Question

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Why are there sometimes meaningless do/while and if/else statements in C/C++ macros?
What's the use of do while(0) when we define a macro?
how does do{} while(0) work in macro?

I wonder what the use do{ ... } while(0) (... as a place-holder for other code) is, as it would, as far as I know, be exactly the same as just using ....

You can find code like this in the official CPython source. As an example, the Py_DECREF macro:

#define Py_DECREF(op)                                   \
    do {                                                \
        if (_Py_DEC_REFTOTAL  _Py_REF_DEBUG_COMMA       \
        --((PyObject*)(op))->ob_refcnt != 0)            \
            _Py_CHECK_REFCNT(op)                        \
        else                                            \
        _Py_Dealloc((PyObject *)(op));                  \
    } while (0)

Answer 1

It makes compiler require ; so the macro looks like a function call:

Py_DECREF(x); // ok
Py_DECREF(x) // error