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The ocamlrpcgen program

The tool [ocamlrpcgen] generates O'Caml modules which greatly simplify the creation and invocation of remote procedures. For example, if we have an XDR definition calculate.x


program P {
  version V {
    int add(int,int) = 1;
  } = 2;
} = 3;

the generation of a corresponding RPC client is done by


ocamlrpcgen -aux -clnt calculate.x

and the tool will generate an RPC server by


ocamlrpcgen -aux -srv calculate.x

The flag -aux causes ocamlrpcgen to create a module Calculate_aux containing types, and constants from the XDR definition, and containing conversion functions doing the language mapping from XDR to O'Caml and vice versa.

Calculate_aux defines the types for the arguments of the procedure and the result as follows:


type t_P'V'add'arg =                      (* Arguments *)
      ( Rtypes.int4 * Rtypes.int4 )
and t_P'V'add'res =                       (* Result *)
      Rtypes.int4

Note that the XDR integer type is mapped to Rtypes.int4 which is an opaque type representing 4-byte signed integers. Rtypes defines conversion functions for int4 to/from other O'Caml types. If Rtypes.int4 is not what you want, you can select a different integer mapping on the command line of ocamlrpcgen. For example, -int int32 selects that you want the built-in int32 integer type, and -int unboxed selects that you want the built-in int integer type. Note (1) that you can also select the integer mapping case-by-case (see below), and (2) that there is a corresponding switch for the XDR hyper type (8-byte integers).

Calculate_aux defines also constants (none in our example), conversion functions, XDR type terms, and RPC programs. These other kinds of definitions can be ignored for the moment.

Generating clients with ocamlrpcgen

The flag -clnt causes ocamlrpcgen to generate the module Calculate_clnt containing functions necessary to contact a remote program as client. Here, Calculate_clnt has the signature:


module P : sig
  module V : sig
    open Calculate_aux
    val create_client :
            ?esys:Unixqueue.event_system ->
            Rpc_client.connector ->
            Rpc.protocol ->
            Rpc_client.t
    val create_portmapped_client :
            ?esys:Unixqueue.event_system ->
            string ->
            Rpc.protocol ->
            Rpc_client.t
    val add : Rpc_client.t -> t_P'V'add'arg -> t_P'V'add'res
    val add'async :
            Rpc_client.t ->
            t_P'V'add'arg ->
            ((unit -> t_P'V'add'res) -> unit) ->
            unit
  end
end

Normally, the function P.V.create_portmapped_client is the preferred function to contact the RPC program. For example, to call the "add" procedure running on host "moon", the following statements suffice:


let client = Calculator_clnt.P.V.create_portmapped_client "moon" Rpc.Tcp in
let n = Calculator_clnt.P.V.add client (m1,m2) in
Rpc_client.shut_down client;

That's all for a simple client!

The invocation of P.V.create_portmapped_client first asks the portmapper on "moon" for the TCP instance of the program P.V, and stores the resulting internet port. Because we wanted TCP, the TCP connection is opened, too. When P.V.add is called, the values m1 and m2 are XDR-encoded and sent over the TCP connection to the remote procedure; the answer is XDR-decoded and returned, here n. Finally, the function Rpc_client.shut_down closes the TCP connection.

Of course, this works for UDP transports, too; simply pass Rpc.Udp instead of Rpc.Tcp.

The function P.V.create_client does not contact the portmapper to find out the internet port; you must already know the port and pass it as connector argument (see Rpc_client for details).

You could have also invoked "add" in an asynchronous way by using P.V.add'async. This function does not wait until the result of the RPC call arrives; it returns immediately. When the result value has been received, the function passed as third argument is called back, and can process the value. An application of asynchronous calls is to invoke two remote procedures at the same time:


let esys = Unixqueue.create_event_system() in
let client1 = Calculator_clnt.P.V.create_portmapped_client 
                ~esys:esys "moon" Rpc.Tcp in
let client2 = Calculator_clnt.P.V.create_portmapped_client 
                ~esys:esys "mars" Rpc.Tcp in
let got_answer1 get_value =
  let v = get_value() in
  print_endline "moon has replied!"; ... in
let got_answer2 get_value =
  let v = get_value() in
  print_endline "mars has replied!"; ... in
Calculator_clnt.P.V.add'async client1 (m1,m2) got_answer1;
Calculator_clnt.P.V.add'async client2 (m3,m4) got_answer1;
Unixqueue.run esys

