error about C macro invalid preprocessing token - c++

Question

// define the macro
#define FATAL(S,...) qDebug("%s,%s,%d,%s:"##S,"[scribble]",__FILE__,__LINE__,__FUNCTION__,##__VA_ARGS__)
// call the macro
FATAL("not supported commandid:%d",cmdid);
While this code works under Windows (Qt with msvc2010), when I try to compile it under macOS (Qt with clang), I get the error:::
pasting formed "%s,%s,%d,%s:""not supported commandid:%d", an invalid
preprocessing token.
How can I make it work under macOS, or is there another way to do the same function?

Answer 1

The token ## only has special meaning of indicating an optional argument when it is placed between a comma and a variable argument: , ##__VA_ARGS__
This is an extension to the C language.
In your example this is done correctly for the second argument, but not for the first.
If you don't need the first argument to be optional simply remove the token ##:
#define FATAL(S,...) printf("%s,%s,%d,%s:"S, \
"[scribble]",__FILE__,__LINE__,__FUNCTION__,##__VA_ARGS__)
But the first argument can be optional too, using another macro to expand the string literal and the first argument, while making the first argument optional:
#include <stdlib.h>
#include <stdio.h>
#define MERGE_EXPAND( a , ... ) a __VA_ARGS__
#define MERGE( a , ... ) MERGE_EXPAND( a , ##__VA_ARGS__ )
#define FATAL(S,...) printf( MERGE( "%s,%s,%d,%s:",S), \
"[scribble]",__FILE__,__LINE__,__FUNCTION__,##__VA_ARGS__)
int main( void )
{
int cmdid = 12345;
FATAL();
puts( "" ) ;
FATAL( "first argument" );
puts( "" ) ;
FATAL( "first and second arguments: %d" , cmdid );
return EXIT_SUCCESS ;
}