(* $Id: netcamlbox.mli 1715 2012-02-20 14:47:01Z gerd $ *)

(** 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.

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

   {b 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.

   {b Integration into event-based programs:} See the section
   below, {!Netcamlbox.events}.
 *)


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.
   *)

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 : Unix.file_descr -> int -> int -> 'a camlbox
  (** [format_camlbox fd n size]: The file [fd] is mapped into memory,
      and formatted as camlbox.
   *)

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 : Unix.file_descr -> 'a camlbox_sender
  (** Gets a sender for a file descriptor from [camlbox_fd]. *)

val camlbox_send : ?prefer:int -> ?slot:int 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.
   *)

(**
   {2:events 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.
  
*)
