[linux-dvb] Re: Bug in ring buffer implementation

[Emard <emard@softhome.net>]

On Sat, Feb 08, 2003 at 04:10:42AM +0100, Oliver Endriss wrote:
> Hi,
> 
> while reading through the driver sources, I found that the free byte 
> count of the ring buffers is not calculated correctly.
> 
> In the current implementation, code like this is used (example taken 
> from dmxdev.c):
> 
>         free=buf->pread-buf->pwrite;    (1)
>         split=0;
>         if (free<=0) {
>                 free+=buf->size;
>                 split=buf->size-buf->pwrite;
>         }
> 
> (1) is not correct. It should read:
>         free=buf->pread-buf->pwrite-1;  (2)
>                                    ^^
>         ...
> 
> Obviously a ring buffer of size <n> cannot hold more than <n-1> bytes.
> Otherwise you cannot distinguish between an empty and a full buffer.

Good point. Better to have one byte less in the buffer than to think
should we have used "<=" or just "<" :)



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