What does __VA_ARGS__ in a macro mean?

Question

/* Debugging */
#ifdef DEBUG_THRU_UART0
#   define DEBUG(...)  printString (__VA_ARGS__)
#else
void dummyFunc(void);
#   define DEBUG(...)  dummyFunc()   
#endif
         


        

Answer 1

The dots are called, together with the __VA_ARGS__, variadic macros

When the macro is invoked, all the tokens in its argument list [...], including any commas, become the variable argument. This sequence of tokens replaces the identifier VA_ARGS in the macro body wherever it appears.

source, bold emphasis of mine.

A sample of usage:

#ifdef DEBUG_THRU_UART0
#   define DEBUG(...)  printString (__VA_ARGS__)
#else
void dummyFunc(void);
#   define DEBUG(...)  dummyFunc()   
#endif
DEBUG(1,2,3); //calls printString(1,2,3) or dummyFunc() depending on
              //-DDEBUG_THRU_UART0 compiler define was given or not, when compiling.