Is a pointer pointing to 0x0000 the same as a pointer set to NULL? If NULL value is defined in the C language, then what location does it physically translate to? Is it the same as 0x0000. Where can I find more details about these concepts?
A point that most of the answers here are not addressing, at least not explicitly, is that a null pointer is a value that exists during execution, and a null pointer constant is a syntactic construct that exists in C source code.
A null pointer constant, as Karlson's answer correctly states, is either an integer constant expression with the value 0 (a simple
0 is the most common example), or such an expression cast to
void* (such as
NULL is a macro, defined in
<stddef.h> and several other standard headers, that expands to an implementation-defined null pointer constant. The expansion is typically either
((void*)0) (the outer parentheses are needed to satisfy other language rules).
So a literal
0, when used in a context that requires an expression of pointer type, always evaluates to a
null pointer, i.e., a unique pointer value that points to no object. That does not imply anything about the representation of a null pointer. Null pointers are very commonly represented as all-bits-zero, but they can be represented as anything. But even if a null pointer is represented as
(void*)0 is still a null pointer constant.
This implies, among other things, that
calloc(), which can set a region of memory to all-bits-zero, will not necessarily set any pointers in that region to null pointers. They're likely to do so on most implementations, but the language doesn't guarantee it.
I don't know why this question isn't considered a duplicate of this one, or how it's topical here.
Every platform out there is free to define NULL as it pleases.
According to the C Standard, if you assign zero to a pointer it will be converted to a NULL value (for that platform.) However, if you take a NULL pointer and cast it to int, there are no guarantees that you will get zero on every platform out there. The fact however is that on most platforms it will be zero.
Information about that stuff you can find in The C Language Specification. One source, the trustworthiness of which I cannot vouch for, is this: http://www.winapi.co.kr/pds/doc/ISO-C-FDIS.1999-04.pdf
It is defined in the C language because there is no one unvarying machine address that it equates to. If it did, we wouldn't need an abstraction from it! Even though on most platforms, NULL might eventually be implemented as 0 of some type or other, it's simply wrong to assume that this is universally so, if you care about portability at all.
According to C Standard Document section
An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant.55) 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.
So far I have not seen a compiler that has broken away from this.