Odtnhj

I have a bit of code where I want to have logging only if DEBUG is defined. So I though I might be able to replace a Token (here: "DEBUGLOG") with the comment string "//". But how to do?

 #ifndef DEBUG
 #define DEBUGLOG //
 #endif

[...] 
 DEBUGLOG printf("Debug String"n);
[...]

There is no definition of DEBUG somwhere else in the code. But my gcc compiles this line and the program itself executes the printf();

Why?

I tried to include it in parantheses like this, but it gets an compile error:

#ifndef DEBUG
 #define DEBUGLOG "//"
#endif

This is the compiler message:

beispiel.c:45:10: error: expected ‘;’ before ‘printf’
 DEBUGLOG printf("Debug String"n);
 ^

Any hints?

If you look up the Phases of translation, you will find that the Phase where preprocessor is executed (Phase 4) is after the phase where comments are replaced by a white space character (Phase 3).

Phase 3

1) The source file is decomposed into comments, sequences of whitespace characters (space, horizontal tab, new-line, vertical tab, and form-feed), and preprocessing tokens, which are the following

...

2) Each comment is replaced by one space character

Phase 4

1) Preprocessor is executed.

So in Phase 3 the line:

#define DEBUGLOG //

becomes:

#define DEBUGLOG 

And in Phase 4 the line:

DEBUGLOG printf("Debug String"n);

becomes:

printf("Debug String"n);

And that is why your printf is executed.

And when you put it quotes ("//"), that line becomes:

"//" printf("Debug String"n);

The quotes ("") will not be removed. And this is a compiler error.