The q_next field in the queue_t structure can be referenced in open, close, put, and service procedures as well as the synchronous callback procedures (scheduled with qtimeout(9F), qbufcall(9F), and qwriter(9F)). However, the value in the q_next field should not be trusted. It is relevant to the STREAMS framework, but may not be relevant to a specific module.
All other module code, such as interrupt routines, timeout(9F) and esballoc(9F) callback routines, cannot dereference q_next. Those routines have to use the "next" version of all functions. For instance, use canputnext(9F) instead of dereferencing q_next and using canput(9F).