Why use apparently meaningless do-while and if-else statements in macros?

Question

In many C/C++ macros I\'m seeing the code of the macro wrapped in what seems like a meaningless do while loop. Here are examples.

#define FOO(X         


        

Answer 1

Explanation

do {} while (0) and if (1) {} else are to make sure that the macro is expanded to only 1 instruction. Otherwise:

if (something)
  FOO(X); 

would expand to:

if (something)
  f(X); g(X); 

And g(X) would be executed outside the if control statement. This is avoided when using do {} while (0) and if (1) {} else.

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Better alternative

With a GNU statement expression (not a part of standard C), you have a better way than do {} while (0) and if (1) {} else to solve this, by simply using ({}):

#define FOO(X) ({f(X); g(X);})

And this syntax is compatible with return values (note that do {} while (0) isn't), as in:

return FOO("X");