Interlocked functions c++

Question

I am developing a system that uses shared memory and interlocked functions.

Let's assume i have volatile unsigned int n, a, b. I want to do the following pseudocode atomicly:

if (a <= n && n < b)
{
    n++;
}
else
{
    //Do nothing
}

How would I do that? Can you use multiple Interlocked functions together?

Here is some good reading to get you started: http://blogs.msdn.com/b/oldnewthing/archive/2004/09/15/229915.aspx — vcsjones, Jul 03 '11 at 16:40
Take everything you think you know about `volatile`, with respect to multithreading, and throw it away. — GManNickG, Jul 03 '11 at 17:16

score 5 · Accepted Answer · answered Jul 03 '11 at 16:36

5

You need a lock or a CAS type operation. No amount of volatile is going to help here. Neither will a true atomic data type.

answered Jul 03 '11 at 16:36

David Heffernan

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CAS is very well-suited for this type of operation. Good call. Here are [some examples](http://blogs.msdn.com/b/oldnewthing/archive/2004/09/15/229915.aspx). – Ben Voigt Jul 03 '11 at 16:38
@Ben - Interesting that we both posted a link to The Old New Thing. – vcsjones Jul 03 '11 at 16:41
2

@vcs @ben just mentioning raymond is usually worth a couple of upvotes – David Heffernan Jul 03 '11 at 16:50

score 2 · Answer 2 · answered Jul 04 '11 at 06:34

Synchronization primitives (such as semaphores, mutexes, etc.) are provided by OS-specific libraries, and not in the language itself. C/C++ have no intrinsic "synchronized" keyword.

If you're programming on Linux, look at Posix threads or the Boost library:

http://www.yolinux.com/TUTORIALS/LinuxTutorialPosixThreads.html

If you're programming on native Windows Win32 (Win98 or higher), you can use APIs like EnterCriticalSection() and InterlockedAdd(), among others:

http://msdn.microsoft.com/en-us/library/ms686353%28v=VS.85%29.aspx

If you're programming Windows with .Net, however, you're back to synchronization primitives being part of the standard .Net libraries:

http://msdn.microsoft.com/en-us/library/ms173179.aspx

'Hope that helps .. PSM

Interlocked functions c++

2 Answers2