Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]
| From | Ben Bacarisse <ben.usenet@bsb.me.uk> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: Objective C |
| Date | 2015-11-15 01:47 +0000 |
| Organization | A noiseless patient Spider |
| Message-ID | <87a8qgvxst.fsf@bsb.me.uk> (permalink) |
| References | (3 earlier) <2e08fc7d-3253-404c-b04f-b624c3550c8f@googlegroups.com> <87fv09xubq.fsf@bsb.me.uk> <e5eccc3a-f834-45ad-9839-9e02a6d39a93@googlegroups.com> <87lha0w7aq.fsf@bsb.me.uk> <bb45cd7e-7d2a-4f28-a850-d75a703027f0@googlegroups.com> |
supercat@casperkitty.com writes:
> On Saturday, November 14, 2015 at 4:22:15 PM UTC-6, Ben Bacarisse wrote:
>> Right, so you don't like any of the rules that C has had in the respect
>> for the last 25 years? Why do you keep suggesting that it's C99 that's
>> the problem?
>
> The thread at http://thread.gmane.org/gmane.linux.file-systems/28361/focus=29271
> would suggest that gcc does not feel an obligation to honor the sequence of
> operations:
>
> 1. write some storage as one type that's not a union
> 2. read it as that first type
> 3. write some storage as a different type that's not a union
> 4. read it as that first type
>
> Perhaps the issue is the same in C89 and C99,
I can't see anything between C89 and C99 that would make a difference
here.
> even though the wording is
> different, but I've never seen any indication that any C89 compiler would
> not honor the sequence of operations as written above,
Do you have any evidence other than not having seen one? In particular,
do you know that the version of gcc referred to in that thread does
*not* make the key optimisation when passed -std=c89?
> but it sounds in
> the linked thread as though gcc is deciding that the term "accessed" which
> is not defined within the Standard so far as I can tell
What did you do to find out? For example, did you read the definitions
of terms used in the standard?
> refers to both
> writing and reading, and as a consequence code which uses memory for one
> purpose and then later wants to use it for a different purpose may invoke
> Undefined Behavior if the different purposes involve different types.
>
> Further, the way the C99 is written, using memcpy to copy an object of one
> type to heap storage and then reading that storage as another type will
> yield Undefined Behavior, while in C89 memcpy was usable as a means of
> "scrubbing" type information from a block of memory allowing it to be read
> as any type (example below)
I can't find any support for that idea in the C89 standard. Can you
give a citation?
> Am I being overly alarmist, or am I misunderstanding what's going on, or
> what? The rationale given for C89 was to allow the compiler to keep things
> in registers while performing operations in the given sequence. The apparent
> use of the effective-type rules in gcc is to allow operations to be performed
> out of sequence. Perhaps one could use the C89 wording to justify the same
> sort of crazy behavior, but if code can't use a block of memory for disjoint
> purposes at different times, which is what the thread would suggest, that
> would render many kinds of embedded programming impossible (since malloc/free
> are often not an option, but are instead replaced by functions which return
> fixed-size blocks of memory from a pool).
It sounds alarmist to say that C renders many kinds of embedded
programming impossible because it's so often used for that purpose.
Maybe the programmers are able to use compilers that give them more
assurances than the C standard does? Maybe they have found ways to work
within the aliasing rules?
> In any case, I see no issue why the Standard shouldn't provide some means of
> specifying "memory barriers"? If the Standard included an intrinsic to
> "detach_and_fill_unspecified", and if code which receives a malloc'ed block
> of memory and uses it for two disjoint purposes used such a directive between
> those uses, then a compiler could either allocate one block of memory and
> ensure that all uses of the first type complete before any of the second are
> performed, or could allocate two blocks of memory and overlap the operations
> as it saw fit.
>
>
> C89-vs-99 example with memcpy():
>
> In C89, by my understanding, the following would have been perfectly defined:
>
> struct s1 { int key; };
> struct s2 { int key; int data[4]; };
>
> struct s2 s2_inst = {23, {1,2,3,4}} ;
>
> void *p1 = malloc(sizeof (struct s1*));
> void *p2 = malloc(sizeof (struct s2*));
> *(struct s2**)p2 = &s2_inst;
>
> memcpy(p1,p2, sizeof (struct s1*)); // Pointers to structs have same size
>
> printf("%d", (*(struct s1**)p1)->key);
>
> In c89, the memcpy would allow &s2_inst to be used as a (struct s1*), which
> could in turn be used to access the "key" field of struct s2_inst. In c99,
> however, the memcpy would set the effective type of *p1 to (struct s2) so
> attempting to use it as a (struct s1*) would invoke Undefined Behavior.
