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I'm currently reading Operating System Concepts 7th edition by Silberschartz, Galvin and Gagne. At the end of chapter 2, there is an exercise "Add a system call to the Linux Kernel". Unfortunately, I realized the directory structure that the authors used is completely different with Ubuntu's one. For example, the authors referred to "/usr/src/linux-2.x/include/asm-i386/unistd.h", but on my machine, they are:

  • /usr/src/linux-headers-2.6.38-10
  • /usr/src/linux-headers-2.6.38-10-generic

And inside this folder, I couldn't find anything called "asm-i386" :(. I wonder is there a documentation specified for Ubuntu? Any suggestion would be greatly appreciated.

Thank you,

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Ubuntu doesn't do anything special. Your first difficulty is that you don't have the kernel source installed, only kernel headers. The authors are describing a system with a copy of the kernel source in /usr/src/linux-2.x. If you're only compiling external modules, the headers, which are what you see on your system, are enough. Ubuntu ships kernel headers for that purpose in the linux-headers-* packages (which you would normally install via the dependency from the linux-headers-generic Install linux-headers-generic metapackage). If you need the whole source, get a compressed archive from the linux-source-2.6 Install linux-source-2.6 binary package.

Another thing is that the directory structure has changed a little over time; architecture-dependent headers moved from include/asm-$ARCH to arch/$ARCH/include/asm. Furthermore the i386 and x86_64 architectures were merged into a unified x86 in 2.6.24. (More details here.) So you now need to look in arch/x86/include rather than include/asm-i386.

Here are a few useful resources for Linux kernel hackers:

  • Linux Device Drivers (LDD3)
  • LWN (news about Linux, including many technical articles about the kernel by one of the authors of LDD)
  • LXR to browse and search the kernel source
  • LKML (the Linux kernel mailing list), Stack Overflow to ask questions about the kernel and kernel development

And also read this thread on Unix & Linux, which explains how to locate the implementation of an existing syscall.

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  • Nice answer! I just ran find /usr/src -type f -name unistd.h for my answer :)
    – Andrew Gunnerson
    Jul 13, 2011 at 23:01
  • @Gilles: I was a bit confused with the author's instruction. Thank you so much for your help.
    – roxrook
    Jul 14, 2011 at 4:48
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/usr/src/linux-headers-2.6.38-10-generic/include/linux/unistd.h

or in future kernels:

/usr/src/linux-headers-KERNEL_VERSION/include/asm-generic/unistd.h

:)

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