Re: unsynchronized access to primitive variables

[Steven Schveighoffer]

On Sat, 19 May 2012 04:16:39 -0400, luka8088 luka8...@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

Re: unsynchronized access to primitive variables

[Dejan Lekic]

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.

Re: unsynchronized access to primitive variables

[David Nadlinger]

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