>
> Am I incorrect in thinking that the above code was 100% legal and defined in
> C89, and Undefined in C99?
C89's rules are all about the declared types of objects, so I don't
think it has anything to say on the matter of allocated ones. C99's
contribution to this area was to specify what type allocated objects can
be assumed to have and that does not appear to the thing you keep
complaining about. Your beef seems to be with C89 as well as with C99.
--
Ben.
Back to comp.lang.c | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread
Objective C Alla _ <modelling.data@gmail.com> - 2015-11-10 09:10 -0800
Re: Objective C "Rick C. Hodgin" <rick.c.hodgin@gmail.com> - 2015-11-10 09:16 -0800
Re: Objective C Richard Heathfield <rjh@cpax.org.uk> - 2015-11-10 17:17 +0000
Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-10 09:32 -0800
Re: Objective C Mark Storkamp <mstorkamp@yahoo.com> - 2015-11-10 11:38 -0600
Re: Objective C James Kuyper <jameskuyper@verizon.net> - 2015-11-10 12:36 -0500
Re: Objective C Alla _ <modelling.data@gmail.com> - 2015-11-10 11:15 -0800
Re: Objective C Richard Heathfield <rjh@cpax.org.uk> - 2015-11-10 21:51 +0000
Re: Objective C Alla _ <modelling.data@gmail.com> - 2015-11-10 23:13 -0800
Re: Objective C supercat@casperkitty.com - 2015-11-13 16:01 -0800
Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-14 01:07 +0000
Re: Objective C supercat@casperkitty.com - 2015-11-14 06:26 -0800
Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-14 22:22 +0000
Re: Objective C supercat@casperkitty.com - 2015-11-14 15:46 -0800
Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-15 01:47 +0000
Re: Objective C BartC <bc@freeuk.com> - 2015-11-15 10:43 +0000
Re: Objective C supercat@casperkitty.com - 2015-11-15 13:19 -0800
Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-14 15:43 -0800
Re: Objective C David Brown <david.brown@hesbynett.no> - 2015-11-15 23:50 +0100
Re: Objective C gazelle@shell.xmission.com (Kenny McCormack) - 2015-11-16 00:05 +0000
Re: Objective C supercat@casperkitty.com - 2015-11-15 19:10 -0800
Re: Objective C David Brown <david.brown@hesbynett.no> - 2015-11-16 09:11 +0100
Re: Objective C supercat@casperkitty.com - 2015-11-16 11:57 -0800
Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-16 20:38 +0000
Re: Objective C supercat@casperkitty.com - 2015-11-16 13:20 -0800
Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-16 22:28 +0000
Re: Objective C supercat@casperkitty.com - 2015-11-16 15:33 -0800
Re: Objective C supercat@casperkitty.com - 2015-11-16 15:56 -0800
Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-17 01:43 +0000
Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-17 01:21 +0000
Re: Objective C supercat@casperkitty.com - 2015-11-16 18:11 -0800
Re: Objective C supercat@casperkitty.com - 2015-11-16 20:12 -0800
Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-19 00:59 +0000
Re: Objective C supercat@casperkitty.com - 2015-11-18 22:46 -0800
Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-19 14:25 +0000
Re: Objective C supercat@casperkitty.com - 2015-11-19 07:58 -0800
Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-19 22:13 +0000
Re: Objective C supercat@casperkitty.com - 2015-11-19 15:29 -0800
Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-27 08:44 -0800
Re: Objective C glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-11-17 01:26 +0000
Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-16 21:48 -0600
Re: Objective C supercat@casperkitty.com - 2015-11-16 20:27 -0800
Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-17 13:58 -0600
Re: Objective C supercat@casperkitty.com - 2015-11-17 12:28 -0800
Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-18 13:00 -0600
Re: Objective C supercat@casperkitty.com - 2015-11-18 12:31 -0800
Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-18 16:43 -0600
Re: Objective C supercat@casperkitty.com - 2015-11-18 15:57 -0800
Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-19 19:09 -0600
Re: Objective C supercat@casperkitty.com - 2015-11-19 19:05 -0800
Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-20 15:00 -0600
Re: Objective C supercat@casperkitty.com - 2015-11-20 15:30 -0800
Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-22 17:11 -0600
Re: Objective C supercat@casperkitty.com - 2015-11-22 15:53 -0800
Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-23 13:51 -0600
Re: Objective C raltbos@xs4all.nl (Richard Bos) - 2015-11-24 12:00 +0000
Re: Objective C David Brown <david.brown@hesbynett.no> - 2015-11-17 10:26 +0100
Re: Objective C supercat@casperkitty.com - 2015-11-17 06:20 -0800
