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Re: What is the data structure of a NULL?

From T <T@invalid.invalid>
Newsgroups comp.lang.c++
Subject Re: What is the data structure of a NULL?
Date 2019-12-28 17:14 -0800
Organization A noiseless patient Spider
Message-ID <qu8umf$fur$2@dont-email.me> (permalink)
References <qu5spt$ad8$1@dont-email.me> <h6nbqkFctnvU3@mid.individual.net> <qu6ce2$sa9$1@dont-email.me>

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On 2019-12-27 17:50, James Kuyper wrote:
> On 12/27/19 4:36 PM, Ian Collins wrote:
>> On 28/12/2019 10:24, T wrote:
>>> Hi All,
>>>
>>> In C++, when you make a call to Windows Kernel32.dll
>>> and it wants a NULL for one or more of its parameters,
>>> what exactly is the structure of a NULL?  A 32 bit
>>> word full of zeros?
>>
>> The C standard defines a null pointer as
>>
>> "An integer constant expression with the value 0, or such an expression
>> cast to type void *, is called a null pointer constant."
> 
> That's the definition of a "null pointer constant". The definition of a
> null pointer comes on the immediately following line:
> 
> "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."
> Note that the second occurrence of the phrase "null pointer" in that
> sentence is in italics, an ISO convention indicating that this sentence
> constitutes the official definition of the italicized phrase. In this
> case, it's a little unclear; the first part of that sentence is not
> actually the definition, it's a one of several ways to obtain a null
> pointer (other ways include calling various standard library functions
> that are documented as returning null pointers under certain
> circumstances). It's actually the end of that sentence that defines what
> null pointer is: a pointer guaranteed to compare unequal to a pointer to
> any object or function.
> 
> The C standard says nothing about how null pointers are represented. A
> null pointer of type T* has a size in bytes equal to sizeof(T*), a
> number that is implementation-defined, not standard-defined. It is
> commonplace for null pointers to have all bits 0, but that's not
> guaranteed, and there have been a fair number of real-world
> implementations that have used other representations. At least one
> popular system I've used had a segment-offset structure for addresses,
> and any pointer with an offset of 0 was defined by that implementation
> to be a null pointer, regardless of what the segment portion of the
> address was.
> 
> Finally, note that the OP asked about NULL. NULL is the name of a macro,
> #defined in several standard headers, which is required to expand into a
> null pointer constant. There's a huge variety of other ways to write a
> null pointer constant, so you shouldn't confuse "NULL" with "null
> pointer constant". While any null pointer constant (and therefore, in
> particular, NULL) will be converted into a null pointer if used in a
> context where it will be converted to a pointer type, it's also
> important to understand that  null pointer constants are not the same
> thing as null pointers.
> 


Thank you!

