Can we use recursive macro to create varadic max/min?

Question

Need max/min with unknown number of arguments, like :

#define MAX_N(first, second, remain...) MAX_N(((first)>(second)?(first):(second)), ##remain)

In my opinion, this will not keep expanding infinitely and should be accepted by compiler?

Unfortunately, I have to use pure C rather than C++.

No, this will not work. C macros are not expanded recursively. — Thomas Jager, Feb 08 '21 at 02:32
Though if you do want this behavior, use a function that take a `va_list`. — Alexander, Feb 08 '21 at 02:37
But no termination condition is different from this situation which is limited to decrease varadic params — Abyss Cybort Gin, Feb 08 '21 at 02:37
@AbyssCybortGin Whether or not it finds an end to the recursion has no influence on whether or not its allowed. The macro currently being expanded is simply not recognized in further expansion. — Thomas Jager, Feb 08 '21 at 02:39
@Alexander thx, but I need to use this at compile time, like to define arrays. — Abyss Cybort Gin, Feb 08 '21 at 02:39
@Thomas Jager Well I'm talking about the reason it's not allowed. If it can actually work, why not allowed? — Abyss Cybort Gin, Feb 08 '21 at 02:48
@AbyssCybortGin That's just the way the language is defined. The way macros are evaluated may also not lend itself well to this. C macros aren't functions, they're just fancy text replacement. — Thomas Jager, Feb 08 '21 at 02:56
@AbyssCybortGin Also, what you have would in fact infinetly recurse. — Thomas Jager, Feb 08 '21 at 02:58

score 2 · Answer 1 · answered Feb 08 '21 at 03:08

C 2018 6.10.3.4 2 says, about rescanning the result of macro replacement for further macros:

If the name of the macro being replaced is found during this scan of the replacement list (not including the rest of the source file’s preprocessing tokens), it is not replaced. Furthermore, if any nested replacements encounter the name of the macro being replaced, it is not replaced…

Therefore, recursive macros are not possible, not even just for two levels, let alone indefinitely many.

It is possible to cause macros to be expanded multiple times by using other macros to expand them. As a simple example, after #define X Y Y, X will be replaced by two occurrences of Y, each of which, if it is a defined macro, will be replaced—but separately, not recursively. This can be exploited to create macros that cause sone finitely limited number of expansions, but indefinitely many expansions are not possible.

score 0 · Answer 2 · answered Feb 08 '21 at 07:27

Why do you need a recursive macro? you can write a variadic min/max function like this:

#include <stdio.h>
#include <stdarg.h>

// credit for NUMARGS: https://stackoverflow.com/a/2124433/2889478
#define NUMARGS(...)  (sizeof((int[]){__VA_ARGS__})/sizeof(int))
#define MAX(...) (max_variadic(NUMARGS(__VA_ARGS__), __VA_ARGS__))

int max_variadic(int narg, ...)
{
    va_list args;
    int pos = 0, max_val = 0;

    va_start(args, narg);
    while (narg--) {
        int i = va_arg(args, int);
        max_val = !pos ? i : i > max_val ? i : max_val;
        pos++;
    }
    va_end(args);
    return max_val;
}

int main(void)
{
    int mx = MAX(4, 6, -1, 9, 2);
    printf("%d", mx);
}

Can we use recursive macro to create varadic max/min?

2 Answers2