# Module Netmcore_mutex

    module Netmcore_mutex: sig .. end

Mutexes

Mutexes 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 mutex 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`.

Mutexes are special values, and cannot be copied or moved.

Mutexes are implemented on top of semaphores. For mutex types with deadlock handling, the process is considered as the owner, and process ID is the identifier.

Only mutexes of type ``Normal` are thread-safe, i.e. can be used when the worker processes also use threads internally.

    type mutex_type = [ `Errorcheck | `Normal | `Recursive ]

Types:

- ``Normal` mutexes have no checks for deadlocks.
- ``Errorcheck` mutexes check whether the process locks the mutex again, and fail in this case. Also, only the owning process can unlock a mutex.
- ``Recursive` mutexes allow that the owner locks several times. The same number of unlock requests need to be issued to give up the ownership

    type mutex

    val dummy : unit -> mutex

A dummy mutex is non-functional, but can be used to put something into `mutex`-typed variables

    val create : Netmcore_heap.mutator -> mutex_type -> mutex

`create m t`: Creates a mutex of type `t`, and pushes it to the heap, using the mutator `m`.

After being pushed to the heap, the mutex can be used. It is nonsense to copy it outside the heap.

    val lock : mutex -> unit

Locks the mutex

    val unlock : mutex -> unit

Unlocks the mutex

    val destroy : mutex -> unit

Destroys the mutex