How can I pass any number of arguments in User define function in C?

Look here for an example.

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

int maxof(int, ...) ;
void f(void);

main(){
        f();
        exit(EXIT SUCCESS);
}

int maxof(int n args, ...){
        register int i;
        int max, a;
        va_list ap;

        va_start(ap, n args);
        max = va_arg(ap, int);
        for(i = 2; i <= n_args; i++) {
                if((a = va_arg(ap, int)) > max)
                        max = a;
        }

        va_end(ap);
        return max;
}

void f(void) {
        int i = 5;
        int j[256];
        j[42] = 24;
        printf("%d\n",maxof(3, i, j[42], 0));
}

Such a function is calle variadic, but such functions are much less useful than they might at first seem. The wikipedia page on the topic is not bad, and has C code.

The basic problem with such functions is that the number of parameters cannot in fact be variable - they are must be fixed at compile time by a known parameter. This is obvious in printf:

printf( "%s %d", "Value is", 42 );

The number of % specifiers must match the number of actual values, and this is true for all other uses of variadic functions in C, in one form or another.