Issue title: lib/util/logging.c line 40-42 I was puzzled about the meaning of this code. 
 Issue body: I was studying xdp-tools and found that in lib/util/logging.c line 40-42 I was puzzled about the meaning of this code. 

#define __printf(a, b) __attribute__((format(printf, a, b))) 

__printf(2, 3)
 Comments: 
 Comment 0: sorry I misunderstood.
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