/include/linux/*
at all, so keeping the libc and kernel headers the same aren't
needed for kernel development.
The headers were included in Debian's libc5-dev after a rash
of very buggy alpha kernel releases (1.3.7* or something like that)
that proceeded to break compilations, etc. Kernel versions are
changed far more rapidly than libc is, and there are higher chances
that people install a custom kernel than they install custom libc.
Add to that the fact that few programs really need the more
volatile elements of the header files (that is, things that really
change from kernel version to kernel version), [before you reject
this, consider: programs compiled on one kernel version usually work
on other kernels]. For the few that do need specific kernel headers,
use -I/usr/src/kernel-headers-<version> or some thing for a specific
kernel version, or -I/usr/src/linux/include for the latest set of
headers installed..
Most programs, even if they include <linux/something.h>, do
not really depend on the version of the kernel, as long as the kernel
versions are not too far off, they will work. And the headers
provided in libc5-dev (and libc6-dev) are just that.
The solution is to separate out the two sets of header files:
the kernel uses one frequently changing set, and a set of headers is
provided from a known good kernel version, which is tested, known to
be stable, and that is sufficient for compiling most programs, (it
also makes the compile time environments for programs on Debian
machines a well known one, easing the process of dealing with problem
reports).
Debian's libc5 method
======== ===== ======
It was decided, then, to have libc5 development package include
files found one specific kernel-sources to insulate most systems from
the vagaries of bleeding edge kernel sources, and to prevent subtle
breakage introduced by the difference in the headers a program is
compiled versus headers the libc was compiled with. As mentioned
before, most programs are not coupled tightly enough to kernel data
structures, and would continue to work even if the user upgrades the
kernel (and presumably the libc development package is not allowed to
lag too far behind current kernels).
There are two different capabilities which are the issue, and
the kernel-packages and libc5-dev address different ones:
a) The kernel packages try to provide a stable, well behaved kernel
and modules, and may be upgraded whenever there are significant
advances in those directions (bug fixes, more/better module
support, etc). These, however, may not have include files that
are non-broken as far as non-kernel programs are concerned, and
the quality of the development/compilation environment is not the
kernel packages priority (Also, please note that the kernel
packages are tied together, so kernel-source, headers, and image
are produced in sync)
b) Quality of the development/compilation environment is the priority
of libc development package, and it tries to ensure that the headers it
provides would be stable and not break non-kernel programs. This
assertion may fail for alpha kernels, which may otherwise be
perfectly stable, hence the need for a different set of known-good
kernel include files.
Shortcoming of the libc5 method
=========== == === ===== ======
Unfortunately, the proposal for libc5 is beginning to unravel
at the edges, since Debian is going truly multi-architecture (and there
are murmurs of multi-OS Debian).
The reason I say that the solution is unraveling at the edges
is that the kernel header files are getting to be quite architecture
dependent. We still want to make libc6-dev include headers from a
well known stable kernel source. But there is no one set of headers
to install, since different architectures need different (sometimes
radically different) files.
If libc development packages continued to include kernel
headers explicitly, we would need different headers for different
architectures, resulting in libc6-dev-i386, libc6-dev-m68k, et. al.
Or we could maintain all the files in libc6-dev as a set of (quite
large) arch dependent patches. But those files are precisely the
files contained in kernel-headers-2.0.32.
Even in the case of more than one architecture sharing a
common kernel-header file set, all I say is that this invariant is
not guaranteed for any future releases. Kernel headers need not be the
same for any two given architectures. Any solution would do well to
address that.
However, the following still remains true: the libc
development packages should include a static, stable, tested, known
good kernel headers to insulate developers from the vagaries of
unstable kernel headers (and the subtler pitfall of compiling with
headers that do not match the libc being linked with).
Debian's libc6 method
======== ===== ======
The current solution is a variant of the original, bad,
symlink solution. The variation is that we link to header files from
a specific kernel version, namely 2.0.32, which are the headers that
libc was compiled with. Whereas we used to link to any old kernel in
the original libc5 symlink, the new libc6 symlink is to
/usr/src/linux-2.0.32, which is a symbolic link that holds headers
from a *static, well known*, *supported*, tested kernel version, and
we let the kernel-headers package handle architecture dependencies
(which it had to anyway).
