Re: Boost.Threads - once functions

From: williamkempf_at_[hidden]
Date: 2001-08-01 08:56:06

--- In boost_at_y..., tneumann_at_w... wrote:
> > POSIX uses pthread_once to do this, 
> > but there's no Win32 equivalent.  Instead, Win32 programmers are 
> > expected to make use of DllMain to handle this sort of 
> initialization 
> in Win32 pthread_once can be simulated by using something similar 
to 
> the the following:

I know there's ways to simulate the functionality.  Other wise the 
question wouldn't have been posed for whether or not Boost.Threads 
should have once methods ;).

> struct OnceInfo {
>    DWORD counter;
>    bool  done;
> };
> 
> static OnceInfo info = {-1,false};
> 
> void doOnce(OnceInfo& info,void (*foo)())
> {
>    if (InterlockedIncrement(&info.counter)==0) {
>       foo();
>       info.done=true;
>    } else while (!info.done) Sleep(0);
> }

This version is very similar to the pthreads-win32 implementation and 
suffers from two problems, IMHO.  First, like I said in another post 
the pseudo spin lock can cause poor performance if foo() does some 
lengthy operations (and by lengthy I just mean enough operations to 
be longer than a single time slice).  Second, the InterlockedIncrement
() approach may well wrap and cause major problems.  This mistake is 
easily fixed by using InterlockedExchange() instead.

Bill Kempf