There's the "##" operator with which you can glue together identifiers with the preprocessor. #define yell(i,l,o,v,e,u) o##v##i##l yell(i,l,o,v,e,u) will t

There's the "##" operator with which you can glue together identifiers with the preprocessor. #define yell(i,l,o,v,e,u) o##v##i##l yell(i,l,o,v,e,u) will turn into the identifier ovil. And billy_bot just uses the yell-macro to produce a "main" identifier. #define billy_bob yell(i,n,m,a,t,e) yell(i,l,o,v,e,u) --> o ## v ## i ## l yell(i,n,m,a,t,e) --> m ## a ## i ## n So billy_bop is main. Why is this features useful at all? When you actually want to programmatically create function names, type names, struct names in preprocessor generated code. #define ENCAPS_STRUCT(type,name) struct encaps_##type { type name; } ENCAPS_STRUCT(int,a); will generate... $ cc -E test.c # 1 "test.c" # 1 "<built-in>" # 1 "<command-line>" # 1 "test.c" struct encaps_int { int a; };