• luka8088 (15/15) May 19 2012 Hello to all,
• Dejan Lekic (4/19) May 20 2012 This proggy will always print "1" because writeln() prints the
• David Nadlinger (6/9) May 20 2012 This is wrong. _Global_ (or static) variables are in TLS by
• Steven Schveighoffer (8/23) May 23 2012 It depends on hardware architecture. loads and stores of word sizes are...

luka8088 <luka8088 owave.net> writes:

Hello to all,

I would like to know if D guarantees that access to primitive variable 
is atomic ?

I was looking for any source of information that says anything about 
unsynchronized access to primitive variables. What I want to know is if 
it is possible (in any way and any OS / hardware) for the following code 
to output anything other then 1 or 2:

import std.stdio;
import core.thread;

void main () {
   int value = 1;
   (new Thread({ value = 2; })).start();
   writeln(value);
}

Thanks !

"Dejan Lekic" <dejan.lekic gmail.com> writes:

On Saturday, 19 May 2012 at 08:16:39 UTC, luka8088 wrote:
 Hello to all,

 I would like to know if D guarantees that access to primitive 
 variable is atomic ?

 I was looking for any source of information that says anything 
 about unsynchronized access to primitive variables. What I want 
 to know is if it is possible (in any way and any OS / hardware) 
 for the following code to output anything other then 1 or 2:

 import std.stdio;
 import core.thread;

 void main () {
   int value = 1;
   (new Thread({ value = 2; })).start();
   writeln(value);
 }

 Thanks !

This proggy will always print "1" because writeln() prints the 
value from the main thread. Spawned thread will have own value. 
Remember TLS is the default storage.

"David Nadlinger" <see klickverbot.at> writes:

On Sunday, 20 May 2012 at 13:51:45 UTC, Dejan Lekic wrote:
 This proggy will always print "1" because writeln() prints the 
 value from the main thread. Spawned thread will have own value. 
 Remember TLS is the default storage.

This is wrong. _Global_ (or static) variables are in TLS by 
default, but local variables aren't automagically »thread 
local« in any sense – the fact that value is implicitly 
heap-allocated due to the closure does not change that.

David

"Steven Schveighoffer" <schveiguy yahoo.com> writes:

On Sat, 19 May 2012 04:16:39 -0400, luka8088 <luka8088 owave.net> wrote:

 Hello to all,

 I would like to know if D guarantees that access to primitive variable  
 is atomic ?

 I was looking for any source of information that says anything about  
 unsynchronized access to primitive variables. What I want to know is if  
 it is possible (in any way and any OS / hardware) for the following code  
 to output anything other then 1 or 2:

 import std.stdio;
 import core.thread;

 void main () {
    int value = 1;
    (new Thread({ value = 2; })).start();
    writeln(value);
 }

 Thanks !

It depends on hardware architecture.  loads and stores of word sizes are  
generally atomic.  A better (more portable) version would be to make value  
size_t or ptrdiff_t, which should mimic CPU word size.

In very crude multithreading apps, I rely on this all the time.  But you  
have to be careful not to change the value in more than one thread, or you  
are subject to racing.

-Steve