Thread: Null

Hi, I'm wondering about NULL pointer how its exactly work in C!!!!!!

In brief, NULL pointer means that there's no deferencing to any other byte's addresses, so what I can conclude is this following syntax writing as:

Code:

char *str="CBOARD"
while (*str!=NULL) 
{/*..do something..*/
str++
}

is valid in C, but I can't write and change the loop's while condition with

Code:

 while (str==NULL)

Am I totally write or just missed up the concept of NULL pointer??!!!

Read this: Question 5.1
More here: Null Pointers

Why not make a complete example?

Code:

#include <stdio.h>

int main(void)
{
    char *str = "CBOARD";

    while (*str != NULL)
    {
        /*..do something..*/
        str++;
    }

    return 0;
}

/*
    main.c|7|warning: comparison between pointer and integer
*/

As you can see, your first example generates a warning because you're dereferencing the pointer for the comparison.

Your second example should actually work:

Code:

#include <stdio.h>

int main(void)
{
    char *str = "CBOARD";

    while (str == NULL)
    {
        printf("In the loop\n");
        str++;
    }

    return 0;
}

There is no output because "str" is pointing somewhere - it is not NULL.

*"abc" == 'a'
*"bc" == 'b'
*"c" == 'c'
*"" == '\0'

Now if you understand that, understand that you're guaranteed "" != NULL and that any address of any object is guaranteed to compare unequal to NULL. So there are no null pointers here, not even in ""+1.

For what it's worth, I prefer to call a null pointer NULL, and the C end of string mask (or null character) NUL, with just one L. This matches the C usage for the pointer, and the ASCII (and Unicode) tables for the character.

Originally Posted by laserlight

NULL isn't actually a null pointer

True, it just expands to one. (I hope that was your point, because this is C and not C++ we're talking about. )

Originally Posted by laserlight

'\0' is a null pointer constant

Not really; it's an integer (and thus not a pointer) constant, which you can use as a pointer constant.

The difference is subtle, like saying that a cream pie is a comic device. To me, it's not, it's just a delicious dessert. (I don't find food fights or pieing or similar antics funny at all, just a waste of sweet edibles to me.)

Originally Posted by laserlight

It doesn't always hold true in C because NULL could be of type void* and hence actually a null pointer

Yeah; I've only seen it defined as ((void *)0) (even on old Unix boxes), so I didn't catch that.

Originally Posted by laserlight

No, '\0' really is a null pointer constant

Is it? Because "An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant" obviously requires the cast?

(At this point, we're so deep in the semantics and language interpretation that my English skills are definitely not up to this. I think I might just be having fun teasing you. If so, apologies. )

Originally Posted by laserlight

Anyway, no: because of the "or", the statement says that there are two kinds of expressions that are called null pointer constants:

Okay, that makes sense. (That is, you're right. The standard does not make much sense, but that's nothing new.)

To RyanC, your initial post indicates you've probably confused the two (NULL and '\0').

Consider this "safe" variant of strlen():

Code:

size_t length(const char *const s)
{
    size_t n = 0;

    if (s != NULL) {
        while (s[n] != '\0')
            n++;
    }

    return n;
}

It differs from the strlen() function in that this one checks for the NULL pointer (and returns 0 in that case).

I guess the reason NULL is defined, in C99 at least, as the less type strict version of the null pointer constant because implicit casting makes it okay. How unfortunate.