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Groups > comp.lang.c++ > #70714
| From | James Kuyper <jameskuyper@alumni.caltech.edu> |
|---|---|
| Newsgroups | comp.lang.c++ |
| Subject | Re: What is the data structure of a NULL? |
| Date | 2019-12-27 20:50 -0500 |
| Organization | A noiseless patient Spider |
| Message-ID | <qu6ce2$sa9$1@dont-email.me> (permalink) |
| References | <qu5spt$ad8$1@dont-email.me> <h6nbqkFctnvU3@mid.individual.net> |
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.
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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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