Subj : Concurrent write undefined?
To   : comp.programming.threads
From : Jonas Latt
Date : Mon May 23 2005 08:33 am

Dear all

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

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.

Many thanks for your appreciated help

Jonas

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