Re: Are lock free algorithms possible in C++?

cpp4ever <>
Mon, 12 Jul 2010 16:26:05 CST
On 07/12/2010 02:22 PM, Andy wrote:

Assuming I've got an atomic & fenced CAS instruction available via a
library; is it possible to use this to write a lock free algorithm in C

Although the memory fence would prevent hardware re-ordering I'm
worried that compiler could reorder memory access rendering the fence
useless. For example consider the below, grossly simplified example:

bool continue = true;
while(continue) {
    while(isLocked) { }
    // atomically set isLocked to true if it has the value false
    if(AtomicTestAndSet(&isLocked, true, false)) {
         continute = false;
SharedStdVector[3] += 10;

Is there anything that prevents the compiler from reordering the
access to SharedStdVector so that it is executed before mutex is
aquired? Is there anything I can do to prevent such reordering?

You need to look up the thread library implementation for the C++ you
are using. This will provide you with various mechanisms for creating
thread safe code. In your case you'd need a mutex, (short for mutual



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