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Re: setting pointers to NULL and testing question.

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On 6/16/16 9:27 AM, supercat@casperkitty.com wrote:
> On Thursday, June 16, 2016 at 7:27:49 AM UTC-5, Richard Damon wrote:
>> This is your common fallacy, something 'worked' a given way which you
>> liked, so you assume that that result of undefined behavior must be th
>> definition, and when it changes you blame the implementation for
>> breaking a promise it never made. This attitude has caused more problems
>> in the history of programming, resulting in the canonical faults like
>> the assumption that int = 16 bits, int smaller than long, int = 32 bits,
>> int equivalent to long, ints could hold pointers, longs could hold
>> pointers, etc.
>
>> All of these were commonly held beliefs, and many programmers wrote
>> code, erroneously, assuming them, only to have the code break on the
>> next generation of machines.
>
> If a simplistic compiler for a language for a particular platform, fed a
> particular piece of source code X, will yield 100% reliable machine code
> to perform that task with a certain degree of performance, then if the
> language is any good there should be a piece of source code Y which is
> can be guaranteed to yield those results without significant sacrifices
> in performance or readability.
>
> If one regards the C language as inheriting the traits of the platforms
> upon which it runs, it's a good language.  If one does not, it's a
> crummy language.
>

100% Reliable requires something to PROMISE that it will always work. 
The hardware documentation for the processor may make the promise that 
the given sequence of assembly instructions are 'Reliable', but unless 
you have something which PROMISES that a given Statement will generate a 
given assembly sequence (which the Standard doesn't, and in fact CAN'T) 
you are not 'Reliable', but just counting on happenstance.

>> If you want promises beyond what the standard provides, fine, document
>> the language extensions you need and use implementations that are
>> DOCUMENTED to provide those. Just realize that you now can only use
>> implementation that are documented to provide those extensions.
>
> Many things which are undefined today were well-defined in C89.  Why should
> a compiler have been expected to document that of struct "foo" has a field
> "x" of type "int" at offset 12, and struct "bar" has a field with the same
> name and offset, then casting to "foo" a pointer to either type and accessing
> field "x" will access the same thing, given that
>
>   1. That behavior was specified in C74
>   2. No compilers since had deviated from it
>
> With regard to two's-complement integers, they are mathematically
> incapable of overflow in the same way as unsigned integers, since the
> two's-complement integers of any given size form an abstract algebraic
> ring which is isomorphic to the unsigned integers of the same size.

I have worked on machines that for 16 bit two's complement numbers, 
32767+1 was NOT -32768, on one machine it was 32767 and on another it 
was a trap. The definition (that the standard uses) of signed numbers 
does NOT define them as a ring (like it does for unsigned)
>
>> The strangest part of the complaint, is that the implementations that
>> you rail against actually do DOCUMENT ways to invoke them to have most
>> of the things you want them to have, but you seem to refuse to use that.
>> They also are specifically NOT 'Standards Compliant' in their default
>> mode of execution, but have some non-compliant extensions to the
>> language enabled by default, so you really NEED to read the
>> documentation to properly use them.
>
> I object in part to the notion that things were never defined, when in
> fact programmers were merely expecting that traditional defined behaviors
> would continue, at least until the better alternatives behave available.

For something to be DEFINED, someone has to give a DEFINITION. Just 
because something has seemed to work, doesn't make it a law that it MUST 
work. History is full of people who assume that things that 'just seem 
to work' will continue to work to their detriment.

>
> I would also suggest that in many cases the level of optimization that
> can be achieved by completely revoking the guarantees would be inferior to
> the level that could be achieved by weakening them but not eliminating
> them altogether.
>
> Do you disagree with either point?
>

I don't know well enough what options are really available for 
optimization, as that is not my field of expertise. It does seem logical 
to me that making no promises about behavior in a situation should give 
the compiler more options than a partial promise, as anything that could 
be done with the latter, could also have been done in the former. Maybe 
in the strange cases you like to propose where you don't care about 
getting right answers, just limited undefined behavior, the partial 
promises might be useful. I will observe that the writers of the 
compilers seem to disagree with you, and think that the typical 
programmer wants the better optimizations and is willing to avoid 
undefined behavior to get it.

