On 2020-02-24 23:35, Mouse wrote:
Unless I remember wrong, older C standards explicitly say that the integer 0 can be converted to a pointer, and that will be the NULL pointer, and a NULL pointer cast as an integer shall give the value 0.The only one I have anything close to a copy of is C99, for which I have a very late draft. Based on that: You are not quite correct. Any integer may be converted to a pointer, and any pointer may be converted to an integer - but the mapping is entirely implementation-dependent, except in the integer->pointer direction when the integer is a "null pointer constant", defined as "[a]n integer constant expression with the value 0" (or such an expression cast to void *, though not if we're talking specifically about integers), in which case "the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function". You could have meant that, but what you wrote could also be taken as applying to the _run-time_ integer value 0, which C99's promise does not apply to. (Quotes are from 6.3.2.3.) I don't think there is any promise that converting a null pointer of any type back to an integer will necessarily produce a zero integer.
Maybe we are reading things differently...? Looking at 6.3.2.3... As far as I read, paragraph 3 says:"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."
Essentially, the integer constant 0 can be casted to a pointer, and that pointer is then a null pointer constand, also called a null pointer. And footnote 55 says:
"The macro NULL is defined in <stddef.h> (and other headers) as a null pointer constant; see 7.17."
So, 0 casted as a pointer gives a NULL pointer. And paragraph 6 says:"Any pointer type may be converted to an integer type. Except as previously specified, the result is implementation-defined. If the result cannot be represented in the integer type, the behavior is undefined. The result need not be in the range of values of any integer type."
And I can only read the "previously specified" to refer to the equivalence between a NULL pointer and integer 0, because nothing before paragraph 6 talks about pointer to integer, so I can't see how it can be read as something more specific than all the things mentioned in the prebious 6 paragraphs.
Johnny
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