Use #define preprocessor directive in a weird way

Question

Today I just finish reading and experimenting on C about how to use #define to create a manifest constant, after that something came into my mind, and below is the code.

#include <stdio.h>
#define dummy main
#define yam {
#define apple }

int dummy(void)        //constant substitution main with dummy
yam                          // constant substitution { with yam
  printf("It works!!\n");
  return 0;
apple                           //constant substitution } with apple

As expected, it works like charm, I just wonder why something like that didn't cause any error, maybe I could understand why the main() could be substituted because main is an identifier (name given to a function, variable and constant), but why the {} can be substituted with a symbolic name too? The second thing is, what data type C use to store this symbolic constant which is not a character enclosed in single quote "" nor an integer or floating-point number.

For the love of all that's holy, don't abuse the preprocessor in such awful ways! — Philip
– Philip, Commented Apr 22, 2011 at 9:57
haha @Philip , i just feel curious about it so i did this,i don't think i'm not going to code any software with this kind of method — caramel1995
– caramel1995, Commented Apr 22, 2011 at 10:32
So , in what datatype does the text { and } stored??because i didn't enclosed it in single quotation — caramel1995
– caramel1995, Commented Apr 22, 2011 at 10:47

Chris Cooper · Accepted Answer · 2014-05-09 17:59:28Z

7

The #define statements are evaluated by a preprocessor before the program is actually compiled, so the compiler never sees yam. The preprocessor performs a direct text substitution.

That is to say, when the compiler sees your code, it looks like this:

int main(void)        //constant substitution main with dummy
{                          // constant substitution { with yam
  printf("It works!!\n");
  return 0;
}                           //constant substitution } with apple

edited May 9, 2014 at 17:59

answered Apr 22, 2011 at 8:56

Chris Cooper

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PrettyPrincessKitty FS · Accepted Answer · 2011-04-22 08:54:58Z

2

Define is literally text replacement. In the preprocessor steps, the compiler will go through your code and replace all dummy with main, yam with {, and apple with }.

answered Apr 22, 2011 at 8:54

PrettyPrincessKitty FS

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Will Dean · Accepted Answer · 2011-04-22 08:55:23Z

2

This works because the preprocessor is a *PRE*processor, i.e. it's something that happens before the real processing.

So the preprocessor does its crude text substitution stuff before the real compiler gets to look at the code.

answered Apr 22, 2011 at 8:55

Will Dean

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Mario · Accepted Answer · 2011-04-22 08:58:04Z

2

#define is handled by the preprocessor. The stuff will be replaced character by character before the compiler compiles the code. You're essentially able to obscure your whole code using #define but anyone with a precompiler can unobscure it later on. The preprocessor replaces all occurances of the "constant" you defined, except when it's enclosed in quotes (e.g. char *test="dumy yam apple"; will stay unchanged).

answered Apr 22, 2011 at 8:58

Mario

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