Custom Linux: A Porting Guide:
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3.2. Modifying code using compiler flags

Although it is not a porting issue, the way you modify your code matters. It's easier if you do it right the first time. The Linux kernel uses standard configuration flags CONFIG_XXXX (like CONFIG_PPC, CONFIG_ISA etc), which are used to mark a certain machine, architecture or device. We defined ourselves a new flag (let's call it CONFIG_TESTMACH), and surrounded our new/modified code with these flags: 
....original code....
#ifdef CONFIG_TESTMACH
....modified code....
#else
....original code....
#endif /* CONFIG_TESTMACH */
To "activate" our code, we added the new flag to the kernel configuration file - .config - by adding CONFIG_TESTMACH=y to it. In the first stage, this solution allows you a quick way to find the code you changed, but later the flag you chose will allow you to add your code into the kernel tree and into the configuration program (make xconfig).

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