Thread: explain me about NULL

NULL is a macro defined in <stddef.h> to be a null pointer constant. This means that it is typically defined as 0 or ((void*)0).

It's also defined in <stdio.h>

EDIT: FAQ

Originally Posted by RockyMarrone

Extention of this
C
#define NULL 0

C++

#define NULL ((void*)0)

No, NULL is implementation defined, and a standard conforming C++ implementation will not define NULL as ((void*)0) in <cstddef>.

Originally Posted by firedream

but I still don't understand what is utility of NULL

What do you understand of pointers and null pointers?

Originally Posted by firedream

hmm
thx u all
but I still don't understand what is utility of NULL
sry
but I'm still beginner in programming ^^

NULL simply means that no memory is allocated to the pointer. For eg.

Code:

int *p; //uninitialized pointer which is dangerous
int *p=NULL; // no memory is allocated better than the previous one

Originally Posted by BEN10

NULL simply means that no memory is allocated to the pointer.

Right -- because the value of a pointer is a memory address. Guess where 0 is?

You could also assign "2" to a pointer and it will respond like it is NULL, because 2 is not a valid address either.

One useful thing about NULL is that it is 0. 0 is the only value in C that will fail a truth test:

Code:

char *ptr = NULL;
if (ptr) {
    blah blah

"blah blah" will not happen if ptr == 0 (NULL). Notice that many C functions will return a NULL pointer to indicate an error or some kind of failure. Eg, if you use strstr(), which searches for a substring (a pattern, such as "wo") in a longer string (such as "hello world") you get a pointer to the beginning of "wo" if it is found, and a NULL pointer if it is not. If it is found you probably want to do something that you will not want to do otherwise, so the truth test is very useful -- really, it is the basic and fundamental concept in programming.

Nb., a pointer assigned and invalid address (like 2) is effectively NULL and will behave that way but it will pass a truth test. So will an uninitialized pointer. Which is why we use NULL

@MK27

Code:

int *ptr;
ptr=10; //or any number

Will ptr be allocated no memory in this case too? It's surprising!!

Originally Posted by MK27

Right -- because the value of a pointer is a memory address. Guess where 0 is?

No, semantically a null pointer does not point to any memory location, not even address 0.

Originally Posted by MK27

You could also assign "2" to a pointer and it will respond like it is NULL, because 2 is not a valid address either.
(...)
Nb., a pointer assigned and invalid address (like 2) is effectively NULL and will behave that way but it will pass a truth test. So will an uninitialized pointer. Which is why we use NULL

Not quite:

Originally Posted by C99 Section 6.3.2.3 Paragraphs 3 to 5

An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant. If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.

Conversion of a null pointer to another pointer type yields a null pointer of that type. Any two null pointers shall compare equal.

An integer may be converted to any pointer type. Except as previously specified, the result is implementation-defined, might not be correctly aligned, might not point to an entity of the referenced type, and might be a trap representation.

Originally Posted by BEN10

Will ptr be allocated no memory in this case too? It's surprising!!

Technically, looking at the quote from laserlight (the result is implementation-defined, might not be correctly aligned, might not point to an entity of the referenced type, and might be a trap representation) I suppose it is more or less "undefined" what will happen, but realistically 2 is not a valid address so there can be no memory there.

Another interesting point about that correction is that two NULL pointers will not == one another, which I had not noticed this, so there is bit more to NULL than it just being

#define NULL 0

which I thought it was exactly that.

Originally Posted by MK27

Another interesting point about that correction is that two NULL pointers will not == one another

Eh, I think you misread. The standard states that "any two null pointers shall compare equal", not that they shall not compare equal.

Originally Posted by laserlight

Eh, I think you misread. The standard states that "any two null pointers shall compare equal", not that they shall not compare equal.

I guess that's why I hadn't noticed. I was thinking I had done that, too (compared NULL pointers). It seems to me that is more useful, so good!