[LAU] Simple, easy multithreaded circular buffer library for Linux?

[Olivier Guilyardi]

Fons Adriaensen wrote:
> On Fri, Oct 17, 2008 at 08:06:55PM +0200, Joern Nettingsmeier wrote:
> 
>> excuse my chiming in here, i'm not really much of a c programmer... but
>> how can this
>>   -  rb->read_ptr += n1;
>>   -  rb->read_ptr &= rb->size_mask;
>>   +  rb->read_ptr = (rb->read_ptr + n1) & rb->size_mask;
>> fix anything?
>>
>> iiuc, both versions are equivalent. a context switch could happen just
>> as well after the parenthesis has been computed..!?
>> putting stuff on one line doesn't make it atomic. maybe you are now
>> getting another compiler optimization that helps to hide the bug?
> 
> The idea is that it is very unlikely that the compiler
> would store the intermediate result in rb->read_ptr,
> and so this value is updated only once.
> 
> I agree it is fragile, there is nothing stopping
> the compiler from doing it wrong. Unless the
> the pointers are made volatile again.

I don't see why it's fragile. There's no reason for the compiler to modify
rb->read_ptr if not asked to. Buf if that's the case, because of some sort of
dark voodoo optimization, then we could use a temporary variable to hold the
intermediary result. Better, all computation could be done on this temporary
variable and transfered into rb->read_ptr once finished, so that modifying the
later only involves memory copy.

> One real solution is to *not* use the size_mask
> at this point, but apply it only when read_ptr
> and write_ptr are read. So only a single addition
> remains. The pointers are allowed to grow, they
> will wrap around at the word size, but since the
> buffer size is a power of two they are still
> correct when that happens.
> 
> This is what is done in the C++ class I
> posted earlier.

You class looks good, but some work is needed to provide a Jack backward
compatible API based on it.

IMO, one important thing is to include unit/regression tests, for the ringbuffer
as well as general Jack operation, in the official jack distribution, that one
can run with a simple "make test". I'll try to do this for this test suite I
wrote, once cleaned up.

-- 
  Olivier Guilyardi / Samalyse