Standard idiom for walking along a list in C

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 ponde开发者_开发百科r 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,

The C FAQ has a whole section on NULL pointers. A short summary:

• the constant int value zero (0), in a context where the compiler knows a pointer is needed, is treated as the "null pointer": a pointer value known to not point at a valid memory address
• this only applies to constants, not any integer values; for example, assigning zero to a variable, and then using that as a pointer, does not guarantee the value will be a null pointer
• the real address may or may not be zero; the compiler may translate it to another address
• several standard library headers define the macro NULL, either as a plain 0 or cast into a pointer: ((void *) 0) (the C standard allows either)

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");
}