why are there so many versions of header files in my system?

Question

I learned to program with Pascal in high school, and more recently I decided to get out of the sandbox and try to figure out how my computer actually works. So I installed ubuntu on my iMac (i686) and started learning C, which seemed like a good way to get "under the hood."

One of the basic things I'm trying to figure out is where the kernel ends and the standard libraries begin. A book told me that the linux system calls (which I understand to be the interface between the kernel and the libraries) could be found in the header file unistd.h, so this seemed like a good place to start. But when I tried to find the header on my system (using locate unistd.h), I got this result:

/usr/include/unistd.h
/usr/include/asm-generic/unistd.h
/usr/include/i386-linux-gnu/asm/unistd.h
/usr/include/i386-linux-gnu/bits/unistd.h
/usr/include/i386-linux-gnu/sys/unistd.h
/usr/include/linux/unistd.h
/usr/lib/syslinux/com32/include/unistd.h
/usr/src/linux-headers-3.5.0-27/arch/alpha/include/asm/unistd.h
/usr/src/linux-headers-3.5.0-27/arch/arm/include/asm/unistd.h
/usr/src/linux-headers-3.5.0-27/arch/avr32/include/asm/unistd.h
/usr/src/linux-headers-3.5.0-27/arch/blackfin/include/asm/unistd.h
/usr/src/linux-headers-3.5.0-27/arch/c6x/include/asm/unistd.h
/usr/src/linux-headers-3.5.0-27/arch/cris/include/arch-v10/arch/unistd.h
/usr/src/linux-headers-3.5.0-27/arch/cris/include/arch-v32/arch/unistd.h
/usr/src/linux-headers-3.5.0-27/arch/cris/include/asm/unistd.h
/usr/src/linux-headers-3.5.0-27/arch/frv/include/asm/unistd.h
/usr/src/linux-headers-3.5.0-27/arch/h8300/include/asm/unistd.h
/usr/src/linux-headers-3.5.0-27/arch/hexagon/include/asm/unistd.h
/usr/src/linux-headers-3.5.0-27/arch/ia64/include/asm/unistd.h
/usr/src/linux-headers-3.5.0-27/arch/m32r/include/asm/unistd.h
/usr/src/linux-headers-3.5.0-27/arch/m68k/include/asm/unistd.h
/usr/src/linux-headers-3.5.0-27/arch/microblaze/include/asm/unistd.h
/usr/src/linux-headers-3.5.0-27/arch/mips/include/asm/unistd.h
/usr/src/linux-headers-3.5.0-27/arch/mn10300/include/asm/unistd.h
/usr/src/linux-headers-3.5.0-27/arch/openrisc/include/asm/unistd.h
/usr/src/linux-headers-3.5.0-27/arch/parisc/include/asm/unistd.h
/usr/src/linux-headers-3.5.0-27/arch/powerpc/include/asm/unistd.h
/usr/src/linux-headers-3.5.0-27/arch/s390/include/asm/unistd.h
/usr/src/linux-headers-3.5.0-27/arch/score/include/asm/unistd.h
/usr/src/linux-headers-3.5.0-27/arch/sh/include/asm/unistd.h
/usr/src/linux-headers-3.5.0-27/arch/sparc/include/asm/unistd.h
/usr/src/linux-headers-3.5.0-27/arch/tile/include/asm/unistd.h
/usr/src/linux-headers-3.5.0-27/arch/unicore32/include/asm/unistd.h
/usr/src/linux-headers-3.5.0-27/arch/x86/include/asm/ia32_unistd.h
/usr/src/linux-headers-3.5.0-27/arch/x86/include/asm/unistd.h
/usr/src/linux-headers-3.5.0-27/arch/xtensa/include/asm/unistd.h
/usr/src/linux-headers-3.5.0-27/include/asm-generic/unistd.h
/usr/src/linux-headers-3.5.0-27/include/linux/unistd.h
/usr/src/linux-headers-3.5.0-27-generic/include/linux/unistd.h

Why the heck are there so many versions of this file--and other header files--in my system? Some of them seem to be for other CPUs (like sparc), so why did ubuntu bother to install them on my computer? And how does the all of this fit with what Eric Raymond calls the SPOT rule: "every piece of knowledge must have a single, unambiguous, authoritative representation within a system." (The Art of Unix Programming, p. 91.)

Thanks in advance for any help. I'm happy to read big books if necessary.

Answer 1

I think these header files are directly from linux-3.5.0-27 source code. Ubuntu developers didn't know what kind of target they are dealing with. Maybe Intel x86/powerPC/ or even a mobile hand set(ARM), so they just copy all the head files and make a simple link.