Difficulty in NULL concept in C?

Question

NULL in C programming can anyone tell me how NULL is handled in C? And the output of this program is 3, how with NULL concept?

#include <stdio.h>

int main(void) {
    int i;
    static int count;
    for(i = NULL; i <= 5;) {
        count++;
        i += 2;
    }

    printf("%d\n",count);
    return 0;
}

Answer 1

For C, "NULL" is traditionally defined to be (void *)0 - in other words, it's a pointer alias to address 0. For C++, "NULL" is typically defined to be "0". The problem with NULL in C++ and C is that it's not type safe - you can build bizarre constructs like the one you included in your code sample.

For C++, the language designers fixed this in C++0x by adding a new "nullptr" type which is implicitly convertable to any pointer type but which cannot be converted to an integer type.

Answer 2

NULL is just a macro defined in stdio.h or a file that stdio includes. It can variously be defined as some variation of zero.

If you run your code through the C pre-processor (usually cc -E) you can see what it translates to on your implemnentation:

void main(){
    int i;
    static int count;
    for(i=((void *)0); i<=5 ;){
       count++;
       i+=2;
    }
    printf("%d",count);
}

which is not only an unnecessary use of NULL but is wildly un-idiomatic C code, more ordinary would be:

int main(){
    int i;
    int count = 0;
    for(i = 0; i <= 5; i += 2){
       count++;
    }
    printf("%d\n",count);
    return 0;
}