about #ifndef and macro_function

i have changed complier from msvc to mingw,

macro function : _countof was not inluded in mingw <stblib.h> or <stdio.h>

#define _countof(array) (sizeof(array) / sizeof(array[0]))

maybe it's only for msvc,

so,i have to add the defination into header file myself,

i'm not sure about the code below:

#ifndef _countof(array)
#define _countof(array) (sizeof(array) / sizeof(array[0]))
#endif

It seems that this way of writing is not common.

Solution

The underscore prefix indicates that it is proprietary and not and ISO standard name.

It is the macro name that is tested in #ifndef, so just:

#ifndef _countof
    #define _countof(array) (sizeof(array) / sizeof(array[0]))
#endif

Though personally I prefer:

#if !defined _countof
    #define _countof(array) (sizeof(array) / sizeof(*array))
#endif

Also a macro is just a macro. There is no such thing as a macro function, rather you might refer to a function-like macro, being a macro that takes arguments, but even then semantically they are very different from functions.