C language syntax: if { ... } while(0);

Question

I am looking at https://github.com/iputils/iputils/blob/s20161105/ping.c and I see from lines 608-713:

if (source.sin_addr.s_addr == 0) {
... // Omitted
} while(0);

Two questions:

1. How it compiles

2. What is the value of adding while (0) after if statement

Answer 1

It's two statements in succession. Entirely equivalent to this

if(/* ... */) {
  // Body
}
while(0) {
  // empty
}

while(0); is just a loop with a single empty statement for a body.

There is no value in adding it after the if. I suspect it's leftover from a previous refactoring, but the git history in that repository does not go that far.

Answer 2

1. It compiles because it is two statements, first

   if (...) { ... }
   

   followed by another statement

   while (0);
   

   i.e. a loop with empty body that is never run.

2. There is no value. while (0); as a separate statement is utterly useless. do { ... } while (0) is not useless but it is a completely different thing.

---

Probably the code was refactored from a form that initially used do { ... } while (0) and using breaks to exit the flow early (from the linked question above). Currently the source file does not have any instances of do statement, there are only 3 times that the verb do exists in some strings.