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Module Netmcore_sem

module Netmcore_sem: sig .. end

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 semaphore 
val dummy : unit -> semaphore
A dummy semaphore is non-functional, but can be used to put something into semaphore-typed variables
val create : Netmcore_heap.mutator -> int -> semaphore
create 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 -> unit
Destroys the semaphore
val getvalue : semaphore -> int
Returns the value of the semaphore
val post : semaphore -> unit
Increments the semaphore by one, and notifies one of the waiting processes.
val wait : semaphore -> Netsys_posix.sem_wait_behavior -> unit
Decrements 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 posted
  • Netsys_posix.SEM_WAIT_NONBLOCK: Raise EAGAIN
Waiting is restarted after running a signal handler.
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