Subj : Re: buffer implementation using semaphores
To   : comp.programming
From : Joe Seigh
Date : Sat Sep 17 2005 09:45 am

metrix007@yahoo.com wrote:
> I have a Semaphore and Buffer class, but I am not sure if I have
> implemented it correctly, it is posted here now, I was just after tips
> and suggestions..or pointing out erros which are not obvious(to me).
> /*
>         This class implements a bounded buffer that stores Objects and
> uses
>         blocking primitives based on semaphores when the buffer is full
> 
> or
>         empty. The buffer can be accessed by any number of threads.
> */
> public class Buffer
> {
> 
> 
> //      ...
>  // private Object store[] = new Object[size];
>   private int inptr = 0;
>   private int outptr = 0;
>     private int size = 0;
>   Semaphore spaces  = new Semaphore(size);
>   Semaphore elements = new Semaphore(0);
> 
> 
>         // creates a bounded buffer with capacity cap
>         public Buffer()
>         {
> 
> 
>         }
> 
> 
>         // places the object o in the buffer; if no space is left in
>         // the buffer, block until a space becomes available
>         public void put(Object o)
>         {
>     spaces.down() ;
>    // store[inptr] = o ;
>     inptr = (inptr+1) % size ;
>     elements.up() ;

This won't work for more than one producer. E.g.
thread 1:  spaces.down();
thread 2:  spaces.down();
thread 1:  store[inptr] = o;
thread 2:  store[inptr] = o;  // overlays store by thread 1

You're going to lose objects and/or object stores.  There
are other problems that can occur also.  I only gave one
example.  You could lose the increment of inptr.   Etc, etc, ...

-- 
Joe Seigh

When you get lemons, you make lemonade.
When you get hardware, you make software.

.