Macro expand in C programming language

Just yesterday, a question came into my mind:

When a C compiler preprocess the macro define?

 

So comes a test in Ubuntu 10.10

Gcc (Ubuntu/Linaro 4.4.4-14ubuntu5) 4.4.5

 

// Header file

# Ifndef TEST_H __

# Define TEST_H __

 

# Define and +

 

# Endif

 

 

// Source file

# Include "test. h"

 

# Define ADD (X) (X and X)

 

Void f (){

ADD (X );

Return 0;

}

 

Int main (int argc, char ** argv ){

Int I = 20;

F (I );

Return 0;

}

// End of source file

 

Using the-E Option in GCC to preprocess the test. C, the result is

#1 "test. c"

#1 "<built-in>"

#1 "<command-line>"

#1 "test. c"

#1 "test. H" 1

#2 "test. c" 2

 

 

 

Void F (){

(X and X );

Return 0;

}

 

Int main (INT argc, char ** argv ){

Int I = 20;

F (I );

Return 0;

}

 

// End of the process

 

If we exchange the macro define (and) and (ADD) in header file and source file

We can get the following result after preprocessing.

 

#1 "test. c"

#1 "<built-in>"

#1 "<command-line>"

#1 "test. c"

#1 "test. h" 1

#2 "test. c" 2

 

 

 

Void f (){

(X + X );

Return 0;

}

 

Int main (int argc, char ** argv ){

Int I = 20;

F (I );

Return 0;

}

 

// End of the process

 

When I defined all macro in two different header file,

The macro cocould expand correctly, whatever order

Header file.

 

So, I think an answer cocould come out:

We 'd better define macro in header file.