What does __VA_ARGS__ in a macro mean?

c gcc variadic-macros variadic-functions
ganeshredcobra picture ganeshredcobra · Sep 26, 2014 · Viewed 28.8k times · Source 
/* Debugging */
#ifdef DEBUG_THRU_UART0
#   define DEBUG(...)  printString (__VA_ARGS__)
#else
void dummyFunc(void);
#   define DEBUG(...)  dummyFunc()   
#endif

I've seen this notation in different headers of C programming, I basically understood it's passing arguments, but I didn't understand what this "three dots notation" is called?

Can someone explain it with example or provide links also about VA Args?

Answer

nos picture nos · Sep 26, 2014

It's a variadic macro. It means you can call it with any number of arguments. The three ... is similar to the same construct used in a variadic function in C

That means you can use the macro like this

DEBUG("foo", "bar", "baz");

Or with any number of arguments.

The __VA_ARGS__ refers back again to the variable arguments in the macro itself.

#define DEBUG(...)  printString (__VA_ARGS__)
               ^                     ^
               +-----<-refers to ----+

So DEBUG("foo", "bar", "baz"); would be replaced with printString ("foo", "bar", "baz")

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