Why do I keep seeing multiline macros wrapped in (seemingly) pointless loops?

Question

Possible Duplicate:
how does do{} while(0) work in macro?

Example from this blog post:

#define VTAILQ_INSERT_BEFORE(listelm, elm, field) do {              \
    (elm)->field.vtqe_prev = (listelm)->field.vtqe_prev;            \
    VTAILQ_NEXT((elm), field) = (listelm);                          \
    *(listelm)->field.vtqe_prev = (elm);                            \
    (listelm)->field.vtqe_prev = &VTAILQ_NEXT((elm), field);        \
} while (0)

I've come across others, but this one highlights what I mean well enough

score 7 · Accepted Answer · edited May 23 '17 at 12:24

7

Long story short:

If you use a non looped macro in an unbraced if else statement, you will get screwed.

if (a)
    VTAILQ_INSERT_BEFORE(c, d, e);
else
    blah(b);

Would break terribly in this case.

Why do I keep seeing multiline macros wrapped in (seemingly) pointless loops?

1 Answers1