Why use do { } while (0) in macro definition?

Question

Possible Duplicate:
Why are there sometimes meaningless do/while and if/else statements in C/C++ macros?

I met code like below:

#define ev_io_init(ev,cb,fd,events) \
do { \
  ev_init ((ev), (cb)); \
  ev_io_set ((ev),(fd),(events)); \
} while (0)

I want to know why the author use do { } while (0) here. Is there any difference with this?

#define ev_io_init(ev,cb,fd,events) { \
  ev_init ((ev), (cb)); \
  ev_io_set ((ev),(fd),(events)); \
}

BTW: the code is from libev, ev_local.h

Answer 1

Consider if( something ) function1(); else function2();

If function1() is actually a macro, just using { } requires you to omit the semicolon at the point of use, but do { } while(0) lets you use exactly the same syntax as for a real function.

(Not using any kind of block construct at all would just generate completely broken code, natch)