Here, the two clients can coexist because they share the same event system (see the Unixqueue module); this system manages it that every network event on the connection to "moon" will be forwarded to client1 and that the network events on the connection to "mars" will be forwarded to client2. The add'async calls do not block; they only register themselves with the event system and return immediately. Unixqueue.run starts the event system: The XDR-encoded values (m1,m2) are sent to "moon", and (m3,m4) to "mars"; replies are recorded. Once the reply of "moon" is complete, got_answer1 is called; once the reply of "mars" has been fully received, got_answer2 is called. These functions can now query the received values by invoking get_value; note that get_value will either return the value or raise an exception if something went wrong. When both answers have been received and processed, Unixqueue.run will return.

Obviously, asynchronous clients are a bit more complicated than synchronous ones; however it is still rather simple to program them. For more information on how the event handling works, see the Equeue User's Guide [2].

Generating servers with ocamlrpcgen

The flag -srv causes ocamlrpcgen to generate the module Calculate_srv containing functions which can act as RPC servers. Here, Calculate_srv has the signature:


module P : sig
  module V : sig
    open Calculate_aux
    val create_server :
            ?limit:int ->
            proc_add : (t_P'V'add'arg -> t_P'V'add'res) ->
            Rpc_server.connector ->
            Rpc.protocol ->
            Rpc.mode ->
            Unixqueue.event_system ->
            Rpc_server.t
    val create_async_server :
            ?limit:int ->
            proc_add : (Rpc_server.session ->
                        t_P'V'add'arg ->
                        (t_P'V'add'res -> unit) ->
                        unit) ->
            Rpc_server.connector ->
            Rpc.protocol ->
            Rpc.mode ->
            Unixqueue.event_system ->
            Rpc_server.t
    end
end

There are two functions: P.V.create_server acts as a synchronous server, and P.V.create_async_server works as asynchronous server. Let's first explain the simpler synchronous case.

P.V.create_server accepts a number of labeled arguments and a number of anonymous arguments. There is always an optional ?limit parameter limiting the number of pending connections accepted by the server (default: 20); this is the second parameter of the listen system call. For every procedure p realized by the server there is a labeled argument proc_p passing the function actually computing the procedure. For synchronous servers, this function simply gets the argument of the procedure and must return the result of the procedure. In this example, we only want to realize the "add" procedure, and so there is only a proc_add argument. The anonymous Rpc_server.connector argument specifies the internet port (or the file descriptor) on which the server will listen for incoming connections. The Rpc.protocol argument defines whether this is a TCP-like (stream-oriented) or a UDP-like (datagram-oriented) service. The Rpc.mode parameter selects how the connector must be handled: Whether it acts like a socket or whether is behaves like an already existing bidirectional pipeline. Finally, the function expects the event system to be used as last argument.

For example, to define a server accepting connections on the local loopback interface on TCP port 6789, the following statement creates such a server:


let esys = Unixqueue.create_event_system in
let server = 
  Calculate_srv.P.V.create_server
    ~proc_add: add
    (Rpc_server.Localhost 6789)            (* connector *)
    Rpc.Tcp                                (* protocol *)
    Rpc.Socket                             (* mode *)
    esys

Note that this statement creates the server, but actually does not serve the incoming connections. You need an additionally


Unixqueue.run esys

to start the service.

Not all combinations of connectors, protocols, and modes are sensible. Especially the following values work:

  1. TCP internet servers: One of the connectors Localhost or Portmapped; the protocol Rpc.Tcp; the mode Rpc.Socket
  2. UDP internet servers: One of the connectors Localhost or Portmapped; the protocol Rpc.Udp; the mode Rpc.Socket
  3. Stream-based Unix domain socket servers: The connector Unix, the protocol Rpc.Tcp; the mode Rpc.Socket
  4. Datagram-based Unix domain socket servers: These are not supported
  5. Serving an already accepted (inetd) stream connection: The connector Descriptor; the protocol Rpc.Tcp; the mode Rpc.BiPipe
  6. Serving an already received (inetd) datagram: XXX (status unclear)

The connector Portmapped registers the service at the local portmapper, and is the connector of choice.