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setting pointers to NULL and testing question. G G <gdotone@gmail.com> - 2016-06-12 16:21 -0700
  Re: setting pointers to NULL and testing question. Richard Heathfield <rjh@cpax.org.uk> - 2016-06-13 00:50 +0100
    Re: setting pointers to NULL and testing question. G G <gdotone@gmail.com> - 2016-06-12 17:03 -0700
    Re: setting pointers to NULL and testing question. Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2016-06-12 20:25 -0600
      Re: setting pointers to NULL and testing question. James Kuyper <jameskuyper@verizon.net> - 2016-06-12 22:57 -0400
        Re: setting pointers to NULL and testing question. Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2016-06-13 10:03 -0600
          Re: setting pointers to NULL and testing question. Tim Rentsch <txr@alumni.caltech.edu> - 2016-06-15 14:43 -0700
      Re: setting pointers to NULL and testing question. supercat@casperkitty.com - 2016-06-13 08:06 -0700
        Re: setting pointers to NULL and testing question. Richard Damon <Richard@Damon-Family.org> - 2016-06-16 08:27 -0400
          Re: setting pointers to NULL and testing question. supercat@casperkitty.com - 2016-06-16 06:27 -0700
            Re: setting pointers to NULL and testing question. supercat@casperkitty.com - 2016-06-16 07:31 -0700
            Re: setting pointers to NULL and testing question. Richard Damon <Richard@Damon-Family.org> - 2016-06-18 13:39 -0400
              Re: setting pointers to NULL and testing question. supercat@casperkitty.com - 2016-06-18 11:37 -0700
                Re: setting pointers to NULL and testing question. Richard Damon <Richard@Damon-Family.org> - 2016-06-18 17:17 -0400
                Re: setting pointers to NULL and testing question. supercat@casperkitty.com - 2016-06-18 17:22 -0700
      Re: setting pointers to NULL and testing question. James Kuyper <jameskuyper@verizon.net> - 2016-06-13 14:10 -0400
        Re: setting pointers to NULL and testing question. Keith Thompson <kst-u@mib.org> - 2016-06-13 11:28 -0700
        Re: setting pointers to NULL and testing question. Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2016-06-13 12:31 -0600
    Re: setting pointers to NULL and testing question. Keith Thompson <kst-u@mib.org> - 2016-06-12 19:38 -0700
      Re: setting pointers to NULL and testing question. Richard Heathfield <rjh@cpax.org.uk> - 2016-06-13 07:35 +0100
    Re: setting pointers to NULL and testing question. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-06-13 11:40 +0100
      Re: setting pointers to NULL and testing question. BartC <bc@freeuk.com> - 2016-06-13 11:56 +0100
        Re: setting pointers to NULL and testing question. Martin Shobe <mashobe9@gmail.com> - 2016-06-13 06:22 -0500
          Re: setting pointers to NULL and testing question. BartC <bc@freeuk.com> - 2016-06-13 12:39 +0100
            Re: setting pointers to NULL and testing question. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-06-13 15:22 +0100
              Re: setting pointers to NULL and testing question. BartC <bc@freeuk.com> - 2016-06-13 17:34 +0100
                Re: setting pointers to NULL and testing question. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-06-13 19:31 +0100
                Re: setting pointers to NULL and testing question. BartC <bc@freeuk.com> - 2016-06-13 20:17 +0100
                Re: setting pointers to NULL and testing question. Keith Thompson <kst-u@mib.org> - 2016-06-13 12:47 -0700
                Re: setting pointers to NULL and testing question. Tim Rentsch <txr@alumni.caltech.edu> - 2016-06-15 16:44 -0700
                Re: setting pointers to NULL and testing question. Keith Thompson <kst-u@mib.org> - 2016-06-15 17:20 -0700
      Re: setting pointers to NULL and testing question. Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-06-13 04:01 -0700
        Re: setting pointers to NULL and testing question. Tim Rentsch <txr@alumni.caltech.edu> - 2016-06-16 12:57 -0700
      Re: setting pointers to NULL and testing question. Richard Heathfield <rjh@cpax.org.uk> - 2016-06-13 12:18 +0100
        Re: setting pointers to NULL and testing question. David Brown <david.brown@hesbynett.no> - 2016-06-13 14:01 +0200
          Re: setting pointers to NULL and testing question. Richard Heathfield <rjh@cpax.org.uk> - 2016-06-13 14:15 +0100
            Re: setting pointers to NULL and testing question. David Brown <david.brown@hesbynett.no> - 2016-06-13 15:37 +0200
              Re: setting pointers to NULL and testing question. Richard Heathfield <rjh@cpax.org.uk> - 2016-06-13 14:48 +0100
        Re: setting pointers to NULL and testing question. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-06-13 15:43 +0100
  Re: setting pointers to NULL and testing question. Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-06-12 17:54 -0700
  Re: setting pointers to NULL and testing question. John Bode <jfbode1029@gmail.com> - 2016-06-14 08:36 -0700
    Re: setting pointers to NULL and testing question. Richard Heathfield <rjh@cpax.org.uk> - 2016-06-14 16:41 +0100
    Re: setting pointers to NULL and testing question. Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-06-14 09:01 -0700
    Re: setting pointers to NULL and testing question. "James R. Kuyper" <jameskuyper@verizon.net> - 2016-06-14 13:25 -0400
