Module Netcamlbox

module Netcamlbox: sig .. end

Camlboxes are a fast IPC mechanism to send Ocaml values from one process to another. Source and destination processes must run on the same machine (no network). The Ocaml value is copied to a shared memory object where it can be directly accessed by the receiver without unmarshalling step. This means the sender writes the value into the shared memory in a format that can immediately interpreted by the receiver.

A camlbox is owned by the single receiving process. Only this process (or a fork) can look for new messages and can read them. There can be any number of sending processes, i.e. we have a n:1 message passing scenario.

The receiver process creates the camlbox, and is seen as the owner. The receiver is accountible for deleting the camlbox when it is no longer needed.

The sender(s) can send messages to any existing camlbox. There is no notification whether the messages are actually read. The sender, however, blocks when the destination camlbox is full, and will only proceed when the receiver makes room for new messages. If there is space in the camlbox the sender does not need to synchronize with the receiver, i.e. it is possible to put a message into the box when the receiver is busy with something else (asynchronous send operation).

Camlboxes have a fixed capacity of messages, and the message slots have a fixed maximum length. The messages can have any type with only a few restrictions (e.g. no functions and no custom blocks). There is no check whether the sender and the receiver assume the same type of the messages. This is left to the user. Breaking this assumption will lead to unpredictable effects, including program crashes. It is strongly advised to only communicate between processes that run the same executable.

The user is also responsible for keeping only references to existing messages. It is possible to get a value pointer for a certain message and then to delete the message. The user must no longer access the value - once the value is deleted it may be overwritten, and the program may crash. Another danger is that message values are modified so that pointers to heap values are put into the message. This may lead to delayed crashes when the heap value is moved to a different location or is even deleted by the garbage collector. There is nothing the camlbox implementation can do about that.

On the system level, camlboxes are stored in POSIX shared memory objects. These objects have kernel persistence and continue to live after the process creating the camlbox has terminated without unlinking the box.

This module requires Ocaml 3.11 or newer. The system must support POSIX shared memory and POSIX semaphores. Camlboxes may be used in multi-threaded programs as long as the values camlbox and camlbox_sender are not used by several threads at the same time.

Camlboxes can be used to gain speed-ups on multi-cores. See examples/camlbox/README in the distribution tarball for an example how to accomplish this.

type camlbox_address = string

The address of a camlbox is a string that does not contain slashes. Addresses are system-global.

type camlbox

A camlbox may receive messages

type camlbox_sender

An endpoint that may send messages to a camlbox

exception Empty

exception Message_too_big

val create_camlbox : camlbox_address -> int -> int -> camlbox

create_camlbox addr n size: Creates a new camlbox for up to n messages of size bytes. The messages are numbered from 0 to n-1. The camlbox is only meaningful for the creating process, and must not be directly accessed by other processes. Other processes can only send using a camlbox_sender.

It is an error if the camlbox already exists.

val unlink_camlbox : camlbox_address -> unit

Removes the global name of the camlbox. All functions requiring a camlbox_address as input will not find the box anymore. The box, however, continues to exist until the receiver and the senders are done with it.

val camlbox_fd : camlbox_address -> Unix.file_descr

Opens a new file descriptor to this address

val camlbox_capacity : camlbox_address -> int

Returns the maximum number of messages n

val camlbox_msg_size : camlbox_address -> int

Returns the max size of a message in bytes

val camlbox_messages : camlbox_address -> int

Returns the number of messages at the moment

val camlbox_get : camlbox -> int -> 'a

camlbox_get box k: Returns message number k from box. The returned value lives in the camlbox, and using it is only safe as long as the camlbox exists and the message is not deleted.

If there is no message at k the exception Empty will be raised.

The result value must have the same type as the sent value. This is not checked, however. Violating this rule is likely to crash the program.

val camlbox_delete : camlbox -> int -> unit

camlbox_delete box k: Deletes the message number k from box. Any value obtained via camlbox_get for a message or a part of a message becomes invalid and must not be used anymore. There is no way to check this - violating this rule is likely to crash the program.

If there is no message at k the exception Empty will be raised.

val camlbox_wait : camlbox -> int list

Waits until new messages arrive, and return the message numbers. A new message is only reported once by camlbox_wait. The order of the messages is not specified.

Only one thread at a time must wait for new messages.

It is allowed that this function returns the empty list.

val camlbox_cancel_wait : camlbox -> unit

Cancels a camlbox_wait operation called by a different thread

val camlbox_sender : camlbox_address -> camlbox_sender

Prepares for sending

val camlbox_sender_of_fd : Unix.file_descr -> camlbox_sender

Gets a sender for a file descriptor from camlbox_fd.

val camlbox_send : camlbox_sender -> 'a -> unit

Sends a message to a camlbox. The value must be boxed (neither char, bool, int, nor a variant type), and a number of restrictions apply:

The size of the representation must not exceed the maximum message size of the camlbox, or the exception Message_too_big is raised.
Objects, closures, and lazy values are not supported
Abstract and custom block values are not supported. This also holds for bigarrays, int32, int64, and nativeint.
(CHECK: atoms like empty arrays)
Values returned by C wrappers that do not use abstract or custom blocks for wrapping data may break this function.

The value is copied to the receiving camlbox.

This function blocks until the receiving camlbox has free space.

Several threads may try to send messages at the same time.

val camlbox_wake : camlbox_sender -> unit

Sends an "empty message" - this only means that if the receiving thread is waiting for new messages it is interrupted and camlbox_wait will return the empty list.

This function is non-blocking.

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