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Groups > comp.lang.c > #101261
| From | BartC <bc@freeuk.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Macro Expansion |
| Date | 2017-01-07 22:50 +0000 |
| Organization | A noiseless patient Spider |
| Message-ID | <o4rr80$pj7$1@dont-email.me> (permalink) |
| References | <o4rcbl$4cp$1@dont-email.me> <o4ri5t$pqj$1@dont-email.me> <874m1appnx.fsf@bsb.me.uk> |
On 07/01/2017 20:42, Ben Bacarisse wrote:
> BartC <bc@freeuk.com> writes:
>> I wish they would just state all this stuff instead of leaving people
>> to either deduce it or eventually discover it for themselves.
>
> "They" (presumably the C committee) have an obligation to be precise
> rather than explanatory. It's the various books and tutorials about C
> that should explain this in simpler terms. If no such thing exists -- a
> lot of C tutorials are either overly simplistic or wrong -- then it's
> not the fault of the C committee.
Who was responsible for standardising such a complicated macro language?
I can't believe most C programmers are aware of all arcane details.
> You are trying to understand it by adding rules. That's often a good
> way to explain things but equally often a bad way to define things.
> Your rule above ("Once a parameter name token such as x has been
> substituted once, a new instance of x resulting from a macro expansion
> is treated like any other identifier") is not needed in the
> specification because it is a consequence of the rules that are needed.
>
> The main fuzziness in your description comes from not explaining the
> boundaries of an expansion (exactly the text from the standard you said
> was not adding anything). It's not the case that the text is repeated
> scanned and the anything expanded already is not expanded again. Every
> macro expansion is a separate, bounded, sequence of tokens, and only
> within that expansion is the expansion of the macro being expanded
> ignored. I'm not suggesting you've got it wrong,
No, if I got it wrong, tell me!
but I did not find
> your explanation clearer that the official one. A test:
>
> #define X Y
> #define Y T
> #define T X Y
>
> <T> => <X><Y>
> <X> => <Y>
> <Y> => <T>
> <Y> => <T>
This introduces new problems. Just take this:
#define X T
#define T X
T
How does it manage to not keep expanding indefinitely? The simplest case
is taken care of:
#define T T
by being explicitly mentioned (somewhere or other). For the T=>X, X=>T
example, I can remember a 'topmost' macro name, and avoid expanding that
even deep inside an unrelated macro expansion. But with this tweak of
your example:
#define X Y
#define Y X
#define T X Y
Now it's X=>Y, Y=>X that threatens to go on forever. I can't add these
names to the list of names that have been expanded, as that is local to
each level of macro. I might need to propagate a whole sequence of
'topmost' macro names to each level.
This only applies to the recursive expansion of one specific macro. It
doesn't apply to expansion of individual macros in a macro argument,
which are processed independently with their own 'name-space' (of a
propagated stack of macro names, and local list of names that have been
processed).
Yikes. I guess whoever devised this scheme didn't think too much about
efficiency in implementing it. To say nothing of people being able to
predict the exact behaviour whenever they write or read macros.
--
Bartc
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Macro Expansion BartC <bc@freeuk.com> - 2017-01-07 18:35 +0000
Re: Macro Expansion Thiago Adams <thiago.adams@gmail.com> - 2017-01-07 11:23 -0800
Re: Macro Expansion Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-01-07 21:28 +0000
Re: Macro Expansion Siri Cruise <chine.bleu@yahoo.com> - 2017-01-07 11:45 -0800
Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-07 20:16 +0000
Re: Macro Expansion Siri Cruise <chine.bleu@yahoo.com> - 2017-01-07 12:23 -0800
Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-07 20:31 +0000
Re: Macro Expansion Siri Cruise <chine.bleu@yahoo.com> - 2017-01-07 12:42 -0800
Re: Macro Expansion Ian Collins <ian-news@hotmail.com> - 2017-01-08 14:42 +1300
Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-08 13:18 +0000
Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-07 20:15 +0000