Note that servers with mode=Socket never terminate; they wait forever for service requests. On the contrary, servers with mode=BiPipe process only the current (next) request, and terminate then.

The resulting server is synchronous because the next request is only accepted after the previous request has been finished. This means that the calls are processed in a strictly serialized way (one after another); however, the network traffic caused by the current and by previous calls can overlap (to maximize network performance).

In contrast to this, an asynchronous server needs not respond immediately to an RPC call. Once the call has been registered, the server is free to reply whenever it likes to, even after other calls have been received. For example, you can synchronize several clients: Only after both clients A and B have called the procedure "sync", the replies of the procedures are sent back:


let client_a_sync = ref None
let client_b_sync = ref None

let sync s arg send_result =
  if arg.name_of_client = "A" then
    client_a_sync := Some send_result;
  if arg.name_of_client = "B" then
    client_b_sync := Some send_result;
  if !client_a_sync <> None && !client_b_sync <> None then (
    let Some send_result_to_a = !client_a_sync in
    let Some send_result_to_b = !client_b_sync in
    send_result_to_a "Synchronized";
    send_result_to_b "Synchronized";
  )

let server =
  Sync.V.create_async_server
    ~proc_sync: sync
    ...

Here, the variables client_a_sync and client_b_sync store whether one of the clients have already called the "sync" service, and if so, the variables store also the function that needs to be called to pass the result back. For example, if "A" calls "sync" first, it is only recorded that there was such a call; because send_result is not invoked, "A" will not get a reply. However, the function send_result is stored in client_a_sync such that it can be invoked later. If "B" calls the "sync" procedure next, client_b_sync is updated, too. Because now both clients have called the service, synchronization has happed, and the answers to the procedure calls can be sent back to the clients. This is done by invoking the functions that have been remembered in client_a_sync and client_b_sync; the arguments of these functions are the return values of the "sync" procedure.

It is even possible for an asynchronous server not to respond at all; for example to implement batching (the server receives a large number of calls on a TCP connection and replies only to the last call; the reply to the last call implicitly commits that all previous calls have been received, too).

To create multi-port servers, several servers can share the same event system; e.g.


let esys = Unixqueue.create_event_system in
let tcp_server = 
  P.V.create_server ... Rpc.Tcp ... esys in
let udp_server = 
  P.V.create_server ... Rpc.Udp ... esys in
Unixqueue.run esys

Command line arguments of ocamlrpcgen

The tool accepts the following options:


usage: ocamlrpcgen [-aux] [-clnt] [-srv]
                   [-int   (abstract | int32 | unboxed) ]
                   [-hyper (abstract | int64 | unboxed) ]  
                   [-cpp   (/path/to/cpp | none) ]
                   [-D var=value]
                   [-U var]
                   file.xdr ...

  1. -aux: Creates for every XDR file the auxiliary module containing the type and constant definitions as O'Caml expressions, and containing the conversion functions implementing the language mapping.
  2. -clnt: Creates for every XDR file a client module.
  3. -srv: Creates for every XDR file a server module.
  4. -int abstract: Uses Rtypes.int4 for signed ints and Rtypes.uint4 for unsigned ints as default integer representation. This is the default.
  5. -int int32: Uses int32 for both signed and unsigned ints as default integer representation. Note that overflows are ignored for unsigned ints; i.e. large unsigned XTR integers are mapped to negative int32 values.
  6. -int unboxed: Uses Pervasives.int for both signed and unsigned ints as default integer representation. XDR values outside the range of O'Camls 31 bit signed ints are rejected (raise an exception).
  7. -hyper abstract: Uses Rtypes.int8 for signed ints and Rtypes.uint8 for unsigned ints as default hyper (64 bit integer) representation. This is the default.
  8. -hyper int64: Uses int64 for both signed and unsigned ints as default hyper representation. Note that overflows are ignored for unsigned ints; i.e. large unsigned XTR hypers are mapped to negative int64 values.
  9. -hyper unboxed: Uses Pervasives.int for both signed and unsigned ints as default hyper representation. XDR values outside the range of O'Camls 31 bit signed ints are rejected (raise an exception).
  10. -cpp /path/to/cpp: Applies the C preprocessor found under /path/to/cpp on the XDR files before these are processed. The default is -cpp cpp.
  11. -cpp none: Does not call the C preprocessor.
  12. -D var=value: Defines the C preprocessor variable var with the given value.
  13. -U var: Undefines the C preprocessor variable var.

