Question

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

Why is C source code I run across sometimes wrapped with a do...while(0) loop?

do {
  parser->http_errno = e;                         
  parser->error_lineno = __LINE__;    
} while (0) 

Why use that, versus this:

parser->http_errno = e;                         
parser->error_lineno = __LINE__;    

I suspect this has something to do with thread safety, but I’m not sure.

Answer 1

It’s done so that you can use it inside a macro, while requiring the macro user to use a semicolon at the end, just like a regular statement.

In other words, whenever you want multiple statements inside a function-like macro FOO(...), you wrap it with do { ... } while (0) so that the user of the macro can (and, in fact, must) call it as FOO(x);, instead of simply FOO(x) (whose lack of semicolon can be weird for both humans and computers).