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<H1><A NAME="SECTION00410000000000000000"> </A><A NAME="kernel-ver"> </A><A NAME="293"> </A>
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Multiple Kernel Versions Source Files
</H1>
<P>
The system calls, which are the major interface the kernel shows to the
processes, generally stay the same across versions. A new system call
may be added, but usually the old ones will behave exactly like they
used to. This is necessary for backward compatibility -- a new kernel
version is <B>not</B> supposed to break regular processes. In most cases,
the device files will also remain the same. On the other hand, the internal
interfaces within the kernel can and do change between versions.
<P>
The Linux kernel versions are divided between the stable versions (n.<even
number>.m) and the development versions (n.<odd number>.m). The
development versions include all the cool new ideas, including those which
will be considered a mistake, or reimplemented, in the next version. As a
result, you can't trust the interface to remain the same in those versions
(which is why I don't bother to support them in this book, it's too much
work and it would become dated too quickly). In the stable versions, on the
other hand, we can expect the interface to remain the same regardless of the
bug fix version (the m number).
<A NAME="295"> </A><A NAME="296"> </A>
<P>
This version of the MPG includes support for both version 2.0.x and version
2.2.x of the Linux kernel. Since there are differences between the two, this
requires conditional compilation depending on the kernel version. The way
to do this to use the macro <TT>LINUX_VERSION_CODE</TT>. In version a.b.c of
the kernel, the value of this macro would be
<!-- MATH: $2^{16}a+2^{8}b+c$ -->
2<SUP>16</SUP><I>a</I>+2<SUP>8</SUP><I>b</I>+<I>c</I>. To get the
value for a specific kernel version, we can use the
<TT>KERNEL_VERSION</TT> macro. Since it's not defined in 2.0.35, we define it
ourselves if necessary.
<A NAME="301"> </A><A NAME="302"> </A><A NAME="303"> </A>
<A NAME="304"> </A><A NAME="305"> </A>
<A NAME="306"> </A> <A NAME="307"> </A>
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<BR><I>1999-05-19</I>
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