Do not understand this do..while loop

Hi everyone,

Could someone explain me how does this do..while work? What is the purpose of while (!m_finished)? How the loop will break if while (0)?

Code:

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#define WAIT_FOR_PERIPH() \
  do {                    \
    while (!m_finished) { \
    }                    \
    m_finished = false;  \
  } while (0)

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Thanks Nick

The outer loop seems useless. Seeing that the inner loop is some kind of busy loop that waits for the value of m_finished to change, and judging by the name WAIT_FOR_PERIPH, I'd say this snippet waits for some kind of user-defined interrupt to be triggered.

It seems possible that the outer loop is there to prevent the compiler from optimizing away the inner one... I can't think of any other reason...

EDIT: Oh yeah, I forgot about the semi-color! Disregard my answer, laserlight is right.

Since this is a function-style macro, the do while (0) loop here is used for the sole purpose of making the "function body" self-contained so that the macro could be used where a single statement was expected and behave in the same way. You could just as well change it to a pair of braces, except then the "caller" could leave out the terminating semi-colon.

Sorry for the late response, thanks guys!!

Ok, so the do..while(0) loop is actually used in order to make a uniform macro function, a function that it can be used anywhere especially in if..else statements and a semicolonafter the macro will always have the same effect.

Quote:

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Originally Posted by laserlight

You could just as well change it to a pair of braces, except then the "caller" could leave out the terminating semi-colon.

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You say "could" leave out the terminating semicolon, but you mean "must" leave out the terminating semicolon, or else the following will not work:

Code:

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    if (b)
        WAIT_FOR_PERIPH();  // the semicolon is an error with just braces
    else
        something_else();

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