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Re: Pointers, constant values

From Bartc <bc@freeuk.com>
Newsgroups comp.lang.c
Subject Re: Pointers, constant values
Date 2015-10-16 21:20 +0100
Organization A noiseless patient Spider
Message-ID <mvrm33$d10$1@dont-email.me> (permalink)
References (1 earlier) <ln4mhq7s4z.fsf@kst-u.example.com> <mvr9nb$ra4$1@dont-email.me> <ln8u726308.fsf@kst-u.example.com> <mvrjar$2ji$1@dont-email.me> <87io66r30y.fsf@bsb.me.uk>

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On 16/10/2015 21:07, Ben Bacarisse wrote:
> Bartc <bc@freeuk.com> writes:
>
>> On 16/10/2015 20:13, Keith Thompson wrote:
>>> Bartc <bc@freeuk.com> writes:
>>
>>>>    int array_sum(int (*array)[], int n);
>>>
>>> Yes, but there's not much you can do with it, since you haven't
>>> specified the length of the array it points to.  (I'm a little surprised
>>> that that declaration is legal.)
>>
>> It's a pointer to an unbounded array.
>
> It's a pointer to an incomplete array type and, as such, is compatible
> with a pointer to an int array of any length.  Talking about an
> unbounded array in C sounds very odd to me (but I did know what you
> mean).

It's the error you get if you compile with g++; it talks about unknown 
bounds.

> In C99 there's no reason not to go the whole hog and write
>
>    int array_sum(int n, int (*data)[n]) { ... }

Possibly, if you're passing the length too.

> Now you *can* ask about sizeof *data, and even the usual ARRAY_SIZE
> macro trick will work[1].  Of course the answer will be n, so there's
> not much point, but then there was not much point in passing a pointer
> to an array in the first place!

Why not? If you want to access the array from inside the function, my 
point is it makes more sense, type-wise, to pass a pointer to the array, 
rather than a pointer to an int.

Because that pointer-to-int might point to an int in some array, but it 
can also point to an int anywhere else. And if you take the function 
definition in isolation (in some library for example), then you are only 
relying on comments to tell you the parameter is meant to be an array; 
it's not enforced by the language.