Re: Objective C David Brown <david.brown@hesbynett.no> - 2015-11-17 16:41 +0100
Re: Objective C supercat@casperkitty.com - 2015-11-17 12:08 -0800
Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-17 16:27 -0600
Re: Objective C supercat@casperkitty.com - 2015-11-17 14:55 -0800
Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-18 13:59 -0600
Re: Objective C supercat@casperkitty.com - 2015-11-18 13:06 -0800
Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-24 14:24 -0600
Re: Objective C supercat@casperkitty.com - 2015-11-24 15:11 -0800
Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-26 17:02 -0600
Re: Objective C supercat@casperkitty.com - 2015-11-27 07:26 -0800
Re: Objective C supercat@casperkitty.com - 2015-11-18 21:56 -0800
Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-22 02:17 -0600
Re: Objective C supercat@casperkitty.com - 2015-11-22 07:16 -0800
Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-26 17:11 -0600
Re: Objective C Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-11-22 19:09 +0000
Re: Objective C Robert Wessel <robertwessel2@yahoo.com> - 2015-11-17 23:28 -0600
Re: Objective C David Brown <david.brown@hesbynett.no> - 2015-11-18 15:33 +0100
Re: Objective C supercat@casperkitty.com - 2015-11-18 23:06 -0800
Re: Objective C David Brown <david.brown@hesbynett.no> - 2015-11-19 09:12 +0100
Re: Objective C supercat@casperkitty.com - 2015-11-19 09:25 -0800
Re: Objective C David Brown <david.brown@hesbynett.no> - 2015-11-19 21:06 +0100
Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-20 13:49 -0800
Re: Objective C Robert Wessel <robertwessel2@yahoo.com> - 2015-11-21 05:01 -0600
Re: Objective C Robert Wessel <robertwessel2@yahoo.com> - 2015-11-21 05:09 -0600
Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-21 03:31 -0800
Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-21 17:43 -0600
Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-21 16:00 -0800
Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-22 01:12 -0600
Re: Objective C raltbos@xs4all.nl (Richard Bos) - 2015-11-27 16:42 +0000
Re: Objective C James Kuyper <jameskuyper@verizon.net> - 2015-11-27 12:38 -0500
Re: Objective C supercat@casperkitty.com - 2015-11-27 11:09 -0800
Re: Objective C James Kuyper <jameskuyper@verizon.net> - 2015-11-21 19:06 -0500
Re: Objective C supercat@casperkitty.com - 2015-11-21 17:00 -0800
Re: Objective C glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-11-22 14:44 +0000
Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-22 01:55 -0600
Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-22 00:03 -0800
Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-22 10:18 +0000
Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-23 03:21 -0800
Re: Objective C James Kuyper <jameskuyper@verizon.net> - 2015-11-22 10:53 -0500
Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-22 20:28 -0600
Re: Objective C supercat@casperkitty.com - 2015-11-22 18:52 -0800
Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-23 11:58 -0600
Re: Objective C supercat@casperkitty.com - 2015-11-23 15:24 -0800
Re: Objective C James Kuyper <jameskuyper@verizon.net> - 2015-11-23 09:46 -0500
Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-23 08:13 -0800
Re: Objective C James Kuyper <jameskuyper@verizon.net> - 2015-11-23 12:04 -0500
Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-23 11:52 -0600
Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-23 16:05 -0800
Re: Objective C supercat@casperkitty.com - 2015-11-24 09:09 -0800
Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-12-08 08:48 -0800
Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-12-08 10:40 -0800
Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-12-09 12:34 -0800
Re: Objective C supercat@casperkitty.com - 2015-12-08 10:50 -0800
Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-12-09 12:42 -0800
Re: Objective C supercat@casperkitty.com - 2015-12-09 13:38 -0800
Re: Objective C raltbos@xs4all.nl (Richard Bos) - 2015-12-10 19:54 +0000
Re: Objective C David Thompson <dave.thompson2@verizon.net> - 2015-12-06 08:02 -0500
Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-22 14:36 -0800
Re: Objective C Robert Wessel <robertwessel2@yahoo.com> - 2015-11-23 00:41 -0600
Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-23 07:39 -0800
Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-22 14:23 -0800
Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-17 10:24 -0800
Re: Objective C David Brown <david.brown@hesbynett.no> - 2015-11-18 15:40 +0100
Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-20 16:02 -0800
Re: Objective C David Brown <david.brown@hesbynett.no> - 2015-11-21 15:53 +0100
