Re: Getting sensible information from gdb

From	Ben Bacarisse <ben.usenet@bsb.me.uk>
Newsgroups	comp.lang.c
Subject	Re: Getting sensible information from gdb
Date	2015-10-14 00:17 +0100
Organization	A noiseless patient Spider
Message-ID	<87oag2bbos.fsf@bsb.me.uk> (permalink)
References	(12 earlier) <mvhl9r$pku$1@speranza.aioe.org> <87h9ludi7t.fsf@bsb.me.uk> <mvj498$upo$1@speranza.aioe.org> <87bnc2d9r1.fsf@bsb.me.uk> <mvjbcf$hi1$1@speranza.aioe.org>

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"Morten W. Petersen" <morphex@gmail.com> writes:

> On 13.10.2015 18:16, Ben Bacarisse wrote:
>> "Morten W. Petersen" <morphex@gmail.com> writes:
> [...]
>>> Well getting rid of all those casts is a good enough reason to use this
>>> type of struct.
>>>
>>> As for having a generic object, I don't quite like the idea of
>>> allocating a full-size struct and then resizing it by some bytes
>>> if they aren't necessary.
>>
>> That's not what I meant, but if you are likely to use the first outline
>> there's no point in drilling down into the second scheme.
>>
>>> But I could create factory functions for this then, that allocate
>>> memory for the largest possible struct and then give it the right
>>> memory allocation before passing it back.  Or not?
>>
>> I don't follow, but, as I said above, if this is about my "other
>> organisation" there is no need to use it if you prefer the one I
>> sketched in code.
>
> Well, a union allocates space for the largest data type it can contain,
> the smallest data type in that union has one pointer, while the largest
> has three.

A union allocates nothing.  The size of a union is sufficient to hold
it's largest member but you are not obliged use the size of the union to
allocate space.

> Being able to address members of a struct without casting is nice, so
> I want to do that.
>
> However, if it is a text element, there is space for two pointers there,
> which are not needed.
>
> So, at the end of the factory function which allocates memory and
> initializes the struct, it can do something like
>
>   realloc(sizeof(xml_item) - (sizeof(void*)*0 or 1 or 2))
>
> Right?

It's not a very good idea to realloc to a slightly smaller size.  For
one thing, it make make no change at all, but even it does why take the
time and risk the fragmentation and/or copying when you can allocate the
right size to start with?

-- 
Ben.

