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Groups > comp.unix.programmer > #802
| From | Nobody <nobody@nowhere.com> |
|---|---|
| Subject | Re: threads, signals, and atomicity of basic types |
| Date | 2011-06-15 03:07 +0100 |
| Message-Id | <pan.2011.06.15.02.07.20.219000@nowhere.com> |
| Newsgroups | comp.unix.programmer |
| References | (5 earlier) <871uz5zwhs.fsf@sapphire.mobileactivedefense.com> <e389f163-ab53-433a-b3db-95761c349ed5@f31g2000pri.googlegroups.com> <87d3ipy94h.fsf@sapphire.mobileactivedefense.com> <5b52c4e5-087c-48cd-a289-eacbb25b9924@h36g2000pro.googlegroups.com> <jCQJp.36914$675.21379@news.usenetserver.com> |
| Organization | Zen Internet |
On Tue, 14 Jun 2011 21:43:44 +0000, Scott Lurndal wrote:
>>Except threading doesn't work that way. The compiler /must/ be
>>threading aware. It needs to know that it may not move code in certain
>>directions past certain kinds of primitives, like pthread_mutex_lock.
>
> Actually, the compiler doesn't know for beans what a function
> called 'pthread_mutex_lock' does, or requires. The C standard
> has certain rules about moving code about, and one of them is that
> code cannot be moved around a sequence point.
That's wrong. Optimising compilers move code around sequence points all
the time. They wouldn't be able to do much optimisation if they didn't.
Bear in mind that lots of things are sequence points beside function
calls, including:
-- The end of the first operand of the following
operators: logical AND && (6.5.13); logical OR ||
(6.5.14); conditional ? (6.5.15); comma , (6.5.17).
-- The end of a full expression: an initializer (6.7.8);
the expression in an expression statement (6.8.3); the
controlling expression of a selection statement (if or
switch) (6.8.4); the controlling expression of a while
or do statement (6.8.5); each of the three expressions
of a for statement (6.8.5.3); the expression in a
return statement (6.8.6.4).
The main significance of sequence points in C is that modifying an lvalue
more than once between sequence points results in undefined behaviour
(e.g. "x = ++x;" is undefined because it modifies x twice without an
intervening sequence point).
The reason why compilers tend to avoid moving code around function calls
is that it's much harder to guarantee equivalence. When moving code around
other statements, the compiler can determine what data those statements
read and write, so it can determine whether there's any interaction with
the code being moved. But it can't normally determine the behaviour of an
external function.
But this doesn't apply to functions which are inlined, or which use
e.g. gcc's __attribute__((pure)). Nor does it apply if the code being
moved doesn't read or write memory which might affect or be affected by
the function (e.g. if the code only reads or writes local variables and
there is no "legitimate" means for the function to obtain a pointer to any
of those variables).
In the case of pthread_mutex_lock(), the compiler doesn't know what that
function does, so it has to assume that it may read and/or write any
global data or any data which could be referenced via its pthread_mutex_t*
argument. Consequently, it can't move any code which reads or writes such
data around the function call.
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threads, signals, and atomicity of basic types Urs Thuermann <urs@isnogud.escape.de> - 2011-05-31 21:07 +0200
Re: threads, signals, and atomicity of basic types Xavier Roche <xroche@free.fr.NOSPAM.invalid> - 2011-05-31 22:44 +0200
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-01 11:58 +0100
Re: threads, signals, and atomicity of basic types Xavier Roche <xroche@free.fr.NOSPAM.invalid> - 2011-06-01 14:15 +0200
Re: threads, signals, and atomicity of basic types Xavier Roche <xroche@free.fr.NOSPAM.invalid> - 2011-06-02 13:49 +0200
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-02 13:40 +0100
Re: threads, signals, and atomicity of basic types Richard Kettlewell <rjk@greenend.org.uk> - 2011-05-31 22:50 +0100
Re: threads, signals, and atomicity of basic types Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-05-31 21:32 -0400
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-01 12:35 +0100
Re: threads, signals, and atomicity of basic types Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-01 23:01 -0400
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-02 11:22 +0100
Re: threads, signals, and atomicity of basic types Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-02 07:52 -0400
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-02 13:38 +0100
Re: threads, signals, and atomicity of basic types Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-02 21:27 -0400
Re: threads, signals, and atomicity of basic types David Schwartz <davids@webmaster.com> - 2011-06-01 12:55 -0700
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-01 22:54 +0100
Re: threads, signals, and atomicity of basic types Alex Fraser <me@privacy.net> - 2011-06-02 21:00 +0100
Re: threads, signals, and atomicity of basic types David Schwartz <davids@webmaster.com> - 2011-06-04 21:53 -0700
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-05 16:39 +0100
Re: threads, signals, and atomicity of basic types David Schwartz <davids@webmaster.com> - 2011-06-06 18:48 -0700
Re: threads, signals, and atomicity of basic types Nobody <nobody@nowhere.com> - 2011-06-07 06:50 +0100
Re: threads, signals, and atomicity of basic types David Schwartz <davids@webmaster.com> - 2011-06-07 06:04 -0700
Re: threads, signals, and atomicity of basic types Nobody <nobody@nowhere.com> - 2011-06-07 22:48 +0100
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-07 23:18 +0100
Re: threads, signals, and atomicity of basic types gordonb.lhpq6@burditt.org (Gordon Burditt) - 2011-06-07 18:28 -0500
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-08 11:09 +0100
Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-08 14:32 -0700
Re: threads, signals, and atomicity of basic types David Schwartz <davids@webmaster.com> - 2011-06-09 18:10 -0700
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-10 15:26 +0100
Re: threads, signals, and atomicity of basic types James Waldby <not@valid.invalid> - 2011-06-10 16:49 +0000
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-10 19:07 +0100
Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-10 13:13 -0700
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-07 12:55 +0100
Re: threads, signals, and atomicity of basic types David Schwartz <davids@webmaster.com> - 2011-06-07 06:02 -0700
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-07 16:05 +0100
Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-07 13:37 -0700
Re: threads, signals, and atomicity of basic types scott@slp53.sl.home (Scott Lurndal) - 2011-06-14 21:43 +0000
Re: threads, signals, and atomicity of basic types Nobody <nobody@nowhere.com> - 2011-06-15 03:07 +0100
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-15 10:22 +0100
Re: threads, signals, and atomicity of basic types Casper H.S. Dik <Casper.Dik@OrSPaMcle.COM> - 2011-06-15 09:40 +0000
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-15 13:20 +0100
Re: threads, signals, and atomicity of basic types Casper H.S. Dik <Casper.Dik@OrSPaMcle.COM> - 2011-06-15 13:19 +0000
Re: threads, signals, and atomicity of basic types Nobody <nobody@nowhere.com> - 2011-06-15 17:41 +0100
Re: threads, signals, and atomicity of basic types David Schwartz <davids@webmaster.com> - 2011-06-15 20:20 -0700
Re: threads, signals, and atomicity of basic types Nobody <nobody@nowhere.com> - 2011-06-15 17:31 +0100
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-15 18:31 +0100
Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-15 13:32 -0700
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-15 22:19 +0100
Re: threads, signals, and atomicity of basic types Nobody <nobody@nowhere.com> - 2011-06-16 05:02 +0100
Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-14 23:23 -0700
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-15 10:12 +0100
Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-15 02:53 -0700
Re: threads, signals, and atomicity of basic types David Schwartz <davids@webmaster.com> - 2011-06-08 10:19 -0700
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-10 16:50 +0100
Re: threads, signals, and atomicity of basic types David Schwartz <davids@webmaster.com> - 2011-06-15 20:24 -0700
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-16 22:11 +0100
Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-17 04:43 -0700
Re: threads, signals, and atomicity of basic types Michael Press <rubrum@pacbell.net> - 2011-06-19 01:43 -0700
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-19 13:35 +0100
Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-19 17:06 -0700
Re: threads, signals, and atomicity of basic types scott@slp53.sl.home (Scott Lurndal) - 2011-06-20 16:22 +0000
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-20 19:16 +0100
Re: threads, signals, and atomicity of basic types scott@slp53.sl.home (Scott Lurndal) - 2011-06-21 17:30 +0000
Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-21 13:02 -0700
Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-20 14:32 -0700
Re: threads, signals, and atomicity of basic types scott@slp53.sl.home (Scott Lurndal) - 2011-06-21 17:05 +0000
Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-19 17:01 -0700
Re: threads, signals, and atomicity of basic types Michael Press <rubrum@pacbell.net> - 2011-06-22 01:55 -0700
Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-22 03:36 -0700
Re: threads, signals, and atomicity of basic types David Schwartz <davids@webmaster.com> - 2011-06-02 21:53 -0700
Re: threads, signals, and atomicity of basic types David Schwartz <davids@webmaster.com> - 2011-06-03 04:24 -0700
Re: threads, signals, and atomicity of basic types Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-03 07:52 -0400
Re: threads, signals, and atomicity of basic types "Ersek, Laszlo" <lacos@caesar.elte.hu> - 2011-06-02 00:23 +0200
Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-01 20:11 -0700
Re: threads, signals, and atomicity of basic types "Ersek, Laszlo" <lacos@caesar.elte.hu> - 2011-06-02 22:54 +0200
Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-02 14:03 -0700
Re: threads, signals, and atomicity of basic types "Ersek, Laszlo" <lacos@caesar.elte.hu> - 2011-06-03 02:49 +0200
Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-02 17:54 -0700
Re: threads, signals, and atomicity of basic types Eric Sosman <esosman@ieee-dot-org.invalid> - 2011-06-02 21:36 -0400
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-03 16:25 +0100
Re: threads, signals, and atomicity of basic types "Ersek, Laszlo" <lacos@caesar.elte.hu> - 2011-06-03 03:12 +0200
Re: threads, signals, and atomicity of basic types "Ersek, Laszlo" <lacos@caesar.elte.hu> - 2011-06-02 00:45 +0200
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-02 11:26 +0100
Re: threads, signals, and atomicity of basic types Alex Fraser <me@privacy.net> - 2011-06-02 21:05 +0100
Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-01 20:21 -0700
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-02 11:31 +0100
Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-02 09:12 -0700
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-02 17:54 +0100
Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-02 14:00 -0700
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-03 15:19 +0100
Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-03 15:50 -0700
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-05 16:46 +0100
Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-05 15:28 -0700
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-06 13:35 +0100
Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-06 09:19 -0700
Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-06 09:26 -0700
Re: threads, signals, and atomicity of basic types Ian Collins <ian-news@hotmail.com> - 2011-06-07 09:43 +1200
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-07 13:29 +0100
Re: threads, signals, and atomicity of basic types Ian Collins <ian-news@hotmail.com> - 2011-06-08 09:04 +1200
Re: threads, signals, and atomicity of basic types Rainer Weikusat <rweikusat@mssgmbh.com> - 2011-06-08 12:34 +0100
Re: threads, signals, and atomicity of basic types Ian Collins <ian-news@hotmail.com> - 2011-06-09 07:37 +1200
Re: threads, signals, and atomicity of basic types Joshua Maurice <joshuamaurice@gmail.com> - 2011-06-07 14:55 -0700
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