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Groups > comp.unix.programmer > #677
| From | Eric Sosman <esosman@ieee-dot-org.invalid> |
|---|---|
| Newsgroups | comp.unix.programmer |
| Subject | Re: threads, signals, and atomicity of basic types |
| Date | 2011-05-31 21:32 -0400 |
| Organization | A noiseless patient Spider |
| Message-ID | <is44tn$lhj$1@dont-email.me> (permalink) |
| References | <ygfwrh6d6yr.fsf@janus.isnogud.escape.de> |
On 5/31/2011 3:07 PM, Urs Thuermann wrote:
> Are basic types like int, long int, and long long int atomic when
> accessed from different threads or from a signal handler, or must they
> be protected using a pthread_mutex?
Some may be atomic, others not. Some may be atomic if
suitably aligned, non-atomic if misaligned. It's up to the
individual implementation.
But atomicity is usually not enough. Even if Thread T1 can
store a new value in variable `foobar' and do so atomically, you
also need some idea of when (or even whether!) Thread T2 will
observe the new value. Atomicity says that T2 will see either
the old value or the new value and not some kind of intermediate
amalgam, but says nothing about how many times T2 will see the
old before noticing the new.
Finally, note that read-modify-write is almost never atomic
unless special machine instructions are used. In particular,
`foobar++' and `foobar >>= 1' and `foobar |= 0x80' are almost
certainly non-atomic.
> I think that those types will be atomic on most platforms if they can
> be accessed in a single machine instruction and single bus cycle,
> i.e. 32 bit ints on ia32, but not 64 bit ints on that platform.
"Most platforms." Well, sure, for suitable values of "most."
> In general, and if code should be portable, you cannot count on
> atomicity and should use locks, right?
Use locks, or use special instructions like compare-and-swap
or store-interlocked; these will usually be packaged inside what
appear to be C functions. The important thing is that they will
provide both the atomicity and the visibility you need for effective
use of the shared variable, using memory barriers or other machine-
specific what-have-you.
> Next, if concurrent access is from different threads you can use
> pthread_mutex_t to protect the access, but how can I protect access
> from a signal handler? AFAIK, if the signal handler can run in the
> context of a thread holding the lock, it cannot take the lock, too.
> Otherwise, you get undefined behavior. Are there other mechanisms in
> POSIX and/or ISO C?
Semaphores are async-signal-safe, I believe. Even so, you can't
use them as a "locking" protocol, lest the signal arrive while the
protected object is "locked." That is, the shared state must be the
semaphore itself, not something the semaphore "protects."
> Yet another question: Do I need to use volatile to prevent compiler
> optimization even if I protect data with a pthread_mutex?
Volatile is neither necessary nor sufficient. In particular,
it doesn't solve the staleness problem mentioned above.
--
Eric Sosman
esosman@ieee-dot-org.invalid
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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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