Hi Kelvin, >I wonder if there is a webpage where all the predefined macros(such as __x86_64__, __i386__) GCC supported are listed ? You can generate your own list of predefined macros GCC provides, since they may vary a bit by version and by platform: For C: echo "" | gcc -E -dM -x c - | sort For C++: echo "" | g++ -E -dM -x c++ - | sort HTH, --Eljay