What is the difference between "NULL", "0" and 0?

Question

Both are worth zero. I can use the 3 interchangeably always?

Answer 1

All 3 are constant literals worth 0, that’s correct.

0

0 is a numerical value and can be used whenever you need the same zero number. In some places it ends up being used for other things. Where a boolean result is expected 0 is considered false, while any other value is considered true.

\0

This is the null character. Compilers often define a macro called NUL with this value, which may even have more than one byte.

It really doesn’t stop being a 0 and it can usually be used where one expects a zero. The slider is used as an escape to define that you want the zero byte there and not the character 0, which is something quite different from the character 0 (is byte 48). So this form is only used within a literal character or string. This character is the terminator of string. This is important since C doesn’t keep the size of string standardized.

In the case of char it is possible to use only the 0 even, as long as you do not use single quotes *not all compilers in some linked options accept). Remember that type char is a numeric type like any other. Actually it can be used to print characters, but it’s just a different representation. Computers only understand numbers, the rest is representation.

NULL

This is just one #define with a zero value. In fact there are some implementations that the value is 0 same. But it is more common the value (void *)0, making it clear that this is actually a pointer to the memory’s zero address, so a null pointer.

A 0 is not that interchangeable. If you try to compare a NULL, that is to say void * 0 even works because it makes a cast automatic. But if you try to compare a NULL with a whole will not happen what you expect. In fact it is generated a Warning and it’s possible that the compiler won’t even let it finish.

A code demonstrating all this:

#include <stdio.h>

int main(void) {
    int i = 0;
    char c = 0;
    char *s = "teste";
    void *p = NULL;
    printf("%d %c %s %p\n", i, c, s, p);
    if (!c) printf("i ok\n"); //verifica se não é zero
    if (!c) printf("c ok\n"); //verifica se não é um caractere nulo
    if (!s[5]) printf("s ok\n"); //verifica se o 6o. caractere é o terminador
    if (!p) printf("p ok\n"); //verifica se o ponteiro não é nulo
//  if (p == i) printf("p NULL ok\n"); //tipos incompatíveis sendo comparados
    printf("%zu", sizeof('x'));
}

Behold working in the ideone. And in the repl it.. Also put on the Github for future reference.

Answer 2

I wanted to make an addendum to @Maniero’s reply.

The Standard C does not require that NULL is set with all bits to 0.

§7.20.3.2

...

2 The calloc Function allocates space for an array of nmemb Objects, each of Whose size is size. The space is initialized to all bits zero.²⁵⁵⁾

...

255) Note that this need not be the same as the representation of floating-point zero or a null Pointer Constant.

Translation:

§7.20.3.2

...

2 The calloc function allocates space to an array of nmemb objects, each with size size. Space is initialized with bits to 0.²⁵⁵⁾

...

255) Note that this does not have to be the same representation of 0 in floating representation, or the null pointer constant.