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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) do { f(X); g(X); } while (0) #define FOO(X) if (1) { f(X); g(X); } else

I can't see what the do while is doing. Why not just write this without it?

#define FOO(X) f(X); g(X)
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    For the example with the else, I would add an expression of void type at the end... like ((void)0). Commented Mar 8, 2018 at 17:59
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    Reminder that the do while construct isn't compatible with return statements, so the if (1) { ... } else ((void)0) construct has more compatible usages in Standard C. And in GNU C, you'll prefer the construct described in my answer. Commented Dec 12, 2018 at 11:50

9 Answers 9

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1009

The do ... while and if ... else are there to make it so that a semicolon after your macro always means the same thing. Let's say you had something like your second macro.

#define BAR(X) f(x); g(x)

Now if you were to use BAR(X); in an if ... else statement, where the bodies of the if statement were not wrapped in curly brackets, you'd get a bad surprise.

if (corge) BAR(corge); else gralt();

The above code would expand into

if (corge) f(corge); g(corge); else gralt();

which is syntactically incorrect, as the else is no longer associated with the if. It doesn't help to wrap things in curly braces within the macro, because a semicolon after the braces is syntactically incorrect.

if (corge) {f(corge); g(corge);}; else gralt();

There are two ways of fixing the problem. The first is to use a comma to sequence statements within the macro without robbing it of its ability to act like an expression.

#define BAR(X) f(X), g(X)

The above version of bar BAR expands the above code into what follows, which is syntactically correct.

if (corge) f(corge), g(corge); else gralt();

This doesn't work if instead of f(X) you have a more complicated body of code that needs to go in its own block, say for example to declare local variables. In the most general case the solution is to use something like do ... while to cause the macro to be a single statement that takes a semicolon without confusion.

#define BAR(X) do { \ int i = f(X); \ if (i > 4) g(i); \ } while (0)

You don't have to use do ... while, you could cook up something with if ... else as well, although when if ... else expands inside of an if ... else it leads to a "dangling else", which could make an existing dangling else problem even harder to find, as in the following code.

if (corge) if (1) { f(corge); g(corge); } else; else gralt();

The point is to use up the semicolon in contexts where a dangling semicolon is erroneous. Of course, it could (and probably should) be argued at this point that it would be better to declare BAR as an actual function, not a macro.

In summary, the do ... while is there to work around the shortcomings of the C preprocessor. When those C style guides tell you to lay off the C preprocessor, this is the kind of thing they're worried about.

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14 Comments

Isn't this a strong argument to always use braces in if, while and for statements? If you make a point of always doing this (as is required for MISRA-C, for example), the problem described above goes away.
The comma example should be #define BAR(X) (f(X), g(X)) otherwise operator precedence may mess up the semantics.
Another reason for this is to prevent the comma being erroneously interpreted as a value separator in a function call or a static initialiser. Also, I'll have to remind myself whether, when invoking a single-parameter macro, you can do something like BAR(42, 69) such that X will be the token sequence 42, ,, 69.
@DawidFerenczy: although both you and me-from-four-and-a-half-years-ago make a good point, we have to live in the real world. Unless we can guarantee that all the if statements, etc, in our code use braces, then wrapping macros like this is a simple way of avoiding problems.
Note: the if(1) {...} else void(0) form is safer than the do {...} while(0) for macros whose parameters are code that is included in the macro expansion, because it doesn't alter the behavior of the break or continue keywords. For example: for (int i = 0; i < max; ++i) { MYMACRO( SomeFunc(i)==true, {break;} ) } causes unexpected behavior when MYMACRO is defined as #define MYMACRO(X, CODE) do { if (X) { cout << #X << endl; {CODE}; } } while (0) because the break affects the macro's while loop rather than the for loop at the macro call site.
@ace void(0) was a typo, I meant (void)0. And I believe this does solve the "dangling else" problem: notice there's no semicolon after the (void)0. A dangling else in that case (e.g. if (cond) if (1) foo() else (void)0 else { /* dangling else body */ }) triggers a compilation error. Here's a live example demonstrating it
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Macros are copy/pasted pieces of text the pre-processor will put in the genuine code; the macro's author hopes the replacement will produce valid code.

