MT STREAMS Perimeters
STREAMS uses a facility known as perimeters for handling thread concurrency in STREAMS modules. Perimeters allow the module writer to select conditions that will result in exclusive access to a queue, a pair of queues, or all the queues in a module. This makes multithreading issues easier to work with in STREAMS.
Perimeters work somewhat like reader/writer locks, where there can be many readers, but only one writer. A synchronous access to the perimeter is similar to holding the writer lock, in that only one thread can be in the perimeter at a time. Synchronous entry will hold a perimeter exclusively until the thread eventually unwinds out of the perimeter (usually when it returns from a put/putnext or a qwriter call that initially invoked the synchronous behavior). While a thread has synchronous access to the perimeter, all other access (synchronous or asynchronous) will be deferred.
An asynchronous access is similar to the reader lock, where many threads can be in the perimeter at a time, including the possible recursive entry of a thread previously entering the perimeter. The asynchronous “claim” is not released until the thread winds out of the put/putnext. Because asynchronous access is similar to the reader lock, any synchronous access will be deferred or blocked until all asynchronous claims are released.
 | Caution - Hardening Information. The perimeter data is private to the STREAMS framework, and should not be modified by the module itself. |
STREAMS enables the definition of two synchronous perimeters. One is an inner perimeter, and is used to define synchronous entry points on a queue or a queue pair (the read and write queue's for a specific module instance). It also identifies an outer perimeter, which is made up of all the inner perimeters for all the queues for a specific module.
There is also a special inner perimeter, PERMOD, that is similar to the outer perimeter, but does not have the overhead of the outer perimeter. PERMOD identifies a single synchronous entry point for all queues for this module. Because PERMOD is like a hybrid of an inner perimeter and outer perimeter, the PERMOD perimeter cannot have an outer perimeter.