Subj : Re: Boss Told Me You Can't Use Semaphore, But I Have To Do Something
To   : comp.programming.threads
From : Scott McPhillips [MVP]
Date : Tue Jul 12 2005 11:52 am

clusardi2k@aol.com wrote:
> Hello,
> 
>    I have a assignment just thrown onto my desk by my boss who is
> condescending at times. I'm lost! Am I missing something?
> 
>    Anyway, what is the easiest way to solve the below task. Do I
> have alternatives. I'm looking for a detailed description and maybe
> code!
> 
>    The job is:
> 
>    There are multible programs which use the same library routine
> which is an interface to what I'll call a service program.
> 
>    The problem is that only 1 request at a time can be serviced by
> the service program. The service program cannot be changed at all.
> 
>    My boss told me to just use somehting such as shared memory
> (via memmap) and set a flag in there to 1 when a program wants
> service. The various programs are to wait in a loop until that
> flag becomes 0 or a time-out occurs. If the flag becomes 0 then
> one process will set it to 1 along with setting other parameters
> to send to the service program. If a process times-out it will
> return without receiving any service. But, if a process received
> service the process will set the flag to 0 for the next process.
> 
>    The problem is, woun't this lead to programs setting that
> shared memory flag to 1 at the same time causing an eventual loss
> of service etc. I.E.:They clobber each other.
> 
>    The way I see it is the flag described above is a critical
> section and must be protected by a mutual exclusion semaphore.
> 
>    I don't see anything wrong with using shared memory for sending
> parameters to the service routine.
> 
>    My boss says I am not allowed to use semaphores due to
> unexplained reasons.
> 
>    O'kay what should I do? 
> 
>    Thank you, 
>    Christopher Lusardi
> 

Sounds like Windows?  If so look up InterlockedCompareExchange.  In one 
call it can synchronize and change the bool.

-- 
Scott McPhillips [VC++ MVP]

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