Macro Preprocessor, what is actually happening in here?

[Adrian20XX]

Hi,

I don't understand why the first C (c, not c++) program compiles and the second one does not compile.

Any idea what is happening with the macro pre-processor in here, why StateStart(0) can be used in the printf statement, but not in the #define?

TIA & Regards ...

First version: compiles

#include <stdio.h>
 
#define FIRST 4
 
#define StateStart(n) (STATE##n##_START)
 
#define STATE0_START (FIRST)
#define STATE1_START (STATE0_START+5)
 
main()
{
    printf("State 0 start: %d\n", STATE0_START);
    printf("State 1 start: %d\n", STATE1_START);
    printf("State 0 start: %d\n", StateStart(0));
    printf("State 1 start: %d\n", StateStart(1));
}
 

Second Version: Does not compile

Error: warning C4013: 'StateStart' undefined; assuming extern returning int

#include <stdio.h>
 
#define FIRST 4
 
#define StateStart(n) (STATE##n##_START)
 
#define STATE0_START (FIRST)
#define STATE1_START (StateStart(0)+5)
 
main()
{
    printf("State 0 start: %d\n", STATE0_START);
    printf("State 1 start: %d\n", STATE1_START);
    printf("State 0 start: %d\n", StateStart(0));
    printf("State 1 start: %d\n", StateStart(1));
}

[weaknessforcats]

This is the translation unit I got from Visual Studio.NET 2005:

main()
{
    printf("State 0 start: %d\n", (4));
    printf("State 1 start: %d\n", (((4))+5));
    printf("State 0 start: %d\n", ((4)));
    printf("State 1 start: %d\n", ((StateStart(0)+5)));
}
 

A StateStart(1) generates the token STATE1_START and that symbol is defined as (StateStart(0)+5). So that's what you get.

There is no rescan after a macro expansion unless a ## operator or a #define was involved and here none were so StateStart(0) was not seen as a macro.

You now die in the link with an unresolved external reference to StateStart.