| 1 | <sect2>
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| 2 | <title>Why we copy the kernel headers and don't symlink them</title>
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| 3 |
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| 4 | <para>In the past it was common practice to symlink the
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| 5 | <filename class="directory">/usr/include/{linux,asm}</filename> directories
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| 6 | to <filename class="directory">/usr/src/linux/include/{linux,asm}</filename>.
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| 7 | This was a <emphasis>bad</emphasis> practice, as the following extract from a
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| 8 | post by Linus Torvalds to the Linux Kernel Mailing List points out:</para>
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| 9 |
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| 10 | <screen>I would suggest that people who compile new kernels should:
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| 11 |
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| 12 | - not have a single symbolic link in sight (except the one that the
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| 13 | kernel build itself sets up, namely the "linux/include/asm" symlink
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| 14 | that is only used for the internal kernel compile itself)
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| 15 |
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| 16 | And yes, this is what I do. My /usr/src/linux still has the old 2.2.13
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| 17 | header files, even though I haven't run a 2.2.13 kernel in a _loong_
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| 18 | time. But those headers were what glibc was compiled against, so those
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| 19 | headers are what matches the library object files.
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| 20 |
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| 21 | And this is actually what has been the suggested environment for at
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| 22 | least the last five years. I don't know why the symlink business keeps
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| 23 | on living on, like a bad zombie. Pretty much every distribution still
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| 24 | has that broken symlink, and people still remember that the linux
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| 25 | sources should go into "/usr/src/linux" even though that hasn't been
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| 26 | true in a _loong_ time.</screen>
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| 27 |
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| 28 | <para>The essential part is where Linus states that the header files should be
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| 29 | <emphasis>the ones which glibc was compiled against</emphasis>. These are
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| 30 | the headers that should be used when you later compile other packages, as they
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| 31 | are the ones that match the object-code library files. By copying the headers,
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| 32 | we ensure that they remain available if later you upgrade your kernel.</para>
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| 33 |
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| 34 | <para>Note, by the way, that it is perfectly all right to have the kernel sources
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| 35 | in <filename class="directory">/usr/src/linux</filename>, as long as you don't
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| 36 | have the <filename class="directory">/usr/include/{linux,asm}</filename>
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| 37 | symlinks.</para>
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| 38 |
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| 39 | </sect2>
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| 40 |
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