There are three good "tips" to succeed in that:

Help the macro behave like genuine code

Normal code is usually ended by a semi-colon. Should the user view code not needing one...

doSomething(1) ; DO_SOMETHING_ELSE(2) // <== Hey? What's this? doSomethingElseAgain(3) ;

This means the user expects the compiler to produce an error if the semi-colon is absent.

But the real real good reason is that at some time, the macro's author will perhaps need to replace the macro with a genuine function (perhaps inlined). So the macro should really behave like one.

So we should have a macro needing semi-colon.

Produce a valid code

As shown in jfm3's answer, sometimes the macro contains more than one instruction. And if the macro is used inside a if statement, this will be problematic:

if(bIsOk) MY_MACRO(42) ;

This macro could be expanded as:

#define MY_MACRO(x) f(x) ; g(x) if(bIsOk) f(42) ; g(42) ; // was MY_MACRO(42) ;

The g function will be executed regardless of the value of bIsOk.

This means that we must have to add a scope to the macro:

#define MY_MACRO(x) { f(x) ; g(x) ; } if(bIsOk) { f(42) ; g(42) ; } ; // was MY_MACRO(42) ;

Produce a valid code 2

If the macro is something like:

#define MY_MACRO(x) int i = x + 1 ; f(i) ;

We could have another problem in the following code:

void doSomething() { int i = 25 ; MY_MACRO(32) ; }

Because it would expand as:

void doSomething() { int i = 25 ; int i = 32 + 1 ; f(i) ; ; // was MY_MACRO(32) ; }

This code won't compile, of course. So, again, the solution is using a scope:

#define MY_MACRO(x) { int i = x + 1 ; f(i) ; } void doSomething() { int i = 25 ; { int i = 32 + 1 ; f(i) ; } ; // was MY_MACRO(32) ; }

The code behaves correctly again.

Combining semi-colon + scope effects?

There is one C/C++ idiom that produces this effect: The do/while loop:

do { // code } while(false) ;

The do/while can create a scope, thus encapsulating the macro's code, and needs a semi-colon in the end, thus expanding into code needing one.

The bonus?

The C++ compiler will optimize away the do/while loop, as the fact its post-condition is false is known at compile time. This means that a macro like:

#define MY_MACRO(x) \ do \ { \ const int i = x + 1 ; \ f(i) ; g(i) ; \ } \ while(false) void doSomething(bool bIsOk) { int i = 25 ; if(bIsOk) MY_MACRO(42) ; // Etc. }

will expand correctly as

void doSomething(bool bIsOk) { int i = 25 ; if(bIsOk) do { const int i = 42 + 1 ; // was MY_MACRO(42) ; f(i) ; g(i) ; } while(false) ; // Etc. }

and is then compiled and optimized away as

void doSomething(bool bIsOk) { int i = 25 ; if(bIsOk) { f(43) ; g(43) ; } // Etc. }
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5 Comments

