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| From | "BartC" <bc@freeuk.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Compatibility question |
| Date | 2012-10-23 00:38 +0100 |
| Organization | A noiseless patient Spider |
| Message-ID | <k64ler$48l$1@dont-email.me> (permalink) |
| References | (3 earlier) <OYMHVs.wix.4bcJD@gmail.com> <0.a87a1c8a472d870b0466.20121022022701BST.87y5izxpne.fsf@bsb.me.uk><JLOhFS.Dvd.FY9Iy@gmail.com> <0.d779297035d2edccd1b4.20121022173052BST.87ehkqbhab.fsf@bsb.me.uk> <2nYjPo.Krd.2aRbe@gmail.com><lnfw56fl74.fsf@nuthaus.mib.org> <qRFu8I.lcH.7XEwQ@gmail.com> |
"Steve Thompson" <stevet810@gmail.com> wrote in message
news:qRFu8I.lcH.7XEwQ@gmail.com...
> On Mon, Oct 22, 2012 at 10:52:47AM -0700, Keith Thompson wrote:
>> I'm not sure what you mean when you say `a` "is a pointer in
>> the implementation", especially after you mention "the fact that
>> variables defined as structures were not pointers".
>
> Meaning that under the hood in the C compiler, 'a' is really a
> pointer.
Not really. &a is a pointer, but a is just a value (of the struct).
I suppose if the struct was huge, a compiler might see if it could get away
with using a pointer to the struct instead. But for smaller structs it's not
a problem to manipulate them by value. (And it's anyway up to the programmer
if he wanted large structs handled by value or not.)
Also consider a function like this:
struct point add(struct point a,struct point b) {
struct point c={a.x+b.x,a.y+b.y};
return c;
}
and calling it like this:
struct point p,q,r,s,t;
p=add(q,add(r,add(s,t)));
how much work do you think it's going to be to manage all those behind the
scenes pointers, which might even require memory allocations to keep track
of? It's easier to keep intermediate values on the call stack.
> True enough, but the difference is that a structure cannot be used for
> arithmetic operations of any kind in C. That makes it fundamentally
> different from simple type variables. It may be meaningful to say that
> structures passed to functions as values is a hack in the C language.
> It fills a use-case scenario that will be found (or should be found)
> only in trivial programs, IMHO.
See above example..
>> > These days I'm thinking more and more about cache-line usage, so I am
>> > concerned with preserving the 'hotness' of my data. Copying
>> > structures around a whole lot is hostile to this paradigm, so that is
>> > one language feature I am happy to avoid.
>>
>> If a structure is smaller than a pointer, or not much bigger, it makes
>> perfect sense to pass it around by value.
>
> Most structures are considerably larger than a pointer in practice.
> My personal opinion is that it is a waste to pass them around by
> value in most cases.
It depends. If there are 100 fields and you only access 1 or 2, then it
probably is wasteful. If you have to access most of the fields anyway, then
it's not so bad (and you can avoid a level of indirection.)
--
bartc
Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread
Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-20 03:08 +0000
Re: Compatibility question Ian Collins <ian-news@hotmail.com> - 2012-10-20 16:20 +1300
Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-20 11:50 +0100
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-20 11:41 -0700
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-21 23:04 +0000
Re: Compatibility question Barry Schwarz <schwarzb@dqel.com> - 2012-10-21 20:27 -0700
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-20 04:01 +0000
Re: Compatibility question Ian Collins <ian-news@hotmail.com> - 2012-10-21 09:26 +1300
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-21 22:54 +0000
Re: Compatibility question Ian Collins <ian-news@hotmail.com> - 2012-10-22 12:29 +1300
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-21 23:50 +0000
Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-22 02:27 +0100
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-22 02:28 +0000
Re: Compatibility question Eric Sosman <esosman@comcast-dot-net.invalid> - 2012-10-22 11:18 -0400
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-22 16:19 +0000
Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-22 17:37 +0100
Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-22 17:40 +0100
Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-22 17:30 +0100
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-22 17:23 +0000
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-22 10:52 -0700
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-22 18:42 +0000
Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-22 21:06 +0100
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-22 22:52 +0000
Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-23 03:33 +0100
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-23 12:28 +0000
Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-23 16:45 +0100
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-23 17:53 +0000
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-23 12:41 -0700
Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-23 21:28 +0100
Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-23 20:57 +0100
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-23 12:26 -0700
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-23 20:39 +0000
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-23 15:53 -0700
Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-24 00:57 +0100
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-23 20:06 -0700
Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-24 11:18 +0100
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-24 12:04 -0700
Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-24 23:10 +0100
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-24 16:58 -0700
Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-25 01:46 +0100
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-24 19:40 -0700
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-24 20:20 +0000
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-24 14:38 -0700
Re: Compatibility question James Kuyper <jameskuyper@verizon.net> - 2012-10-24 17:55 -0400
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-24 16:29 -0700
Re: Compatibility question Greg Martin <greg@softsprocket.com> - 2012-10-24 15:21 -0700
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-26 03:30 +0000
Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-26 13:02 +0100
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-27 00:04 +0000
Re: Compatibility question Ben Bacarisse <ben.usenet@bsb.me.uk> - 2012-10-27 20:15 +0100
Re: Compatibility question Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2012-10-27 16:36 -0600
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-28 17:36 +0000
Re: Compatibility question Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-10-27 22:42 +0300
Re: Compatibility question Greg Martin <greg@softsprocket.com> - 2012-10-27 14:30 -0700
Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-27 22:57 +0100
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-27 17:04 -0700
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-27 17:14 -0700
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-27 18:04 -0700
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-24 16:46 -0700
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-25 01:18 +0000
Re: Compatibility question Phil Carmody <thefatphil_demunged@yahoo.co.uk> - 2012-10-27 21:08 +0300
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-28 17:38 +0000
Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-23 00:38 +0100
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-22 16:50 -0700
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-23 12:33 +0000
Re: Compatibility question Öö Tiib <ootiib@hot.ee> - 2012-10-22 14:23 -0700
Re: Compatibility question James Kuyper <jameskuyper@verizon.net> - 2012-10-22 17:26 -0400
Re: Compatibility question Ian Collins <ian-news@hotmail.com> - 2012-10-23 10:55 +1300
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-22 23:27 +0000
Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-22 22:44 +0100
Re: Compatibility question Les Cargill <lcargill99@comcast.com> - 2012-10-22 17:15 -0500
Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-23 00:13 +0100
Re: Compatibility question James Kuyper <jameskuyper@verizon.net> - 2012-10-22 18:35 -0400
Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-23 00:14 +0100
Re: Compatibility question Steve Thompson <stevet810@gmail.com> - 2012-10-22 23:42 +0000
Re: Compatibility question Ian Collins <ian-news@hotmail.com> - 2012-10-23 13:35 +1300
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-22 17:01 -0700
Re: Compatibility question "BartC" <bc@freeuk.com> - 2012-10-23 01:47 +0100
Re: Compatibility question Keith Thompson <kst-u@mib.org> - 2012-10-21 19:27 -0700
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