Re: [boost] [singleton] call_once

From: Anthony Williams (anthony_w.geo_at_[hidden])
Date: 2005-09-13 11:00:09

Jason Hise <chaos_at_[hidden]> writes:

> John Maddock wrote:
>
>>The mutex is unique to both the calling process and the call_once flag, but 
>>not the function pointer (however there should always be a one to one 
>>correspondence between the once-flag and the function pointer).
>>
> I should probably elaborate on my problem a little bit more.  I wanted 
> to create a function that makes a given call atomic, but doesn't enforce 
> that the function be called only once.  I thought I could do so by using 
> a once_flag allocated on the stack immediately before invoking 
> call_once, so the flag would always be reinitialized to BOOST_ONCE_INIT 
> and call_once could just offer me it's atomic guarantees, but apparently 
> that won't work.  I guess I'll just have to find some place to put my 
> own mutex where it can be constructed safely.

Calls through call_once are only atomic with respect to other calls that
reference the same once_flag. If you want mutual exclusion, you need a shared
mutex --- why not just create a function-local static mutex, lock it before
your function call, and unlock it afterwards?

Anthony
-- 
Anthony Williams
Software Developer
Just Software Solutions Ltd
http://www.justsoftwaresolutions.co.uk