Why "do ... while (0)" can't be replaced by simple curly brackets?

Question

I recently saw a piece of C code including a macro of the following style:

#define TOTO()              \
do {                        \
  do_something();           \
  do_another_something();   \
} while (0)

I was at first wondering of the purpose of the do while (0) here, but this answer explained to me: it is in case the macro is used just after an if or else without curly brackets, like this:

if (something)
    TOTO();
else
    do_something_else();

So here, without the do while (0) statement, the code would be expanded to:

if (something)
    do_something();
    do_another_something();
else
    do_something_else();

Which is syntactically wrong, because the else is not directly following an if scope anymore.

But I thought it would work as well by declaring the macro in its own scope, without necessary a do while around it, so I tested the same code with just curly brackets. My entire code looks like this:

#include <stdio.h>

#define HELLO_WORLD()       \
{                           \
    printf("hello ");       \
    printf("world!\n");     \
}

int     main(int argc, char** argv)
{
    if (argc == 1)
        HELLO_WORLD();
    else
        fprintf(stderr, "nope\n");
    return 0;
}

But GCC give me the following error:

error: ‘else’ without a previous ‘if’

However, the code of the main function should be expanded to:

if (argc == 1)
    {
        printf("hello ");
        printf("world!\n");
    }
else
    fprintf(stderr, "nope\n");
return 0;

Which is valid.

So what am I missing here?

As always when facing macro-related problems: your compiler may have a switch to output the preprocessed file, so you can see what the actual input to the code compiler looks like. — Jongware, Jan 15 '16 at 09:21
@Downvoters How is this a bad question? It is a duplicate, yes, but then mark it as one at least instead of downvoting and telling everybody it's a bad question. Or what's the reason for this? — cadaniluk, Jan 15 '16 at 09:23
The first part `do{} while()` part has nothing to do with the his code I think . ( However I didn't downvote I just think about the reason ) — , Jan 15 '16 at 09:24
The whole do-while-zero macro trick was always kind of retarded, because it assumes that code without braces is good and should be used. The proper solution is to adopt a coding standard which always enforces braces after each and every statement, and then check the code with static analysis to ensure that it does. — Lundin, Jan 15 '16 at 09:32
For that reason the `{ }` macro is **better** practice than `do {} while(0)`, because it creates a compiler error if you don't have any braces in the caller. — Lundin, Jan 15 '16 at 09:33
@Aracthor No! `if(something) { { stuff(); } ; } else` is perfectly fine C code, as opposed to `if(something) { stuff(); } ; else`. Where `{ stuff(); }` is your expanded macro. — Lundin, Jan 15 '16 at 09:39

cadaniluk · Accepted Answer · 2016-01-16T10:32:20.050

It's the semicolon after the macro.

The macro is expanded to this instead:

if (argc == 1)
    {
        printf("hello ");       \
        printf("world!\n");     \
    };
else        // HERE, SYNTAX ERROR
    fprintf(stderr, "nope\n");
return 0;

Since there should be no semicolon between the body of an if-statement and an else-clause, it's a syntax error.
OTOH, a do-while loop allows (and needs) a semicolon.

The misconception regarding the compiler output can be easily avoided by just printing the real preprocessor output. That's possible by using

the -E switch of the gcc executable. From man 1 gcc:

-E Stop after the preprocessing stage; do not run the compiler proper. The output is in the form of preprocessed source code, which is sent to the standard output.

the preprocessor directly in shape of the cpp executable.

_{Thanks to

- @dhke for correcting the line the error occurs at.

- @kakeh for the suggestion to view the preprocessor output beforehand.}

score 2 · Answer 2 · answered Jan 15 '16 at 09:20

Look CAREFULLY at how you call the macro.

You'd have to write

if (argc == 1)
    HELLO_WORLD() // NO SEMICOLON!!!!!!!
else
    fprintf(stderr, "nope\n");

which would be godawful because you could never replace the HELLO_WORLD macro with a real function.

Why "do ... while (0)" can't be replaced by simple curly brackets?

2 Answers2