# Module Netsys_sem

    module Netsys_sem: sig .. end

## Generic anonymous semaphores

This module purely exists to also support "kind of anonymous" sempahores on MacOS X (which only has named semaphores). On other OS, it is just a wrapper for the functions in `Netsys_posix`.

Note that on OS X named semaphores have a max name length of 31 characters (including the / at the beginning), and that `Netsys_sem` uses 9 characters for its own purposes, leaving 22 characters for the prefix. (On other OS this is less restricted.)

    val have_anon_semaphores : unit -> bool

Returns `true` if anonymous semaphores are supported on this system, either natively or emulated via named semaphores.

Constants.

    val sem_value_max : int

The maximum value of a semaphore, but at most `max_int`

    val sem_size : int

The size of an anonymous semaphore in bytes (`sizeof(sem_t)`)

Types.

    type container

The container of the semaphore is the shared memory object

    type prefix = string

A name starting with a slash. Must not contain further slashes

    type anon_semaphore

    type sem_open_flag = Netsys_posix.sem_open_flag =

`|` | `SEM_O_CREAT`
`|` | `SEM_O_EXCL`

Container functions.

    val container : prefix -> container

`container prefix`: The prefix shall identify the container uniquely. Once can e.g. use the path of the shared memory object. The prefix is used to construct names for persistent objects.

Note that containers have kernel persistence! They are not automatically deleted when the process finishes. Call `drop` to delete containers, or `create_container` to force their creation as fresh objects.

If the container does not exist yet, it is created. Otherwise the container is just opened.

    val create_container : prefix -> container

`create_container prefix`: Like `container`, but the container is always created. A previous instance is first deleted.

    val prefix : container -> prefix

Return the prefix

    val drop : container -> unit

Drop the semaphores in this container, and delete the container.

This function is a no-op if the OS supports anonymous semaphores directly (because in this case the deletion of the container will automatically destroy the semaphores).

Note that there is a general problem when a container is deleted or dropped by a process while it is still being used by another one. This will generally not generate errors, but can cause a lot of confusion, because the processes will partly access the same semaphores, and partly different semaphores.

    val unlink : prefix -> unit

Unlinks the identified container if it exists, and unlinks all possible named semaphores.

Semaphore functions.

    val sem_init : container ->
           Netsys_types.memory -> int -> bool -> int -> anon_semaphore

`sem_init cont mem pos pshared init_value`: Initializes the memory at position `pos` to `pos + sem_size() - 1` as anonymous semaphore. If `pshared` the semaphore is shared between processes. `init_value` is the initial non-negative value (max is `sem_value_max`).

    val sem_destroy : container -> anon_semaphore -> unit

Destroys the anonymous semaphore

    val as_sem : container ->
           Netsys_types.memory -> int -> anon_semaphore

`as_sem mem pos`: Interprets the memory at position `pos` to `pos + sem_size() - 1` as anonymous semaphore. The memory region must already have been initialized.

    val sem_getvalue : anon_semaphore -> int

Returns the value of the semaphore. If the value is bigger than what can be represented as `int`, an `EINVAL` error is returned.

The returned value is non-negative - if the underlying POSIX function reports a negative value zero is returned instead.

Unavailable on MacOS.

    val sem_post : anon_semaphore -> unit

Unlocks the semaphore (increases the value by 1)

    type sem_wait_behavior = Netsys_posix.sem_wait_behavior =

`|` | `SEM_WAIT_BLOCK`
`|` | `SEM_WAIT_NONBLOCK`

    val sem_wait : anon_semaphore -> sem_wait_behavior -> unit

Locks the semaphore (decreases the value by 1). If the semaphore value is already zero, and `SEM_WAIT_BLOCK` is given, the function waits until another process or thread posts. If `SEM_WAIT_NONBLOCK` the error `EAGAIN` is returned.

`sem_wait` may be interrupted by signals.

    module Debug: sig .. end