Thread

Getting sensible information from gdb "Morten W. Petersen" <morphex@gmail.com> - 2015-10-11 21:21 +0200
  Re: Getting sensible information from gdb Geoff <geoff@invalid.invalid> - 2015-10-11 14:47 -0700
    Re: Getting sensible information from gdb "Morten W. Petersen" <morphex@gmail.com> - 2015-10-12 00:07 +0200
      Re: Getting sensible information from gdb Geoff <geoff@invalid.invalid> - 2015-10-11 16:15 -0700
        Re: Getting sensible information from gdb James Kuyper <jameskuyper@verizon.net> - 2015-10-11 21:02 -0400
          Re: Getting sensible information from gdb Geoff <geoff@invalid.invalid> - 2015-10-11 18:55 -0700
            Re: Getting sensible information from gdb "Morten W. Petersen" <morphex@gmail.com> - 2015-10-12 04:04 +0200
              Re: Getting sensible information from gdb Geoff <geoff@invalid.invalid> - 2015-10-11 19:37 -0700
                Re: Getting sensible information from gdb "Morten W. Petersen" <morphex@gmail.com> - 2015-10-12 05:16 +0200
            Re: Getting sensible information from gdb James Kuyper <jameskuyper@verizon.net> - 2015-10-12 00:01 -0400
              Re: Getting sensible information from gdb "Morten W. Petersen" <morphex@gmail.com> - 2015-10-12 06:06 +0200
                Re: Getting sensible information from gdb James Kuyper <jameskuyper@verizon.net> - 2015-10-12 00:48 -0400
                Re: Getting sensible information from gdb "Morten W. Petersen" <morphex@gmail.com> - 2015-10-12 07:08 +0200
          Re: Getting sensible information from gdb Keith Thompson <kst-u@mib.org> - 2015-10-11 18:56 -0700
    Re: Getting sensible information from gdb Eric Sosman <esosman@comcast-dot-net.invalid> - 2015-10-11 18:14 -0400
      Re: Getting sensible information from gdb "Morten W. Petersen" <morphex@gmail.com> - 2015-10-12 00:20 +0200
        Re: Getting sensible information from gdb James Kuyper <jameskuyper@verizon.net> - 2015-10-11 21:07 -0400
          Re: Getting sensible information from gdb "Morten W. Petersen" <morphex@gmail.com> - 2015-10-12 03:59 +0200
            Re: Getting sensible information from gdb Geoff <geoff@invalid.invalid> - 2015-10-11 23:21 -0700
              Re: Getting sensible information from gdb Geoff <geoff@invalid.invalid> - 2015-10-11 23:32 -0700
                Re: Getting sensible information from gdb "Morten W. Petersen" <morphex@gmail.com> - 2015-10-12 08:42 +0200
                Re: Getting sensible information from gdb Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-12 17:13 +0100
                Re: Getting sensible information from gdb "Morten W. Petersen" <morphex@gmail.com> - 2015-10-12 23:50 +0200
                Re: Getting sensible information from gdb Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-13 01:39 +0100
                Re: Getting sensible information from gdb "Morten W. Petersen" <morphex@gmail.com> - 2015-10-13 03:04 +0200
                Re: Getting sensible information from gdb Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-13 14:13 +0100
                Re: Getting sensible information from gdb "Morten W. Petersen" <morphex@gmail.com> - 2015-10-13 16:26 +0200
                Re: Getting sensible information from gdb Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-13 17:16 +0100
                Re: Getting sensible information from gdb "Morten W. Petersen" <morphex@gmail.com> - 2015-10-13 18:27 +0200
                Re: Getting sensible information from gdb Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-14 00:17 +0100
                Re: Getting sensible information from gdb "Morten W. Petersen" <morphex@gmail.com> - 2015-10-14 02:26 +0200
                Re: Getting sensible information from gdb "Morten W. Petersen" <morphex@gmail.com> - 2015-10-14 03:59 +0200
                Re: Getting sensible information from gdb Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-14 13:04 +0100
                Re: Getting sensible information from gdb "Morten W. Petersen" <morphex@gmail.com> - 2015-10-15 12:48 +0200
              Re: Getting sensible information from gdb "Morten W. Petersen" <morphex@gmail.com> - 2015-10-12 08:41 +0200
      Re: Getting sensible information from gdb Geoff <geoff@invalid.invalid> - 2015-10-11 16:18 -0700
        Re: Getting sensible information from gdb "Morten W. Petersen" <morphex@gmail.com> - 2015-10-12 01:22 +0200
          Re: Getting sensible information from gdb Geoff <geoff@invalid.invalid> - 2015-10-11 16:42 -0700
            Re: Getting sensible information from gdb "Morten W. Petersen" <morphex@gmail.com> - 2015-10-12 02:16 +0200
  Getting sensible information from gdb "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-10-11 18:59 -0700
    Re: Getting sensible information from gdb "Morten W. Petersen" <morphex@gmail.com> - 2015-10-12 04:09 +0200
      Re: Getting sensible information from gdb "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-10-11 19:28 -0700
        Re: Getting sensible information from gdb "Morten W. Petersen" <morphex@gmail.com> - 2015-10-12 04:59 +0200
          Re: Getting sensible information from gdb "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-10-12 07:10 -0700
            Re: Getting sensible information from gdb Rosario19 <Ros@invalid.invalid> - 2015-10-12 19:41 +0200
  Re: Getting sensible information from gdb Ian Collins <ian-news@hotmail.com> - 2015-10-12 17:59 +1300
    Re: Getting sensible information from gdb "Morten W. Petersen" <morphex@gmail.com> - 2015-10-12 07:36 +0200
      Re: Getting sensible information from gdb Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-10-11 22:59 -0700
        Re: Getting sensible information from gdb "Morten W. Petersen" <morphex@gmail.com> - 2015-10-12 08:27 +0200
          Re: Getting sensible information from gdb Johann Klammer <klammerj@NOSPAM.a1.net> - 2015-10-12 10:55 +0200
          Re: Getting sensible information from gdb Keith Thompson <kst-u@mib.org> - 2015-10-12 08:04 -0700
          Re: Getting sensible information from gdb Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-10-12 17:35 +0100
      Re: Getting sensible information from gdb Keith Thompson <kst-u@mib.org> - 2015-10-12 08:00 -0700
      Re: Getting sensible information from gdb James Kuyper <jameskuyper@verizon.net> - 2015-10-12 12:25 -0400

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