Thread: Concatenate with preprocessing function

Hi guys,

I am learning how to write macro style fuctions.
I wrote this simple function in a header file

Code:

#ifndef CONCATENATE_H
#define CONCATENATE_H


#define CONCAT_2 (p1,p2)  p1##p2


#endif

and I call the function from main file

Code:

#include <stdio.h>
#include <stdint.h>
#include "Concatenate.h"


int main()
{
  
    printf("%d",CONCAT_2(10,20));


    return 0;
}

I use Visual Studio Code is a code editor

When I run the code it returns the following errors and I cannot figure out what I am doing wrong

Code:

[Running] cd "c:\Users\n.antoniou\Dropbox\IoT\C_Projects\Macros\" && gcc Concatenate.c -o Concatenate && "c:\Users\n.antoniou\Dropbox\IoT\C_Projects\Macros\"Concatenate
In file included from Concatenate.c:6:
Concatenate.c: In function 'main':
Concatenate.h:4:19: error: 'p1' undeclared (first use in this function)
    4 | #define CONCAT_2 (p1,p2)  p1##p2
      |                   ^~
Concatenate.c:11:17: note: in expansion of macro 'CONCAT_2'
   11 |     printf("%d",CONCAT_2(10,20));
      |                 ^~~~~~~~
Concatenate.h:4:19: note: each undeclared identifier is reported only once for each function it appears in
    4 | #define CONCAT_2 (p1,p2)  p1##p2
      |                   ^~
Concatenate.c:11:17: note: in expansion of macro 'CONCAT_2'
   11 |     printf("%d",CONCAT_2(10,20));
      |                 ^~~~~~~~
Concatenate.h:4:22: error: 'p2' undeclared (first use in this function)
    4 | #define CONCAT_2 (p1,p2)  p1##p2
      |                      ^~
Concatenate.c:11:17: note: in expansion of macro 'CONCAT_2'
   11 |     printf("%d",CONCAT_2(10,20));
      |                 ^~~~~~~~
Concatenate.h:4:27: error: expected ')' before 'p1p2'
    4 | #define CONCAT_2 (p1,p2)  p1##p2
      |                           ^~
Concatenate.c:11:17: note: in expansion of macro 'CONCAT_2'
   11 |     printf("%d",CONCAT_2(10,20));
      |                 ^~~~~~~~

I found the error, there was a space in CONCAT_2 (p1,p2)

The preprocessor gets used for some truly weird things. In particular, it is considered "good form" when defining a macro that will be used as part of an (arithmetic) expression to wrap the contents of the macro in parentheses. Thus:

#define sum (a + b)

Because of this, the rule for "object-like macros" (like "sum") versus "function-like macros" (like your "CONCAT_2") is very simple: a function-like macro is defined with parens immediately after the name:


#define sum (a + b) // defines an object-like macro you use like "y = sum * 4;"

#define sum(a, b) (a + b) // defines a function-like macro you use like "y = sum(m * x, b);"

Note that this only applies to the definition of the macro. Once the macros are defined, you can use spaces or not: sum (a, 4).