Question

Possible Duplicates:
Why are there sometimes meaningless do/while and if/else statements in C/C++ macros?
do { … } while (0) what is it good for?

I’m working on some C code filled with macros like this:

#define SAFE_FREE(x) do { if ((x) != NULL) {free(x); x=NULL;} } while(0)

Can anyone explain what this macro does, and why do {} while(0) is needed? Wouldn’t that just execute the code once?

Answer 1

do { stuff() } while(0);

is doing the exact same thing as stuff(). So what’s the big deal, then? The issue is with the syntax of macros. Suppose we defined the macro like:

#define SAFE_FREE(x) if ((x) != NULL) { free(x); x=NULL; }

Then, there are two issue. The first is relatively minor: uses of SAFE_FREE no longer require a trailing semi-colon. More importantly, though, code like:

if (...)
  SAFE_FREE(x)
else
   stuff();

Will expand to:

if (...)
  if ((x) != NULL) {
    free(x);
    x = NULL;
  } else
    stuff();

Note, how the else now matches the wrong if statement. This completely alters the program-flow, even though it does not look like that in the source code.

Defining the macro as above prevents weird behavior as above, since do { ... } while(0) acts just like a statement without its semicolon.