Pasting macro on linux not working

Question

Hi,

Program below runs fine on windows. But compilation error occurs on linux "error: pasting "." and "config" does not give a valid preprocessing token"

Any reason??? i cant understand why....

#include <stdio.h>

typedef struct pr {
 int config;
}pr_t;

#define JOIN(x,y) x.##y 

void main()
{

 pr_t temp = {5};

 printf("Value %d\n", JOIN(temp, config)); //temp.config

 return 0;
}

Answer 1

Try without the ## :)

#define JOIN(x,y) x.y

Answer 2

It's to do with trying to paste a literal string and a token together. This behaviour was changed in gcc 2.7 onwards, see for example the info here: http://weblog.pell.portland.or.us/~orc/2004/12/30/000/index.html

You should be able to remove the ## and simply concatenate the operators:

#define JOIN(x,y) x.y

Answer 3

The macro concatenation operator, ##, should only be used between two macro parameters. You have a period between them which serves to delimit the two parameter names. So as ykatchou suggested, just edit out the ## operator from the macro definition:

#define JOIN(x,y) x.y 

which should still work fine in your windows compiler.

Answer 4

Two tokens that don't together form a valid token cannot be pasted together using ##.
The compiler error says that clearly:

error: pasting "." and "config" does not give a valid preprocessing token

As suggested by others you can drop ## altogether.

More info here.