§ of the C99 standard says 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. § 7.17 also says [.] NULL which expands to an implementation-defined null pointer constant [.] The address of the NULL pointer might be different from 0, while it will behave like it was in most cases. ( This should be the same as in older C standards, which I don't have at hand right now).
0 is a number, you use it to check if a numeric value (int, short, float, double, etc.) is the number 0. null is the value of a reference that points nowhere, you use it to make sure a reference does indeed reference something. nothing is not part of Java. The closest thing for nothing (for: no information at all) is the void declaration of a method. It states that the method returns literally nothing. Note that 0, null and the empty string are values and thus contain information. For example, 0 is the answer to What is 5 subtracted from 5? (among others). null is the negative response to Does this thing point to an object?. could be the answer to What is the longest sequenc of ' X's in your name?, unless your name is Xanthippe.
Null Pointers The integer constant literal 0 has different meanings depending upon the context in which it's used. In all cases, it is still an integer constant with the value 0, it is just described in different ways. If a pointer is being compared to the constant literal 0, then this is a check to see if the pointer is a null pointer. This 0 is then referred to as a null pointer constant. The C standard defines that 0 cast to the type void * is both a null pointer and a null pointer constant. Additionally, to help readability, the macro NULL is provided in the header file stddef.h. Depending upon your compiler it might be possible to undef NULL and redefine it to something wacky. Anyone doing this deserves to be shot. Therefore, here are some valid ways to check for a null pointer: if (pointer = NULL) NULL is defined to compare equal to a null pointer. It is implementation defined what the actual definition of NULL is, as long as it is a valid null pointer constant. if (pointer = 0) 0 is another representation of the null pointer constant. if (!pointer) This if statement implicitly checks is not 0, so we reverse that to mean is 0. The following are INVALID ways to check for a null pointer: int mynull = 0; if (pointer = mynull) To the compiler this is not a check for a null pointer, but an equality check on two variables. This might work if mynull never changes in the code and the compiler optimizations constant fold the 0 into the if statement, but this is not guaranteed and the compiler has to produce at least one diagnostic message (warning or error) according to the C Standard. Note that what is a null pointer in the C language. It does not matter on the underlying architecture. If the underlying architecture has a null pointer value defined as address 0x DEADBEEF, then it is up to the compiler to sort this mess out. As such, even on this funny.
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