3 ms·

Would you please explain the code?

• deleted 14y ago
  [deleted]
• kevinnk 14y ago
  >#include <stdio.h> >#define yell(i,l,o,v,e,u) o##v##i##l defines a macro called yell. The preprocessor operator ## concatenates it's operands together so something like ma##in gets transformed into main. >#define billy_bob yell(i,n,m,a,t,e) defines a macro called billy_bob that simply expands to the macro yell. When we call yell with the given arguments we get m##a##i##n (look at the order of the arguments for yell) which gets transformed into main. >billy_bob() Expanding the macro turns this line into main(). >{ > printf("no main in da house!"); >} So now we've declared a main function that just prints "no main in da house!".
• dubya 14y ago
  Hint: the 't' and 'e' in yell(i,n,m,a,t,e) are irrelevant.
• cnvogel 14y ago
  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; };