module Netmcore_sem:sig..end
Semaphores
Semaphores are here defined as values that reside in shared heaps
(Netmcore_heap), for example in the header field of
Netmcore_array or somewhere else in heap-allocated
data structures.
In order to ensure that the semaphore is in the heap, the
special function create must be used to initialize it
there. As create requires a mutator as argument, this is
only possible by calling create from the callback of
Netmcore_heap.modify.
Semaphores are special values, and cannot be copied or moved.
Note that Netsys_posix provides wrappers for direct use
of the semaphore functionality of the OS. These wrappers, however,
cannot be used together with heaps, because semaphores are
there represented as bigarrays, which cannot be pushed to heaps.
This module here uses a trick to make it possible nevertheless.
type
val dummy : unit -> semaphoreA dummy semaphore is non-functional, but can be used to put something
into semaphore-typed variables
val create : Netmcore_heap.mutator -> int -> semaphorecreate m value: Creates a semaphore with initial value, and
pushes it to the heap, using the mutator m.
After being pushed to the heap, the semaphore can be used. It is nonsense to copy it outside the heap.
val destroy : semaphore -> unitDestroys the semaphore
val getvalue : semaphore -> intReturns the value of the semaphore
val post : semaphore -> unitIncrements the semaphore by one, and notifies one of the waiting processes.
val wait : semaphore -> Netsys_posix.sem_wait_behavior -> unitDecrements the semaphore by one. Semaphores cannot become negative. If the semaphore is already 0, the wait behavior defines how to go on:
Netsys_posix.SEM_WAIT_BLOCK: Wait until the semaphore is
postedNetsys_posix.SEM_WAIT_NONBLOCK: Raise EAGAINWaiting is restarted after running a signal handler.
