Ocamlnet 3 Reference Manual : Netcamlbox

# 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 via `camlbox_get` 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. If this is a problem, it is advised to use `camlbox_get_copy` instead which is not dangerous in this respect.

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.

Examples. There a few examples in the distribution tarball (examples/camlbox).

Multi-core: 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.

Integration into event-based programs: See the section below, Integration into event-based programs.

Since OCaml-4.01: This OCaml version changed the semantics of the built-in primitives `caml_modify` and `caml_initialize`. Essentially, it is no longer possible to modify OCaml values residing outside the regular OCaml heap. As we do this inside Netcamlbox, this change affects this library. Fortunately, there is a workaround on systems supporting weak symbols (all ELF systems and OS X): Here, `caml_modify` and `caml_initialize` are overridden by Netcamlbox so that they are again compatible. Note that this is a global modification of the runtime system!

Future versions of Ocamlnet may solve this problem differently.

    type camlbox_address = string

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

    type 'a camlbox

A `camlbox` may receive messages of type `'a`

    type 'a camlbox_sender

An endpoint that may send messages of type `'a` to a camlbox

    exception Empty

    exception Message_too_big

    val create_camlbox : camlbox_address -> int -> int -> 'a 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.

It is suggested that the result of `create_camlbox` is immediately coerced to the right type `t`, e.g.

            let box = (create_camlbox addr n size : t camlbox)

as this ensures type safety for all following operations.

Note that camlboxes have kernel persistence! They are not automatically deleted when the process finishes. Call `unlink_camlbox` to delete camlboxes.

    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 format_camlbox : camlbox_address ->
           Unix.file_descr -> int -> int -> 'a camlbox

`format_camlbox addr fd n size`: The file `fd` is mapped into memory, and formatted as camlbox.

In Ocamlnet-3.6, the function got the extra `camlbox_address` argument.

    val camlbox_addr : 'a camlbox -> camlbox_address

returns the address

    val camlbox_saddr : 'a camlbox_sender -> camlbox_address

returns the address

    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_bcapacity : 'a camlbox -> int

same for an already opened box

    val camlbox_scapacity : 'a camlbox_sender -> int

same for a box already opened for sending

    val camlbox_msg_size : camlbox_address -> int

Returns the max size of a message in bytes

    val camlbox_bmsg_size : 'a camlbox -> int

same for an already opened box

    val camlbox_smsg_size : 'a camlbox_sender -> int

same for a box already opened for sending

    val camlbox_messages : camlbox_address -> int

Returns the number of messages at the moment

    val camlbox_bmessages : 'a camlbox -> int

same for an already opened box

    val camlbox_smessages : 'a camlbox_sender -> int

same for a box already opened for sending

    val camlbox_get : 'a 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_get_copy : 'a camlbox -> int -> 'a

`camlbox_get box k`: Returns a deep copy of message number `k` from `box`. This is safer than `camlbox_get`, because the returned value remains valid when the message is deleted from the box.

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 : 'a 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. (In doubt use `camlbox_get_copy` instead which cannot interfer with `camlbox_delete`.)

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

    val camlbox_wait : 'a 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 : 'a camlbox -> unit

Cancels a `camlbox_wait` operation called by a different thread

    val camlbox_sender : camlbox_address -> 'a camlbox_sender

Prepares for sending.

It is suggested that the result of `camlbox_sender` is immediately coerced to the right type `t`, e.g.

            let box = (camlbox_sender addr : t camlbox_sender)

as this ensures type safety for all following operations.

    val camlbox_sender_of_fd : camlbox_address -> Unix.file_descr -> 'a camlbox_sender

Gets a sender for a file descriptor from `camlbox_fd`.

Ocamlnet-3.6: new arg `camlbox_address`

    val camlbox_send : ?prefer:int ->
           ?slot:int Pervasives.ref -> 'a 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 except bigarrays, `int32`, `int64`, and `nativeint`.
- Atoms (like empty arrays) may cause problems when the message is extracted by `camlbox_get` because atoms are duplicated, and no longer unique. For example, a test `if array=[||] then...` is likely not to work. Use `if Array.length array = 0 then...`, or use `camlbox_get_copy` for extraction.

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.

`prefer`: suggests a slot for sending `slot`: the actual slot number is stored here

    val camlbox_wake : 'a 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.

## Integration into event-based programs

The functions `camlbox_wait` and `camlbox_send` may both block the execution of the program when no message has arrived, and no space is available, respectively. This is a challenge for event-based programs where all waiting is bound to events on file descriptors.

Generally, Camlboxes use semaphores for speed. The results are good, often only 4 microseconds for sending and receiving a short message. This is only possible because semaphores implement a fast path where the help of the kernel is not needed, i.e. no context switch happens. This is basically incompatible with the style of waiting implemented for file descriptors, because this kind of waiting for an event must always go through the kernel, and is thus slower by design.

But anyway, what to do if Camlboxes need to be integrated into a program that bases already on file descriptor polling? Of course, speed will decrease, but maybe not dramatically. We assume here that the program uses `Unixqueue`s as the basic data structure for organizing polling.

If the program can use the Netmulticore library, there is a very simple solution. The condition variables provided by this library allow the integration into Unixqueues, see `Netmcore_condition`. The condition variable is signalled whenever a new message is put into the Camlbox, and the receiver waits until this signal arrives. The function `Netmcore_condition.wait_e` permits it to integrate waiting into a Unixqueue.

Otherwise, if Netmulticore is no option (e.g. because the processes are unrelated that communicate via Camlboxes), the other workaround is to use threads. A special thread is set up which waits for new Camlbox messages. Whenever a message arrives, an engine is notified via a `Uq_engines.signal_engine` (which is thread-safe). This engine can run in a different thread.

    module Debug: sig .. end