Subj : Re: Concurrent write undefined?
To   : comp.programming.threads
From : David Schwartz
Date : Mon May 23 2005 01:06 pm


"Jonas Latt" <Jonas.Latt@cui.unige.ch> wrote in message 
news:4291886f$1@nntp.unige.ch...

> I have a question on the memory model underlying the pthread library (I am 
> programming in C++ with pthreads on a Linux platform).

    Do you want what you are guaranteed? Or what happens to happen on that 
platform?

> In the code I am concerned about, several threads write concurrently to 
> the same shared variable (I am implementing the PRAM version of a 
> quicksort. In this model, all threads write their ID to the same location, 
> in order to determine in a random way the next pivot). My question is: do 
> I need to protect this shared variable by a Mutex? Or is the result 
> undefined if I don't? I am of course aware that the result is undefined in 
> the sense that it is unpredictable which thread wins the race and writes 
> the number which will finally be contained in the shared variable; this is 
> actually a desired side-effect of the algorithm. But do I at least have 
> the guarantee that the operation results in that a well-defined number 
> (one of those proposed by the threads) is written to the shared variable, 
> or do I have completely undefined behavior?

    You have completely undefined behavior.

> When I implement it, the algorithm works well without Mutex. But I doubt 
> that what I am doing is portable or even reproducible. Can anybody give me 
> a hint on this? Does the answer to this kind of questions depend on the 
> thread library? Or the programming language? Or the hardware? I would also 
> be very grateful for a pointer to some literature on these issues.

    This sounds like a really strange "optimization". If you want a random 
number, why not just use a reasonable random number generator?

    DS

.