how

[seatrix]

if I have a buff like below:

**12
^
|
A
Region B is not used.
Now, if I want to offer more than 2 bytes for example 4 bytes "3456", the bipbuf_offer will return 0.
But there are enough buffer for 4bytes, it can offer region B and the result will like below:

345612**
^ ^
| |
B A

[FlorianFranzen]

Region B is not a second equal sized buffer, that is allocated when the first buffer is full, but just a second consecutive region at the beginning of the ring buffer, that comes into use when the buffer is about to "roll over".
Region B allows the ring buffer to skip the last few bytes at the end of the buffer. This guarantees that all returned memory blocks by the ring buffer are consecutive.