Re: Macro Expansion Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-01-07 20:42 +0000
Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-07 22:50 +0000
Re: Macro Expansion Ian Collins <ian-news@hotmail.com> - 2017-01-08 11:59 +1300
Re: Macro Expansion Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-01-07 15:10 -0800
Re: Macro Expansion supercat@casperkitty.com - 2017-01-07 17:24 -0800
Re: Macro Expansion Tim Rentsch <txr@alumni.caltech.edu> - 2017-01-15 21:55 -0800
Re: Macro Expansion supercat@casperkitty.com - 2017-01-16 09:06 -0800
Re: Macro Expansion Tim Rentsch <txr@alumni.caltech.edu> - 2017-01-31 08:51 -0800
Re: Macro Expansion supercat@casperkitty.com - 2017-01-31 13:06 -0800
Re: Macro Expansion Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-03 07:49 -0800
Re: Macro Expansion supercat@casperkitty.com - 2017-02-03 12:03 -0800
Re: Macro Expansion Tim Rentsch <txr@alumni.caltech.edu> - 2017-02-09 09:00 -0800
Re: Macro Expansion Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-01-08 01:37 +0000
Re: Macro Expansion Jean-Marc Bourguet <jm@bourguet.org> - 2017-01-08 10:22 +0100
Re: Macro Expansion Thiago Adams <thiago.adams@gmail.com> - 2017-01-07 12:18 -0800
Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-07 20:28 +0000
Re: Macro Expansion Thiago Adams <thiago.adams@gmail.com> - 2017-01-07 12:36 -0800
Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-07 22:59 +0000
Re: Macro Expansion Thiago Adams <thiago.adams@gmail.com> - 2017-01-08 00:20 -0800
Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-08 13:30 +0000
Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-10 14:00 +0000
Re: Macro Expansion Thiago Adams <thiago.adams@gmail.com> - 2017-01-10 07:43 -0800
Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-10 17:47 +0000
Re: Macro Expansion Ben Bacarisse <ben.usenet@bsb.me.uk> - 2017-01-10 22:49 +0000
Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-10 23:28 +0000
Re: Macro Expansion "James R. Kuyper" <jameskuyper@verizon.net> - 2017-01-10 18:51 -0500
Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-11 00:06 +0000
Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-10 20:30 +0000
Re: Macro Expansion Thiago Adams <thiago.adams@gmail.com> - 2017-04-26 10:04 -0700
Re: Macro Expansion Keith Thompson <kst-u@mib.org> - 2017-04-26 12:00 -0700
Re: Macro Expansion Thiago Adams <thiago.adams@gmail.com> - 2017-04-26 12:24 -0700
Re: Macro Expansion bartc <bc@freeuk.com> - 2017-04-26 20:12 +0100
Re: Macro Expansion Thiago Adams <thiago.adams@gmail.com> - 2017-04-26 12:34 -0700
Re: Macro Expansion jameskuyper@verizon.net - 2017-04-26 13:46 -0700
Re: Macro Expansion Jean-Marc Bourguet <jm@bourguet.org> - 2017-01-08 10:13 +0100
Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-08 13:10 +0000
Re: Macro Expansion Jean-Marc Bourguet <jm@bourguet.org> - 2017-01-08 15:51 +0100
Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-08 17:17 +0000
Re: Macro Expansion supercat@casperkitty.com - 2017-01-08 11:04 -0800
Re: Macro Expansion Malcolm McLean <malcolm.mclean5@btinternet.com> - 2017-01-08 11:10 -0800
Re: Macro Expansion supercat@casperkitty.com - 2017-01-08 14:08 -0800
Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-08 19:49 +0000
Re: Macro Expansion supercat@casperkitty.com - 2017-01-08 14:17 -0800
Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-08 19:19 +0000
Re: Macro Expansion Jean-Marc Bourguet <jm@bourguet.org> - 2017-01-08 21:43 +0100
Re: Macro Expansion supercat@casperkitty.com - 2017-01-08 10:43 -0800
Re: Macro Expansion Keith Thompson <kst-u@mib.org> - 2017-01-08 12:39 -0800
Re: Macro Expansion antispam@math.uni.wroc.pl - 2017-01-08 22:54 +0000
Re: Macro Expansion "James R. Kuyper" <jameskuyper@verizon.net> - 2017-01-08 18:36 -0500
Re: Macro Expansion antispam@math.uni.wroc.pl - 2017-01-09 01:15 +0000
Re: Macro Expansion "James R. Kuyper" <jameskuyper@verizon.net> - 2017-01-08 20:49 -0500
Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-08 23:53 +0000
Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-09 01:11 +0000
Re: Macro Expansion antispam@math.uni.wroc.pl - 2017-01-09 01:31 +0000
Re: Macro Expansion BartC <bc@freeuk.com> - 2017-01-10 11:28 +0000
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