Re: Objective C supercat@casperkitty.com - 2015-11-21 08:58 -0800
Re: Objective C David Brown <david.brown@hesbynett.no> - 2015-11-22 22:14 +0100
Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-23 03:39 -0800
Re: Objective C raltbos@xs4all.nl (Richard Bos) - 2015-11-23 13:24 +0000
Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-26 09:39 -0800
Re: Objective C Robert Wessel <robertwessel2@yahoo.com> - 2015-11-26 13:16 -0600
Re: Objective C supercat@casperkitty.com - 2015-11-26 12:25 -0800
Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-27 14:34 -0800
Re: Objective C Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-26 12:39 -0800
Re: Objective C raltbos@xs4all.nl (Richard Bos) - 2015-11-27 16:43 +0000
Re: Objective C Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-27 09:29 -0800
Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-27 14:18 -0800
Re: Objective C glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-11-23 22:35 +0000
Re: Objective C James Kuyper <jameskuyper@verizon.net> - 2015-11-23 17:40 -0500
Re: Objective C glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-11-24 05:35 +0000
Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-25 12:15 -0800
Re: Objective C glen herrmannsfeldt <gah@ugcs.caltech.edu> - 2015-11-25 22:49 +0000
Re: Objective C raltbos@xs4all.nl (Richard Bos) - 2015-11-27 16:44 +0000
Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-27 14:03 -0800
Re: Objective C Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-11-29 09:29 +0000
Re: Objective C Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-11-22 19:01 +0000
Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-23 07:31 -0800
Re: Objective C Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-23 07:43 -0800
Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-23 08:17 -0800
Re: Objective C David Brown <david.brown@hesbynett.no> - 2015-11-23 17:34 +0100
Re: Objective C Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-11-23 22:02 +0000
Re: Objective C David Brown <david.brown@hesbynett.no> - 2015-11-24 10:12 +0100
Re: Objective C Tim Rentsch <txr@alumni.caltech.edu> - 2015-11-27 08:39 -0800
Re: Objective C Jorgen Grahn <grahn+nntp@snipabacken.se> - 2015-11-14 18:24 +0000
Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-14 12:23 -0800
Re: Objective C Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-14 12:36 -0800
Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-14 13:13 -0800
Re: Objective C Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-14 14:04 -0800
Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-14 14:15 -0800
Re: Objective C Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-14 14:29 -0800
Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-14 15:39 -0800
Re: Objective C Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-14 16:37 -0800
Re: Objective C Ian Collins <ian-news@hotmail.com> - 2015-11-15 13:46 +1300
Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-15 02:06 +0000
Re: Objective C Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-15 03:25 -0800
Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-15 12:56 +0000
Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-15 07:53 -0600
Re: Objective C Stephen Sprunk <stephen@sprunk.org> - 2015-11-15 07:19 -0600
Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-15 13:29 +0000
Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-15 10:56 -0800
Re: Objective C supercat@casperkitty.com - 2015-11-15 14:35 -0800
Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-15 16:52 -0800
Re: Objective C supercat@casperkitty.com - 2015-11-15 19:49 -0800
Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-16 11:02 +0000
Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-16 10:36 -0800
Re: Objective C David Thompson <dave.thompson2@verizon.net> - 2015-12-06 08:03 -0500
Re: Objective C Melzzzzz <mel@zzzzz.com> - 2015-11-16 02:01 +0100
Re: Objective C supercat@casperkitty.com - 2015-11-15 19:34 -0800
Re: Objective C Chad <cdalten@gmail.com> - 2015-11-15 19:54 -0800
Re: Objective C raltbos@xs4all.nl (Richard Bos) - 2015-11-17 22:36 +0000
Re: Objective C supercat@casperkitty.com - 2015-11-17 15:27 -0800
Re: Objective C Malcolm McLean <malcolm.mclean5@btinternet.com> - 2015-11-17 21:15 -0800
Re: Objective C Ben Bacarisse <ben.usenet@bsb.me.uk> - 2015-11-14 22:08 +0000
Re: Objective C fir <profesor.fir@gmail.com> - 2015-11-27 14:36 -0800
Re: Objective C David Brown <david.brown@hesbynett.no> - 2015-11-10 19:03 +0100
Re: Objective C Keith Thompson <kst-u@mib.org> - 2015-11-10 10:50 -0800
csiph-web