C multi-line macro: do/while(0) vs scope block [duplicate]

Andrey Tarasevich provides the following explanation:

1. On Google Groups
2. On bytes.com

[Minor changes to formatting made. Parenthetical annotations added in square brackets []].

The whole idea of using ‘do/while’ version is to make a macro which will
expand into a regular statement, not into a compound statement. This is
done in order to make the use of function-style macros uniform with the
use of ordinary functions in all contexts.

Consider the following code sketch:

if (<condition>)
  foo(a);
else
  bar(a);

where foo and bar are ordinary functions. Now imagine that you’d
like to replace function foo with a macro of the above nature [named CALL_FUNCS]:

if (<condition>)
  CALL_FUNCS(a);
else
  bar(a);

Now, if your macro is defined in accordance with the second approach
(just { and }) the code will no longer compile, because the ‘true’
branch of if is now represented by a compound statement. And when you
put a ; after this compound statement, you finished the whole if
statement, thus orphaning the else branch (hence the compilation error).

One way to correct this problem is to remember not to put ; after
macro “invocations”:

if (<condition>)
  CALL_FUNCS(a)
else
  bar(a);

This will compile and work as expected, but this is not uniform. The
more elegant solution is to make sure that macro expand into a regular
statement, not into a compound one. One way to achieve that is to define
the macro as follows:

#define CALL_FUNCS(x) \
do { \
  func1(x); \
  func2(x); \
  func3(x); \
} while (0)

Now this code:

if (<condition>)
  CALL_FUNCS(a);
else
  bar(a);

will compile without any problems.

However, note the small but important difference between my definition
of CALL_FUNCS and the first version in your message. I didn’t put a
; after } while (0). Putting a ; at the end of that definition
would immediately defeat the entire point of using ‘do/while’ and make
that macro pretty much equivalent to the compound-statement version.

I don’t know why the author of the code you quoted in your original
message put this ; after while (0). In this form both variants are
equivalent. The whole idea behind using ‘do/while’ version is not to
include this final ; into the macro (for the reasons that I explained
above).