usage of define func() as (void)0

["Ming-Chia Jiang" <jmc@xxxxxxxxxxxxxxxxxxx>]

Hi folks, 

I observed sometimes macros are writen as: 
#define xxx() (void)0 

Then, xxx() is called in some place of code. 
What is the real meaning of doing this? To define xxx() as (void)0. 
And under what conditions would people use this?? 

Thanks & best regards 
Ming-Chia