One should also consider that the libc maintainer has enough
on their plates, and should not be expected to be responsible for the
creation and testing of the arch dependent diffs.
Coupled with the fact that the afore-mentioned diffs are
exactly what is there in kernel-headers package, it exists already,
and requires no work on the part of the libc maintainer.
So, a dependency was created on kernel headers rather than
create kernel header sized architecture dependent diffs. The
dependency is such that it libc6-dev links to the directory
/usr/src/linux-2.0.32. This link is provided by just the packages
kernel-source-2.0.32 or kernel-headers-2.0.32, and *no other* kernel
package! So, libc6-dev depends can now provide 2.0.32 kernel headers
*for all architectures*, with no messy architecture dependent
patches, we always have fixed, static, known good headers in
/usr/include/{linux,asm}, and this is goodness.
This is a working technical solution to having libc
development package contain/depend on a well known static set of
kernel headers (insulating the vast majority of programs that are not
closely tied to kernel version specific internal data structures),
while allowing the kernel headers to vary from architecture to
architecture, and still allowing device driver authors from having
any set of kernel headers they want on the machine through the simple
artifice of adding a -I flag to the compilation flags.
Indeed, had we thought of the current solution in libc5
days it would have been cleaner (and produced less confusion now).
The solution of having libc5-dev depend on kernel-headers-X.XX.XX
wold have worked just as well in the libc5 case as it is in the libc6
case. I don't see a change in the situation, just a switch to an
equivalent, possibly cleaner, solution. If we had had libc5-dev
depend on kernel-headers, then there wouldn't be the argument that
there is now.
Authors response to complaints that Debian is doing it's own thing
======= ======== == ========== ==== ====== == ===== ==== === =====
Yes, Debian is different. It has a package management
system. Different part of Debian work with each other. There are
assumptions part of the distribution make about itself, and because
of this set of co-operating assumptions, or rules, or policy, Debian
is better integrated than most Linux distributions I have seen.
We often do not do things the "standard" way. Like when we
started including /usr/include/{linux,asm} directories instead of
having symlinks. It is different. And was acknowledged to be
technically superior.
Again, we are making changes that shall not be reflected in
other distributions. I think we are making the technically superior
choice. But we shall be different again.
I think this solution works, and is an elegant solution.
______________________________________________________________________
Appendix A
======== =
This document contains comments from Linus Torvalds (made in
an ``off-the-cuff'' personal email) to help clarify the rationale
behind the Debian way of handling symlinks, but this should not be
seen as an official policy statement by Linus. I'm attaching a
disclaimer in his own words.
The only reason that Linus's message is quoted in here is that
he can explain the technical reasons with far more lucidity than I
can, and now that I have permission to include his mail, I am
removing most of my far less facile efforts in that regard.
Need for isolating the C development Library from volatile
kernel headers
----------------------------------------------------------------------------
>> "David" == David Engel <david@sw.ods.com> said on Mon, 24 Feb 1997
>> "Linus" == Linus Torvalds said on Mon, 24 Feb 1997
David> Hi Linus,
David> No matter how well we try to explain ourselves, the symlinks issue
David> keeps coming up. Would you mind if we used your message below in
David> our responses?
Linus> Sure. Don't make it "the word of God" - please point out that
Linus> it was a off-the-bat personal reply to a question concerning
Linus> this, and while I'm more than happy to have the email
Linus> circulated it shouldn't be seen as a "official" document in any
Linus> way..
Linus> Linus
---------------------------------------------------------------------------
>> "Linus" == Linus Torvalds said on Wed, 22 Jan 1997:
Linus> The kernel headers used to make sense exporting to user space,
Linus> but the user space thing has grown so much that it's really not
Linus> practical any more. The problem with Debian is just that they
Linus> are different, not that they are doing anything wrong. That
Linus> leads to differences between the distributions, and that in
Linus> turn obviously can result in subtle problems.
Linus> As of glibc, the kernel headers will really be _kernel_
Linus> headers, and user level includes are user level
Linus> includes. Matthias Ulrich did that partly because I've asked
Linus> him to, but mainly just because it is no longer possible to try
Linus> to synchronize the libc and the kernel the way it used to
Linus> be. The symlinks have been a bad idea for at least a year now,
Linus> and the problem is just how to get rid of them
Linus> gracefully. Personally, I'm counting on glibc, which we are
Linus> already using on alpha.
