Magic numbers of the Linux reboot() system call

The Linux Programming Interface has an exercise in Chapter 3 that goes like this:

When using the Linux-specific reboot() system call to reboot the system, the second argument, magic2, must be specified as one of a set of magic numbers (e.g., LINUX_REBOOT_MAGIC2). What is the significance of these numbers? (Converting them to hexadecimal provides a clue.)

The man page tells us magic2 can be one of LINUX_REBOOT_MAGIC2 (672274793), LINUX_REBOOT_MAGIC2A (85072278), LINUX_REBOOT_MAGIC2B (369367448), or开发者_JS百科 LINUX_REBOOT_MAGIC2C (537993216). I failed to decipher their meaning in hex. I also looked at /usr/include/linux/reboot.h, which didn't give any helpful comment either.

I then searched in the kernel's source code for sys_reboot's definition. All I found was a declaration in a header file.

Therefore, my first question is, what is the significance of these numbers? My second question is, where's sys_reboot's definition, and how did you find it?

EDIT: I found the definition in kernel/sys.c. I only grepped for sys_reboot, and forgot to grep for the MAGIC numbers. I figured the definition must be hidden behind some macro trick, so I looked at the System.map file under /boot, and found it next to ctrl_alt_del. I then grepped for that symbol, which led me to the correct file. If I had compiled the kernel from source code, I could try to find which object file defined the symbol, and go from there.

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Just a guess, but those numbers look more interesting in hex:

672274793 = 0x28121969
 85072278 = 0x05121996
369367448 = 0x16041998
537993216 = 0x20112000

Developers' or developers' children's birthdays?

Regarding finding the syscall implementation, I did a git grep -n LINUX_REBOOT_MAGIC2 and found the definition in kernel/sys.c. The symbol sys_reboot is generated by the SYSCALL_DEFINE4(reboot, ... gubbins, I suspect.

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It's the birthday of Linus Torvalds (The developer of the Linux kernel and the Git version control) and his 3 daughters. works as magic numbers to reboot the system.

http://en.wikipedia.org/wiki/Linus_Torvalds