Note that changing macros to inline function alters some standard predefined macros, e.g. the following code shows a change in FUNCTION and LINE: #include <stdio.h> #define Fmacro() printf("%s %d\n", FUNCTION, LINE) inline void Finline() { printf("%s %d\n", FUNCTION, LINE); } int main() { Fmacro(); Finline(); return 0; } (bold terms should be enclosed by double underscores — bad formatter!)
There are a number of minor but not completely inconsequential issues with this answer. For instance: void doSomething() { int i = 25 ; { int i = x + 1 ; f(i) ; } ; // was MY_MACRO(32) ; } is not the correct expansion; the x in the expansion should be 32. A more complex issue is what is the expansion of MY_MACRO(i+7). And another is the expansion of MY_MACRO(0x07 << 6). There's a lot that's good, but there are some undotted i's and uncrossed t's.
@Gnubie: I case you’re still here and you haven’t figured this out by now: you can escape asterisks and underscores in comments with backslashes, so if you type \_\_LINE\_\_ it renders as __LINE__. IMHO, it’s better just to use code formatting for code; for example, __LINE__ (which doesn’t require any special handling).                     P.S. I don’t know whether this was true in 2012; they’ve made quite a few improvements to the engine since then.
Appreciating that my comment is six years late, but most C compilers do not actually inline inline functions (as permitted by the standard)
Thanks! The most upvoted answer covered that you need a while(0) to behave correctly in unscoped if statements. But what they didn't cover (and you did) is why it isn't simply enough to put the macro body in a scope, like #define FOO(x) { bar(x); }. It seems the real reason that you need while(0) is to require a semi-colon after the statement.
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@jfm3 - You have a nice answer to the question. You might also want to add that the macro idiom also prevents the possibly more dangerous (because there's no error) unintended behavior with simple 'if' statements:

#define FOO(x) f(x); g(x) if (test) FOO( baz);

expands to:

if (test) f(baz); g(baz);

which is syntactically correct so there's no compiler error, but has the probably unintended consequence that g() will always be called.

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Comments

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The above answers explain the meaning of these constructs, but there is a significant difference between the two that was not mentioned. In fact, there is a reason to prefer the do ... while to the if ... else construct.

The problem of the if ... else construct is that it does not force you to put the semicolon. Like in this code:

FOO(1) printf("abc");

Although we left out the semicolon (by mistake), the code will expand to

if (1) { f(X); g(X); } else printf("abc");

and will silently compile (although some compilers may issue a warning for unreachable code). But the printf statement will never be executed.

do ... while construct does not have such problem, since the only valid token after the while(0) is a semicolon.

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6 Comments

@RichardHansen: Still not as good, because from the look at the macro invocation you don't know whether it expands to a statement or to an expression. If someone assumes the later, she may write FOO(1),x++; which will again give us a false positive. Just use do ... while and that's it.
Documenting the macro to avoid the misunderstanding should suffice. I do agree that do ... while (0) is preferable, but it has one downside: A break or continue will control the do ... while (0) loop, not the loop containing the macro invocation. So the if trick still has value.
I don't see where you could put a break or a continue that would be seen as inside your macros do {...} while(0) pseudo-loop. Even in the macro parameter it would make a syntax error.
Another reason to use do { ... } while(0) instead of if whatever construct, is the idiomatic nature of it. The do {...} while(0) construct is widespread, well known and used a lot by many programmers. Its rationale and documentation is readily known. Not so for the if construct. It takes therefore less effort to grok when doing code review.
@tristopia: I've seen people write macros that take blocks of code as arguments (which I don't necessarily recommend). For example: #define CHECK(call, onerr) if (0 != (call)) { onerr } else (void)0. It could be used like CHECK(system("foo"), break;);, where the break; is intended to refer to the loop enclosing the CHECK() invocation.
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Explanation

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

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");
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the use of block-clamping {} in the macro would be sufficient for bundling the macro code so that all is executed for the same if-condition path. the do-while around is used for enforcing a semicolon at places the macro gets used. thus the macro is enforced behaving more function alike. this includes the requirement for the trailing semicolon when used.
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While it is expected that compilers optimize away the do { ... } while(false); loops, there is another solution which would not require that construct. The solution is to use the comma operator:

#define FOO(X) (f(X),g(X))

or even more exotically:

#define FOO(X) g((f(X),(X)))

While this will work well with separate instructions, it will not work with cases where variables are constructed and used as part of the #define :

#define FOO(X) (int s=5,f((X)+s),g((X)+s))

With this one would be forced to use the do/while construct.

