Question

I am reading “The practice of programming” and it says that a standard loop for walking along a list in C is:

for (p=list;p != NULL;p=p->next)
...

My question is how does NULL work? I myself have used this in programs, but never paused to ponder over how it works.

I suppose, NULL must be a standard macro defined in a very basic header file like stdio.h
but what it is defined as? Is it?

#define MACRO 0  

here how does 0 work when we compare a pointer to 0. This 0 could be a valid memory address 0, then how do we distinguish to say that we actually mean invalid address i.e. the pointer has a logical empty value?

Thanks,

Answer 1

It’s defined as 0, yes. The thing is that 0 is not a valid memory address; computers are actually put together such that trying to access that address is an error.

Try running this program on your machine, if you want to insist that 0 is a legal address:

#import <stdio.h>

int main() {
  int *p = 0;
  printf("No read error: %d\n", *p);
  *p = 1;
  printf("No write error either\n");
}