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 should: + 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.