      Re: setting pointers to NULL and testing question. supercat@casperkitty.com - 2016-06-14 10:48 -0700
        Re: setting pointers to NULL and testing question. Ian Collins <ian-news@hotmail.com> - 2016-06-15 08:24 +1200
          Re: setting pointers to NULL and testing question. "James R. Kuyper" <jameskuyper@verizon.net> - 2016-06-14 16:57 -0400
            Re: setting pointers to NULL and testing question. supercat@casperkitty.com - 2016-06-14 14:29 -0700
              Re: setting pointers to NULL and testing question. BartC <bc@freeuk.com> - 2016-06-14 22:41 +0100
                Re: setting pointers to NULL and testing question. supercat@casperkitty.com - 2016-06-14 15:16 -0700
                Re: setting pointers to NULL and testing question. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-06-15 00:25 +0100
                Re: setting pointers to NULL and testing question. Keith Thompson <kst-u@mib.org> - 2016-06-14 16:58 -0700
                Re: setting pointers to NULL and testing question. David Brown <david.brown@hesbynett.no> - 2016-06-15 09:28 +0200
                Re: setting pointers to NULL and testing question. Keith Thompson <kst-u@mib.org> - 2016-06-15 09:57 -0700
              Re: setting pointers to NULL and testing question. David Brown <david.brown@hesbynett.no> - 2016-06-15 09:23 +0200
                Re: setting pointers to NULL and testing question. Richard Heathfield <rjh@cpax.org.uk> - 2016-06-15 08:35 +0100
                Re: setting pointers to NULL and testing question. Richard Heathfield <rjh@cpax.org.uk> - 2016-06-15 08:37 +0100
                Re: setting pointers to NULL and testing question. David Brown <david.brown@hesbynett.no> - 2016-06-15 10:42 +0200
                Re: setting pointers to NULL and testing question. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-06-15 13:28 +0100
                Re: setting pointers to NULL and testing question. supercat@casperkitty.com - 2016-06-15 11:41 -0700
                Re: setting pointers to NULL and testing question. raltbos@xs4all.nl (Richard Bos) - 2016-06-15 19:43 +0000
                Re: setting pointers to NULL and testing question. supercat@casperkitty.com - 2016-06-15 13:09 -0700
                Re: setting pointers to NULL and testing question. David Brown <david.brown@hesbynett.no> - 2016-06-15 22:31 +0200
                Re: setting pointers to NULL and testing question. Eric Sosman <esosman@comcast-dot-net.invalid> - 2016-06-15 17:11 -0400
                Re: setting pointers to NULL and testing question. raltbos@xs4all.nl (Richard Bos) - 2016-06-18 20:44 +0000
                Re: setting pointers to NULL and testing question. Eric Sosman <esosman@comcast-dot-net.invalid> - 2016-06-18 17:20 -0400
        Re: setting pointers to NULL and testing question. Malcolm McLean <malcolm.mclean5@btinternet.com> - 2016-06-14 23:22 -0700
          Re: setting pointers to NULL and testing question. Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-06-15 13:23 +0100
      Re: setting pointers to NULL and testing question. John Bode <jfbode1029@gmail.com> - 2016-06-20 13:36 -0700
    Re: setting pointers to NULL and testing question. Eric Sosman <esosman@comcast-dot-net.invalid> - 2016-06-14 14:59 -0400
      Re: setting pointers to NULL and testing question. "James R. Kuyper" <jameskuyper@verizon.net> - 2016-06-14 15:17 -0400
      Re: setting pointers to NULL and testing question. Richard Heathfield <rjh@cpax.org.uk> - 2016-06-14 20:25 +0100
      Re: setting pointers to NULL and testing question. Tim Rentsch <txr@alumni.caltech.edu> - 2016-06-15 19:48 -0700
        Re: setting pointers to NULL and testing question. Eric Sosman <esosman@comcast-dot-net.invalid> - 2016-06-15 23:25 -0400
          Re: setting pointers to NULL and testing question. James Kuyper <jameskuyper@verizon.net> - 2016-06-16 12:11 -0400
            Re: setting pointers to NULL and testing question. supercat@casperkitty.com - 2016-06-16 09:25 -0700
            Re: setting pointers to NULL and testing question. Tim Rentsch <txr@alumni.caltech.edu> - 2016-06-16 21:46 -0700
              Re: setting pointers to NULL and testing question. Gareth Owen <gwowen@gmail.com> - 2016-06-17 06:13 +0100
                Re: setting pointers to NULL and testing question. Robert Wessel <robertwessel2@yahoo.com> - 2016-06-17 02:03 -0500
                Re: setting pointers to NULL and testing question. Martin Shobe <mashobe9@gmail.com> - 2016-06-17 07:07 -0500
                Re: setting pointers to NULL and testing question. supercat@casperkitty.com - 2016-06-17 03:18 -0700
                Re: setting pointers to NULL and testing question. James Kuyper <jameskuyper@verizon.net> - 2016-06-17 09:16 -0400
                Re: setting pointers to NULL and testing question. Tim Rentsch <txr@alumni.caltech.edu> - 2016-06-21 11:03 -0700
                Re: setting pointers to NULL and testing question. Tim Rentsch <txr@alumni.caltech.edu> - 2016-06-21 10:58 -0700
              Re: setting pointers to NULL and testing question. Keith Thompson <kst-u@mib.org> - 2016-06-21 11:28 -0700
                Re: setting pointers to NULL and testing question. Tim Rentsch <txr@alumni.caltech.edu> - 2016-07-18 08:24 -0700
          Re: setting pointers to NULL and testing question. Tim Rentsch <txr@alumni.caltech.edu> - 2016-06-16 21:39 -0700

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