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5 Comments

thanks, since the comma operator does not guarantee execution order, this nesting is a way to enforce that.
@Marius: False; the comma operator is a sequence point and thereby does guarantee execution order. I suspect you confused it with the comma in function argument lists.
Just wanted to add that compilers are forced to preserve the program observable behavior so optimizing the do/while away isn't much of a big deal (assuming the compiler optimizations are correct).
@MarcoA. while you are correct, I have found in the past that compiler optimization, while exactly preserving the function of code, but by changing around lines which would seem to do nothing in the singular context, will break multithreaded algorithms. Case in point Peterson's Algorithm.
This will also not work for all kinds of constructs, although C, with the ternary operator and this, is rather expressive.
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Jens Gustedt's P99 preprocessor library (yes, the fact that such a thing exists blew my mind too!) improves on the if(1) { ... } else construct in a small but significant way by defining the following:

#define P99_NOP ((void)0) #define P99_PREFER(...) if (1) { __VA_ARGS__ } else #define P99_BLOCK(...) P99_PREFER(__VA_ARGS__) P99_NOP

The rationale for this is that, unlike the do { ... } while(0) construct, break and continue still work inside the given block, but the ((void)0) creates a syntax error if the semicolon is omitted after the macro call, which would otherwise skip the next block. (There isn't actually a "dangling else" problem here, since the else binds to the nearest if, which is the one in the macro.)

If you are interested in the sorts of things that can be done more-or-less safely with the C preprocessor, check out that library.

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5 Comments

While very clever, this causes one to be bombarded with compiler warnings on potential dangling else.
You usually use macros to create a contained environment, that is, you never use a break (or continue) inside a macro to control a loop that started/ended outside, that’s just bad style and hides potential exit points.
There's also a preprocessor library in Boost. What is mind-blowing about it?
The risk with else ((void)0) is that someone might be writing YOUR_MACRO(), f(); and it will be syntactically valid, but never call f(). With do while it's a syntax error.
@melpomene so what about else do; while (0)?
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For some reasons I can't comment on the first answer...

Some of you showed macros with local variables, but nobody mentioned that you can't just use any name in a macro! It will bite the user some day! Why? Because the input arguments are substituted into your macro template. And in your macro examples you've use the probably most commonly used variabled name i.

For example when the following macro

#define FOO(X) do { int i; for (i = 0; i < (X); ++i) do_something(i); } while (0)

is used in the following function

void some_func(void) { int i; for (i = 0; i < 10; ++i) FOO(i); }

the macro will not use the intended variable i, that is declared at the beginning of some_func, but the local variable, that is declared in the do ... while loop of the macro.

Thus, never use common variable names in a macro!

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@Blaisorblade: Actually that's incorrect and illegal C; leading underscores are reserved for use by the implementation. The reason you've seen this "usual pattern" is from reading system headers ("the implementation") which must restrict themselves to this reserved namespace. For applications/libraries, you should choose your own obscure, unlikely-to-collide names without underscores, e.g. mylib_internal___i or similar.
@R.. You're right - I've actually read this in an ''application'', the Linux kernel, but it's an exception anyway since it uses no standard library (technically, a ''freestanding'' C implementation instead of a ''hosted'' one).
@R.. this is not quite correct: leading underscores followed by a capital or second underscore are reserved for the implementation in all contexts. Leading underscores followed by something else are not reserved in local scope.
@Leushenko: Yes, but the distinction is sufficiently subtle that I find it best to tell people just not to use such names at all. The people who understand the subtlety presumably already know that I'm glossing over the details. :-)
While correct, this is not an answer to the question. By site rules, please make this into a comment; if you’re unable to, gather karma from good answers (that actually are answers) first; yes, first steps can be hard, but on SO there’s just so much it’s possible.
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I don't think it was mentioned so consider this

while(i<100) FOO(i++);

would be translated into

while(i<100) do { f(i++); g(i++); } while (0)

notice how i++ is evaluated twice by the macro. This can lead to some interesting errors.

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3 Comments

This has nothing to do with the do ... while(0) construct.
True. But relevent to the topic of macros vs. functions and how to write a macro that behaves as a function...
Similarily to the above one, this is not an answer but a comment. On topic: that’s why you use stuff only once: do { int macroname_i = (i); f(macroname_i); g(macroname_i); } while (/* CONSTCOND */ 0)

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