-- 
Bartc

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Thread

Pointers, constant values Alla _ <modelling.data@gmail.com> - 2015-10-16 00:49 -0700
  Re: Pointers, constant values Philip Lantz <prl@canterey.us> - 2015-10-16 01:49 -0700
  Re: Pointers, constant values James Kuyper <jameskuyper@verizon.net> - 2015-10-16 09:41 -0400
    Re: Pointers, constant values supercat@casperkitty.com - 2015-10-16 08:55 -0700
  Re: Pointers, constant values Keith Thompson <kst-u@mib.org> - 2015-10-16 08:25 -0700
    Re: Pointers, constant values supercat@casperkitty.com - 2015-10-16 09:11 -0700
      Re: Pointers, constant values Keith Thompson <kst-u@mib.org> - 2015-10-16 12:06 -0700
        Re: Pointers, constant values supercat@casperkitty.com - 2015-10-16 12:55 -0700
    Re: Pointers, constant values Bartc <bc@freeuk.com> - 2015-10-16 17:49 +0100
      Re: Pointers, constant values Lew Pitcher <lew.pitcher@digitalfreehold.ca> - 2015-10-16 13:23 -0400
        Re: Pointers, constant values Bartc <bc@freeuk.com> - 2015-10-16 19:34 +0100
          Re: Pointers, constant values supercat@casperkitty.com - 2015-10-16 12:05 -0700
        Re: Pointers, constant values Keith Thompson <kst-u@mib.org> - 2015-10-16 12:18 -0700
      Re: Pointers, constant values Keith Thompson <kst-u@mib.org> - 2015-10-16 12:13 -0700
        Re: Pointers, constant values Bartc <bc@freeuk.com> - 2015-10-16 20:33 +0100
          Re: Pointers, constant values Keith Thompson <kst-u@mib.org> - 2015-10-16 13:01 -0700
            Re: Pointers, constant values Bartc <bc@freeuk.com> - 2015-10-16 21:09 +0100
              Re: Pointers, constant values Keith Thompson <kst-u@mib.org> - 2015-10-16 13:41 -0700
          Re: Pointers, constant values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-16 21:07 +0100
            Re: Pointers, constant values Bartc <bc@freeuk.com> - 2015-10-16 21:20 +0100
              Re: Pointers, constant values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-16 22:02 +0100
                Re: Pointers, constant values Bartc <bc@freeuk.com> - 2015-10-16 22:21 +0100
                Re: Pointers, constant values supercat@casperkitty.com - 2015-10-16 14:39 -0700
                Re: Pointers, constant values Keith Thompson <kst-u@mib.org> - 2015-10-16 15:26 -0700
                Re: Pointers, constant values supercat@casperkitty.com - 2015-10-16 15:44 -0700
                Re: Pointers, constant values Keith Thompson <kst-u@mib.org> - 2015-10-16 16:58 -0700
                Re: Pointers, constant values supercat@casperkitty.com - 2015-10-16 17:28 -0700
                Re: Pointers, constant values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-16 22:50 +0100
                Re: Pointers, constant values supercat@casperkitty.com - 2015-10-16 15:14 -0700
                Re: Pointers, constant values Bartc <bc@freeuk.com> - 2015-10-17 01:23 +0100
                Re: Pointers, constant values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-17 19:39 +0100
                Re: Pointers, constant values Bartc <bc@freeuk.com> - 2015-10-17 22:42 +0100
                Re: Pointers, constant values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-18 00:31 +0100
                Re: Pointers, constant values Bartc <bc@freeuk.com> - 2015-10-18 02:11 +0100
                Re: Pointers, constant values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-18 11:59 +0100
                Re: Pointers, constant values Bartc <bc@freeuk.com> - 2015-10-18 12:12 +0100
                Re: Pointers, constant values Bartc <bc@freeuk.com> - 2015-10-18 12:28 +0100
                Re: Pointers, constant values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-18 23:16 +0100
                Re: Pointers, constant values Bartc <bc@freeuk.com> - 2015-10-18 23:34 +0100
                Re: Pointers, constant values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-19 01:45 +0100
                Re: Pointers, constant values Bartc <bc@freeuk.com> - 2015-10-19 11:51 +0100
                Re: Pointers, constant values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-19 12:55 +0100
                Re: Pointers, constant values Bartc <bc@freeuk.com> - 2015-10-19 13:51 +0100
                Re: Pointers, constant values Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-10-19 06:02 -0700
                Re: Pointers, constant values supercat@casperkitty.com - 2015-10-19 07:02 -0700
                Re: Pointers, constant values Bartc <bc@freeuk.com> - 2015-10-19 15:39 +0100
                Re: Pointers, constant values supercat@casperkitty.com - 2015-10-19 08:54 -0700
                Re: Pointers, constant values Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-10-19 09:14 -0700
                Re: Pointers, constant values supercat@casperkitty.com - 2015-10-19 10:21 -0700
                Re: Pointers, constant values Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-10-19 12:57 -0700
                Re: Pointers, constant values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-19 21:20 +0000
                Re: Pointers, constant values supercat@casperkitty.com - 2015-10-20 20:50 -0700
                Re: Pointers, constant values glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-10-21 04:58 +0000
                Re: Pointers, constant values Keith Thompson <kst-u@mib.org> - 2015-10-19 08:40 -0700
                Re: Pointers, constant values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-19 20:17 +0100
                Re: Pointers, constant values supercat@casperkitty.com - 2015-10-17 16:44 -0700
                Re: Pointers, constant values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-18 11:52 +0100
                Re: Pointers, constant values Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-10-18 05:15 -0700
                Re: Pointers, constant values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-18 21:54 +0100
                Re: Pointers, constant values Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-10-18 16:09 -0700
                Re: Pointers, constant values supercat@casperkitty.com - 2015-10-18 07:55 -0700
                Re: Pointers, constant values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-18 21:47 +0100
                Re: Pointers, constant values supercat@casperkitty.com - 2015-10-18 15:37 -0700
                Re: Pointers, constant values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-19 00:10 +0100
                Re: Pointers, constant values supercat@casperkitty.com - 2015-10-18 19:04 -0700
        Re: Pointers, constant values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-16 21:13 +0100
      Re: Pointers, constant values Ian Collins <ian-news@hotmail.com> - 2015-10-17 11:27 +1300
        Re: Pointers, constant values Bartc <bc@freeuk.com> - 2015-10-17 00:56 +0100
  Re: Pointers, constant values Les Cargill <lcargill99@comcast.com> - 2015-10-16 12:37 -0500
    Re: Pointers, constant values James Kuyper <jameskuyper@verizon.net> - 2015-10-16 14:22 -0400
  Re: Pointers, constant values Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-10-16 18:57 +0000
    Re: Pointers, constant values Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-16 20:40 +0100
      Re: Pointers, constant values Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-10-16 20:46 +0000
        Re: Pointers, constant values Keith Thompson <kst-u@mib.org> - 2015-10-16 14:24 -0700
          Re: Pointers, constant values Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-10-19 12:43 +0000
  Re: Pointers, constant values Siri Cruz <chine.bleu@yahoo.com> - 2015-10-16 13:20 -0700
    Re: Pointers, constant values supercat@casperkitty.com - 2015-10-17 13:59 -0700

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