On 30/10/2019 17.19, Ilya Dryomov wrote: > On Thu, Oct 24, 2019 at 11:49 AM David Howells <dhowells@xxxxxxxxxx> wrote: >> /* >> - * We use a start+len construction, which provides full use of the >> - * allocated memory. >> - * -- Florian Coosmann (FGC) >> - * >> + * We use head and tail indices that aren't masked off, except at the point of >> + * dereference, but rather they're allowed to wrap naturally. This means there >> + * isn't a dead spot in the buffer, provided the ring size < INT_MAX. >> + * -- David Howells 2019-09-23. > > Hi David, > > Is "ring size < INT_MAX" constraint correct? No. As long as one always uses a[idx % size] to access the array, the only requirement is that size is representable in an unsigned int. Then because one also wants to do the % using simple bitmasking, that further restricts one to sizes that are a power of 2, so the end result is that the max size is 2^31 (aka INT_MAX+1). > I've never had to implement this free running indices scheme, but > the way I've always visualized it is that the top bit of the index is > used as a lap (as in a race) indicator, leaving 31 bits to work with > (in case of unsigned ints). Should that be > > ring size <= 2^31 > > or more precisely > > ring size is a power of two <= 2^31 Exactly. But it's kind of moot since the ring size would never be allowed to grow anywhere near that. Rasmus
