1 | <?xml version="1.0" encoding="ISO-8859-1"?>
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2 | <!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.3//EN" "http://www.oasis-open.org/docbook/xml/4.3/docbookx.dtd" [
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3 | <!ENTITY % general-entities SYSTEM "../general.ent">
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4 | %general-entities;
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5 | ]>
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6 | <sect1 id="ch-system-kernel-headers">
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7 | <title>Linux-&linux-version; headers</title>
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8 | <?dbhtml filename="kernelheaders.html"?>
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9 |
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10 | <indexterm zone="ch-system-kernel-headers">
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11 | <primary sortas="a-Linux">Linux</primary>
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12 | <secondary>system, headers</secondary></indexterm>
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13 |
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14 | <screen>&buildtime; 0.1 SBU
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15 | &diskspace; 186 MB</screen>
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16 |
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17 |
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18 | <sect2>
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19 | <title>Installation of the kernel headers</title>
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20 |
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21 | <para>We won't be compiling a new kernel yet -- we'll do that when we have
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22 | finished the installation of all the packages. But the libraries installed in
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23 | the next section need to refer to the kernel header files in order to know how
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24 | to interface with the kernel. Instead of unpacking the kernel sources again,
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25 | making the version file and the symlinks and so on, we will simply copy the
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26 | headers from the temporary tools directory in one swoop:</para>
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27 |
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28 | <screen><userinput>cp -a /tools/include/{asm,asm-generic,linux} /usr/include</userinput></screen>
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29 |
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30 | <para>A few kernel header files refer to the <filename>autoconf.h</filename>
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31 | header file. Since we have not yet configured the kernel, we need to create
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32 | this file ourselves in order to avoid a compilation failure of Sysklogd.
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33 | Create an empty <filename>autoconf.h</filename> file with:</para>
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34 |
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35 | <screen><userinput>touch /usr/include/linux/autoconf.h</userinput></screen>
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36 |
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37 | </sect2>
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38 |
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39 |
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40 | <sect2>
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41 | <title>Why we copy the kernel headers</title>
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42 |
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43 | <para>In the past it was common practice to symlink the
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44 | <filename class="directory">/usr/include/{linux,asm}</filename> directories
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45 | to <filename class="directory">/usr/src/linux/include/{linux,asm}</filename>.
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46 | This was a <emphasis>bad</emphasis> practice, as the following extract from a
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47 | post by Linus Torvalds to the Linux Kernel Mailing List points out:</para>
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48 |
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49 | <screen>I would suggest that people who compile new kernels should:
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50 |
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51 | - not have a single symbolic link in sight (except the one that the
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52 | kernel build itself sets up, namely the <quote>linux/include/asm</quote>
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53 | symlink that is only used for the internal kernel compile itself)
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54 |
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55 | And yes, this is what I do. My /usr/src/linux still has the old 2.2.13
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56 | header files, even though I haven't run a 2.2.13 kernel in a _loong_
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57 | time. But those headers were what Glibc was compiled against, so those
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58 | headers are what matches the library object files.
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59 |
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60 | And this is actually what has been the suggested environment for at
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61 | least the last five years. I don't know why the symlink business keeps
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62 | on living on, like a bad zombie. Pretty much every distribution still
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63 | has that broken symlink, and people still remember that the linux
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64 | sources should go into <quote>/usr/src/linux</quote> even though that hasn't
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65 | been true in a _loong_ time.</screen>
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66 |
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67 | <para>The essential part is where Linus states that the header files should be
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68 | <emphasis>the ones which Glibc was compiled against</emphasis>. These are
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69 | the headers that should be used when you later compile other packages, as they
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70 | are the ones that match the object-code library files. By copying the headers,
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71 | we ensure that they remain available if later you upgrade your kernel.</para>
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72 |
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73 | <para>Note, by the way, that it is perfectly all right to have the kernel sources
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74 | in <filename class="directory">/usr/src/linux</filename>, as long as you don't
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75 | have the <filename class="directory">/usr/include/{linux,asm}</filename>
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76 | symlinks.</para>
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77 |
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78 | </sect2>
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79 |
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80 | </sect1>
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