Correction for article 240

Here is an explanation why it is no longer recommended to create links from /usr/include to the kernel header files:

Symlinks In The Kernel; Kernel/Library/etc Interface Dispute

27 Jul 2000 - 3 Aug 2000 (185 posts) Archive Link: "RLIM_INFINITY
inconsistency between archs" Boszormenyi Zoltan had some trouble
compiling the latest 'egcs' snapshots on a Linux 2.4.0 system, and
traced the problem to the fact that "/usr/include/asm is a symlink
to /usr/src/linux/include/asm, as in the original distribution but
/usr/src/linux is a 2.4.0-testX tree. With a 2.2.X source tree, it does
not produce any warning."  Linus Torvalds replied:     I've asked glibc
maintainers to stop the symlink insanity for the last few years now,
but it doesn't seem to happen.

    Basically, that symlink should not be a symlink. It's a symlink for
    historical reasons, none of them very good any more (and haven't
    been for a long time), and it's a disaster unless you want to be a
    C library developer. Which not very many people want to be.

    The fact is, that the header files should match the library you link
    against, not the kernel you run on.

    Think about it a bit.. Imagine that the kernel introduces a new
    "struct X", and maintains binary backwards compatibility by having
    an old system call in the old place that gets passed a pointer to
    "struct old_X". It's all compatible, because binaries compiled for
    the old kernel will still continue to run - they'll use the same
    old interfaces they are still used to, and they obviously do not
    know about the new ones.

    Now, if you start mixing a new kernel header file with an old binary
    "glibc", you get into trouble. The new kernel header file will use
    the _new_ "struct X", because it will assume that anybody compiling
    against it is after the new-and-improved interfaces that the new
    kernel provides.

    But then you link that program (with the new "struct X") to the binary
    library object archives that were compiled with the old header files,
    that use the old "struct old_X" (which _used_ to be X), and that use
    the old system call entry-points that have the compatibility stuff
    to take "struct old_X".

    Boom! Do you see the disconnect?  In short, the _only_ people
    who should update their /usr/include/linux tree are the people who
    actually make library releases and compile their own glibc, because if
    they want to take advantaged of new kernel features they need those
    new definitions. That way there is never any conflict between the
    library and the headers, and you never get warnings like the above..

He went on:     I would suggest that people who compile new kernels

      + NOT do so in /usr/src. Leave whatever kernel (probably only
      the header
        files) that the distribution came with there, but don't touch it.
      + compile the kernel in their own home directory, as their very own
        selves. No need to be root to compile the kernel. You need to
        be root to _install_ the kernel, but that's different.
      + not have a single symbolic link in sight (except the one that the
        kernel build itself sets up, namely the "linux/include/asm"
        symlink that is only used for the internal kernel compile itself)

    And yes, this is what I do. My /usr/src/linux still has the old
    2.2.13 header files, even though I haven't run a 2.2.13 kernel in a
    _loong_ time.  But those headers were what glibc was compiled against,
    so those headers are what matches the library object files.

    And this is actually what has been the suggested environment for
    at least the last five years. I don't know why the symlink business
    keeps on living on, like a bad zombie. Pretty much every distribution
    still has that broken symlink, and people still remember that the
    linux sources should go into "/ usr/src/linux" even though that
    hasn't been true in a _loong_ time.

    Is there some documentation file that I've not updated and that
    people are slavishly following outdated information in? I don't read
    the documentation myself, so I'd never notice ;)

Mike A. Harris commended, "I very much like the idea of what you describe
below however as it solves NUMEROUS problems indeed. This information
should be put in the top level README file, and emphasis put on the
'dont compile in /usr/ local' part, because it would sure save people a
lot of headaches IMHO." Also in reply to Linus, Kai Henningsen pointed
out that in Debian at least, "/usr/ include/asm is a directory, and its
contents come with the libc6-dev package." In reply to Linus' question
about misleading docs that might be floating around, several folks piped
up. Jeff Lightfoot pointed out that a ton of files in the 'Documentation'
directory referenced '/usr/src/linux', and James Lewis Nance and André
Dahlqvist independently posted patchs to clean that up in the main
README. Adam Sampson added that the 'glibc' installation instructions
had similar problems, and Kai added that in the Linux sources, the
problem existed in "Lots of places, actually. 'find -type f | xargs grep
/usr/include' and shudder." Also in reply to Linus, Theodore Y. Ts'o
suggested having /usr/src/linux be a symlink to the header files of
whatever kernel booted by default. Since only root could actually install
a kernel (even though any user could do the compilation themselves),
the question of where the link should point would always be clear. He
explained, "The problem is that unless you are trying to say that you
want to outlaw external source packages which generate kernel modules,
there needs to be some way for such packages to be able to find the kernel
header files." But Linus replied that this would force kernel header files
to maintain source-level backward compatibility forever, which would cause
big problems. In terms of how external packages could find header files,
Linus replied:     By hand. By the maintainer.  And _independently_
of what random user Joe Blow has on his particular installation.