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What is the data structure of a NULL? T <T@invalid.invalid> - 2019-12-27 13:24 -0800
  Re: What is the data structure of a NULL? Ian Collins <ian-news@hotmail.com> - 2019-12-28 10:36 +1300
    Re: What is the data structure of a NULL? T <T@invalid.invalid> - 2019-12-27 13:55 -0800
      Re: What is the data structure of a NULL? Bo Persson <bo@bo-persson.se> - 2019-12-28 11:28 +0100
    Re: What is the data structure of a NULL? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-12-27 20:50 -0500
      Re: What is the data structure of a NULL? T <T@invalid.invalid> - 2019-12-28 17:14 -0800
      Re: What is the data structure of a NULL? Tim Rentsch <tr.17687@z991.linuxsc.com> - 2020-01-10 20:56 -0800
  Re: What is the data structure of a NULL? "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2019-12-27 22:42 +0100
    Re: What is the data structure of a NULL? T <T@invalid.invalid> - 2019-12-27 13:55 -0800
      Re: What is the data structure of a NULL? "Alf P. Steinbach" <alf.p.steinbach+usenet@gmail.com> - 2019-12-27 23:10 +0100
      Re: What is the data structure of a NULL? Öö Tiib <ootiib@hot.ee> - 2019-12-27 14:15 -0800
        Re: What is the data structure of a NULL? T <T@invalid.invalid> - 2019-12-27 14:24 -0800
          Re: What is the data structure of a NULL? Öö Tiib <ootiib@hot.ee> - 2019-12-27 14:41 -0800
          Re: What is the data structure of a NULL? Manfred <noname@add.invalid> - 2019-12-28 20:11 +0100
            Re: What is the data structure of a NULL? T <T@invalid.invalid> - 2019-12-28 17:16 -0800
              Re: What is the data structure of a NULL? Manfred <noname@invalid.add> - 2019-12-29 17:52 +0100
                Re: What is the data structure of a NULL? T <T@invalid.invalid> - 2019-12-29 12:21 -0800
      Re: What is the data structure of a NULL? Bart <bc@freeuk.com> - 2019-12-28 01:46 +0000
        Re: What is the data structure of a NULL? T <T@invalid.invalid> - 2019-12-28 17:17 -0800
          Re: What is the data structure of a NULL? Bart <bc@freeuk.com> - 2019-12-29 01:42 +0000
            Re: What is the data structure of a NULL? T <T@invalid.invalid> - 2019-12-28 18:39 -0800
  Re: What is the data structure of a NULL? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2019-12-28 02:14 -0800
    Re: What is the data structure of a NULL? Ned Latham <nedlatham@woden.valhalla.oz> - 2019-12-28 08:33 -0600
      Re: What is the data structure of a NULL? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2019-12-28 13:22 -0800
        Re: What is the data structure of a NULL? "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2019-12-28 16:02 -0800
          Re: What is the data structure of a NULL? Ned Latham <nedlatham@woden.valhalla.oz> - 2019-12-28 22:37 -0600
        Re: What is the data structure of a NULL? Ned Latham <nedlatham@woden.valhalla.oz> - 2019-12-28 22:37 -0600
          Re: What is the data structure of a NULL? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2019-12-28 21:12 -0800
            Re: What is the data structure of a NULL? Ned Latham <nedlatham@woden.valhalla.oz> - 2019-12-29 04:45 -0600
              Re: What is the data structure of a NULL? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2019-12-29 13:31 -0800
                Re: What is the data structure of a NULL? Ned Latham <nedlatham@woden.valhalla.oz> - 2019-12-29 21:34 -0600
                Re: What is the data structure of a NULL? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2019-12-29 22:48 -0800
              Re: What is the data structure of a NULL? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-12-29 20:20 -0500
                Re: What is the data structure of a NULL? Ned Latham <nedlatham@woden.valhalla.oz> - 2019-12-29 21:50 -0600
                Re: What is the data structure of a NULL? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2019-12-29 22:46 -0800
                Re: What is the data structure of a NULL? David Brown <david.brown@hesbynett.no> - 2019-12-30 10:25 +0100
                Re: What is the data structure of a NULL? Ned Latham <nedlatham@woden.valhalla.oz> - 2019-12-30 05:20 -0600
                Re: What is the data structure of a NULL? David Brown <david.brown@hesbynett.no> - 2019-12-30 17:24 +0100
                Re: What is the data structure of a NULL? Ned Latham <nedlatham@woden.valhalla.oz> - 2019-12-30 17:22 -0600
                Re: What is the data structure of a NULL? David Brown <david.brown@hesbynett.no> - 2019-12-31 13:57 +0100
                Re: What is the data structure of a NULL? Ned Latham <nedlatham@woden.valhalla.oz> - 2019-12-31 07:33 -0600
                Re: What is the data structure of a NULL? David Brown <david.brown@hesbynett.no> - 2019-12-31 15:45 +0100
                Re: What is the data structure of a NULL? Ben Bacarisse <ben.usenet@bsb.me.uk> - 2019-12-30 18:16 +0000
                Re: What is the data structure of a NULL? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2019-12-30 10:45 -0800
                Re: What is the data structure of a NULL? Ned Latham <nedlatham@woden.valhalla.oz> - 2019-12-30 17:29 -0600
                Re: What is the data structure of a NULL? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2019-12-30 21:48 -0800
                Re: What is the data structure of a NULL? Ned Latham <nedlatham@woden.valhalla.oz> - 2019-12-30 04:19 -0600
                Re: What is the data structure of a NULL? David Brown <david.brown@hesbynett.no> - 2019-12-30 17:27 +0100
                Re: What is the data structure of a NULL? Öö Tiib <ootiib@hot.ee> - 2019-12-30 03:25 -0800
                Re: What is the data structure of a NULL? Jorgen Grahn <grahn+nntp@snipabacken.se> - 2019-12-31 21:54 +0000
                Re: What is the data structure of a NULL? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-12-30 06:03 -0800
                Re: What is the data structure of a NULL? Ned Latham <nedlatham@woden.valhalla.oz> - 2019-12-30 17:01 -0600
                Re: What is the data structure of a NULL? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-12-30 21:42 -0500
                Re: What is the data structure of a NULL? Ned Latham <nedlatham@woden.valhalla.oz> - 2019-12-31 06:03 -0600
                Re: What is the data structure of a NULL? Öö Tiib <ootiib@hot.ee> - 2019-12-31 06:17 -0800
                Re: What is the data structure of a NULL? Ned Latham <nedlatham@woden.valhalla.oz> - 2019-12-31 10:46 -0600
                Re: What is the data structure of a NULL? Melzzzzz <Melzzzzz@zzzzz.com> - 2019-12-31 16:57 +0000
                Re: What is the data structure of a NULL? David Brown <david.brown@hesbynett.no> - 2019-12-31 18:15 +0100
                Re: What is the data structure of a NULL? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2019-12-31 11:20 -0800
                Re: What is the data structure of a NULL? David Brown <david.brown@hesbynett.no> - 2020-01-01 17:01 +0100
                Re: What is the data structure of a NULL? Ned Latham <nedlatham@woden.valhalla.oz> - 2019-12-31 11:21 -0600
                Re: What is the data structure of a NULL? Jorgen Grahn <grahn+nntp@snipabacken.se> - 2019-12-31 21:37 +0000
                Re: What is the data structure of a NULL? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-12-31 15:21 -0800
                Re: What is the data structure of a NULL? Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2019-12-31 17:26 +0000
                Re: What is the data structure of a NULL? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-12-31 23:30 -0500
                Re: What is the data structure of a NULL? Manfred <noname@add.invalid> - 2020-01-01 18:10 +0100
                Re: What is the data structure of a NULL? James Kuyper <jameskuyper@alumni.caltech.edu> - 2020-01-01 12:21 -0500
                Re: What is the data structure of a NULL? Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2019-12-31 15:20 +0000
                Re: What is the data structure of a NULL? Keith Thompson <Keith.S.Thompson+u@gmail.com> - 2019-12-31 11:11 -0800
                Re: What is the data structure of a NULL? Chris Vine <chris@cvine--nospam--.freeserve.co.uk> - 2019-12-31 19:47 +0000
                Re: What is the data structure of a NULL? James Kuyper <jameskuyper@alumni.caltech.edu> - 2019-12-31 10:27 -0800

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