Linus> Just to give you some idea of exactly why the includes really
Linus> can't be handled by simple symlinks: the main problem is
Linus> version skew. Lots of people want to upgrade their library
Linus> without affecting the kernel, and probably even more people
Linus> want to be able to upgrade their kernel without affecting their
Linus> compilation environment. Right now doing that has been
Linus> extremely fragile.
Linus> Just to give _one_ example of why the symlinks are bad: NR_OPEN
Linus> and "fd_set". I have had no end of problems making NR_OPEN
Linus> larger in the kernel, exactly _because_ of the damn
Linus> sym-links. If I just make NR_OPEN larger (the right thing to
Linus> do), the problem is that people with old libraries will now
Linus> compile against a header file that doesn't match the library
Linus> any more. And when the library internally uses another NR_OPEN
Linus> than the new program does, "interesting" things happen.
Linus> In contrast, with separate header files, this doesn't make any
Linus> difference. If I change NR_OPEN in the kernel, the compilation
Linus> environment won't notice UNTIL the library and associated
Linus> header files are changed: thus the user will continue to compile
Linus> with the old values, but because we'll still be binary
Linus> compatible, the worst thing that happens is that new programs
Linus> won't take advantage of new features unless the developer has
Linus> upgraded his library. Compare that to breaking subtly.
Linus> NR_OPEN is just _one_ example, and actually it's one of the
Linus> easier ones to handle (because the only thing that really makes
Linus> much of a difference when it comes to NR_OPEN is the fd_set
Linus> size - but it certainly bit some people). Another major problem
Linus> is name-space pollution: the POSIX/ANSI/XOpen rules are not
Linus> only complex, but they are actually contradictory too. And the
Linus> kernel header files really can't reasonably support all of the
Linus> intricacies very cleanly.
Linus> One specific example of why we want separate header files for
Linus> libraries and kernel is offered by glibc: Matthias wanted to
Linus> have a "sigset_t" that will suffice for the future when the
Linus> POSIX.1b realtime signals are implemented. But at the same time
Linus> he obviously wants to be able to support programming on
Linus> Linux-2.0 and the current 2.1 that do not have that support.
Linus> The _only_ reasonably clean way to handle these kinds of
Linus> problems is to have separate header files: user programs see a
Linus> larger sigset_t, and then the library interaction with the
Linus> kernel doesn't necessarily use all of the bits, for
Linus> example. Then later, when the kernel support is actually there,
Linus> it's just a matter of getting a new shared library, and voila,
Linus> all the realtime signals work.
Linus> The symlink approach simply wouldn't work for the above: that
Linus> would have required everybody who uses the library to have a
Linus> recent enough kernel that whatever magic all the above entails
Linus> would be available in the kernel header files. But not only
Linus> don't I want to pollute the kernel header files with user level
Linus> decisions, it's actually possible that somebody wants to run
Linus> glibc on a 1.2.x kernel, for example. We _definitely_ do not
Linus> want him to get a 32-bit sigset_t just because he is happy with
Linus> an old kernel.
Linus> Anyway, this email got longer than intended, but I just wanted
Linus> to make clear that the symlinks will eventually be going away
Linus> even in non-Debian distributions. Debian just happened to do it
Linus> first - probably because Debian seems to be more interested in
Linus> technical reasons than any old traditions. And technically, the
Linus> symlinks really aren't very good.
Linus> The _only_ reason for the symlinks is to immediately give
Linus> access to new features in the kernel when those happen. New
Linus> ioctl numbers etc etc. That was an overriding concern early on:
Linus> the kernel interfaces expanded so rapidly even in "normal"
Linus> areas that having the synchronization that symlinks offered was
Linus> a good thing.
Linus> However, the kernel interfaces aren't really supposed to change
Linus> all that quickly any more, and more importantly: the technical
Linus> users know how to fix things any way they want, so if they want
Linus> a new ioctl number to show up they can actually edit the header
Linus> files themselves, for example. But having separation is good
Linus> for the non-technical user, because there are less surprises
Linus> and package dependencies.
Linus> Anyway, something like the patch that David suggested will
Linus> certainly go in, although I suspect I'll wait for it to become
Linus> "standard" and the glibc first real release to take place.
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