Is NULL in C required/defined to be zero?

Question

NULL appears to be zero in my GCC test programs, but wikipedia says that NULL is only required to point to unaddressable memory.

Do any compilers make

Answer 1

From the language standard:

6.3.2.3 Pointers
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3 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.
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55) The macro NULL is defined in (and other headers) as a null pointer constant; see 7.17.

Given that language, the macro NULL should evaluate to a zero-valued expression (either an undecorated literal 0, an expression like (void *) 0, or another macro or expression that ultimately evaluates to 0). The expressions ptr == NULL and !ptr should be equivalent. The second form tends to be more idiomatic C code.

Note that the null pointer value doesn't have to be 0. The underlying implementation may use any value it wants to represent a null pointer. As far as your source code is concerned, however, a zero-valued pointer expression represents a null pointer.