Macro using variable arguments

Question

Kindly let me know If there is way where I could use variable arguments inside a macro function and make the below macro code working.

To put it other way, Is there any way where I could use local variables in va_start.

typedef struct
{
    char *p1;
    char *p2;
    char *p3;
}Pointers;

#define NULLCHECK_VR(num, ...)                          \
        {                                               \
            va_list valist;                             \
            int     v_index;                            \
                                                        \
            va_start(valist, num);                      \
                                                        \
            for( v_index=0; v_index < num; v_index++)   \
            {                                           \
                if( NULL == va_arg(valist, void *))     \
                {                                       \
                    printf("NULL \n");                  \
                }                                       \
                else                                    \
                {                                       \
                    printf("NOT NULL \n");              \
                }                                       \
            }                                           \
                                                        \
            va_end(valist);                             \
        }

int main(void)
{
    Pointers ptr;

    memset(&ptr, 0, sizeof(Pointers));

    NULLCHECK_VR(3, ptr.p1, ptr.p2, ptr.p3)

    return (0);
}

MACRO is not function (but text replacement). so va_start(valist, num); is wrong as it applies to .. main. — Jarod42
– Jarod42, Commented Jan 14, 2020 at 10:10
You're mixing C-style variadic functions with variadic macros when you could be using a variadic template (if you're using C++ which I'm not sure you are). — Indiana Kernick
– Indiana Kernick, Commented Jan 14, 2020 at 10:11

Jarod42 · Accepted Answer · 2020-01-14 10:20:06Z

6

You might use regular function:

void NULLCHECK_VR(int num, ...)
{
    va_list valist;
    int     v_index;

    va_start(valist, num);

    for( v_index=0; v_index < num; v_index++)
    {
        if( NULL == va_arg(valist, void *))
        {
            printf("NULL\n");
        }
        else
        {
            printf("NOT NULL\n");
        }
    }

    va_end(valist);
}

or in C++ (17)

template <typename Ts>
void NULLCHECK_VR(const Ts*... ptrs)
{
    ((std::cout << (ptrs == nullptr ? "NULL\n" : "NOT NULL\n")), ...);
}

answered Jan 14, 2020 at 10:20

Jarod42

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Comments

Ian Abbott · Accepted Answer · 2020-01-14 11:19:41Z

You cannot use a va_list to access local variables. The va_start, va_copy and va_end macros are only for handling variadic function arguments. A macro is not a function.

For C variadic macros, the __VA_ARGS__ identifier in the macro replacement text will expand to the variadic arguments, so a possible implementation of your NULLCHECK_VR macro is as follows:

#define NULLCHECK_VR(num, ...)                          \
    do {                                                \
        void *_ptrs[] = { __VA_ARGS__ };                \
        int _num = (num);                               \
        int v_index;                                    \
                                                        \
        for (v_index = 0; v_index < _num; v_index++)    \
        {                                               \
            if (NULL == _ptrs[v_index])                 \
            {                                           \
                printf("NULL \n");                      \
            }                                           \
            else                                        \
            {                                           \
                printf("NOT NULL \n");                  \
            }                                           \
        }                                               \
    } while (0)

Here, __VA_ARGS__ has been used to initialize the local _ptrs array.

(I added local variable _num to avoid expanding the num parameter multiple times, and I used the do while (0) idiom to turn the block into a statement.)

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