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
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)
Enclosing code with a loop allows for a preprocessor directive to execute multiple statements without "breaking" if-else-constructs. Consider the following:
#define DO_SOMETHING() a();b();c(); void foo() { // This is ok... DO_SOMETHING(); } void bar() { // ...whereas this would trigger an error. if (condition) DO_SOMETHING(); else blah(); } The second example breaks the if-else-construct because three statements are followed by an else clause. To allow for it to correctly substitute, the instructions in DO_SOMETHING should be enclosed with a do { ... } while(0).
A do{}while(0) allows you to break from the loop:
do{ expr1; foo(); if ( cond ) break; expr2; goo(); } while (0); It's the same as a simple block {...} except that you can break execution when you want with the break statement. You couldn't do that in a simple code block, unless you have multiple checks, which can get cumbersome. It still gets executed once, because of the condition while(0).
