A simple c preprocessor example. Here #define, #undef, #ifdef, #ifndef, #else, #endif directives are explained.
[c preprocessor example]
#define DEBUG // ON
#undef DEBUG // OFF
#ifdef DEBUG // Enable the block if DEBUG symbol is defined.
puts("haha");
#endif
#ifndef DEBUG // Enable the block if DEBUG symbol is not defined.
puts("haha");
#endif
#ifdef DEBUG // Run puts("haha") if DEBUG symbol is defined.
puts("haha");
#else DEBUG // Otherwise, run puts("hoho")
puts("hoho");
#endif
