C multi-line macro do while 0 vs scope block

Question

Possible Duplicates    What rsquo s the use of do while 0  when we define a macro    Why are there sometimes meaningless do while and if else statements in C C   macros     do    hellip    while  0   what is it good for      I ve seen some multi-line C macros that are wrapped inside a do while 0  loop like     define FOO     do         do stuff here       do more stuff       while  0    What are the benefits  if any  of writing the code that way as opposed to using a basic block     define FOO             do stuff here       do more stuff

User · Accepted Answer

Andrey Tarasevich provides the following explanation:

[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).

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

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