What is the difference between NULL, '\0' and 0?

Question

In C, there appear to be differences between various values of zero -- NULL, NUL and 0.

I know that the ASCII character

Answer 1

NULL is not guaranteed to be 0 -- its exact value is architecture-dependent. Most major architectures define it to (void*)0.

'\0' will always equal 0, because that is how byte 0 is encoded in a character literal.

I don't remember whether C compilers are required to use ASCII -- if not, '0' might not always equal 48. Regardless, it's unlikely you'll ever encounter a system which uses an alternative character set like EBCDIC unless you're working on very obscure systems.

The sizes of the various types will differ on 64-bit systems, but the integer values will be the same.

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Some commenters have expressed doubt that NULL be equal to 0, but not be zero. Here is an example program, along with expected output on such a system:

#include 

int main () {
    size_t ii;
    int *ptr = NULL;
    unsigned long *null_value = (unsigned long *)&ptr;
    if (NULL == 0) {
        printf ("NULL == 0\n"); }
    printf ("NULL = 0x");
    for (ii = 0; ii < sizeof (ptr); ii++) {
        printf ("%02X", null_value[ii]); }
    printf ("\n");
    return 0;
}

That program could print:

NULL == 0
NULL = 0x00000001