The language mapping underlying ocamlrpcgen

Names

Because XDR has a different naming concept than O'Caml, sometimes identifiers must be renamed. For example, if you have two structs with equally named components


struct a {
  t1 c;
  ...;
}

struct b {
  t2 c;
  ...;
}

the corresponding O'Caml types will be


type a = { c : t1; ... }
type b = { c' : t2; ... }

i.e. the second occurrence of c has been renamed to c'. Note that ocamlrpcgen prints always a warning for such renamings that are hard to predict.

Another reason to rename an identifier is that the first letter has the wrong case. In O'Caml, the case of the first letter must be compatible with its namespace. For example, a module name must be uppercase. Because RPC programs are mapped to O'Caml modules, the names of RPC programs must begin with an uppercase letter. If this is not the case, the identifier is renamed, too.

You can specify the O'Caml name of every XDR/RPC identifier manually: Simply add after the definition of the identifier the phrase "=> ocaml_id" where ocaml_id is the preferred name for O'Caml. Example:


struct a {
  t1 c => a_c;
  ...;
}

struct b {
  t2 c => b_c;
  ...;
}

Now the generated O'Caml types are


type a = { a_c : t1; ... }
type b = { b_c : t2; ... }

This works wherever a name is defined in the XDR file.

Integer types

XDR defines 32 bit and 64 bit integers, each in a signed and unsigned variant. As O'Caml does only know 31 bit signed integers (type "int"; the so-called unboxed integers), 32 bit signed integers (type "int32"), and 64 bit signed integers (type "int64"), it is unclear how to map the XDR integers to O'Caml integers.

The module Rtypes defines the opaque types int4, uint4, int8, and uint8 which exactly correspond to the XDR types. These are useful to pass integer values through to other applications, and for simple identification of things. However, you cannot compute directly with the Rtypes integers. Of course, Rtypes also provides conversion functions to the basic O'Caml integer types int, int32, and int64, but it would be very inconvenient to call these conversions for every integer individually.

Because of this, ocamlrpcgen has the possibility to specify the O'Caml integer variant for every integer value (and it generates the necessary conversion invocations automatically). The new keywords "_abstract", "_int32", "_int64", and "_unboxed" select the variant to use:

  1. _abstract int: A signed 32 bit integer mapped to Rtypes.int4
  2. _int32 int: A signed 32 bit integer mapped to int32
  3. _int64 int: A signed 32 bit integer mapped to int64
  4. _unboxed int: A signed 32 bit integer mapped to int
  5. unsigned _abstract int: An unsigned 32 bit integer mapped to Rtypes.uint4
  6. unsigned _int32 int: An unsigned 32 bit integer mapped to int32 (ignoring overflows)
  7. unsigned _int64 int: An unsigned 32 bit integer mapped to int64
  8. unsigned _unboxed int: An unsigned 32 bit integer mapped to int

Note that the 32 bits of the unsigned integer are simply casted to the 32 bits of int32 in the case of unsigned _int32 int (the meaning of the sign is ignored). In contrast to this, the _unboxed specifier causes a language mapping rejecting too small or too big values.

A similar mapping can be specified for the 64 bit integers (hypers):

  1. _abstract hyper: A signed 64 bit integer mapped to Rtypes.int8
  2. _int64 hyper: A signed 64 bit integer mapped to int64
  3. _unboxed hyper: A signed 64 bit integer mapped to int
  4. unsigned _abstract hyper: An unsigned 64 bit integer mapped to Rtypes.uint8
  5. unsigned _int64 hyper: An unsigned 64 bit integer mapped to int64
  6. unsigned _unboxed hyper: An unsigned 64 bit integer mapped to int

Again, unsigned _int64 hyper causes that the 64 bits of the XDR values are casted to int64.

If the keyword specifying the kind of language mapping is omitted, the default mapping applies. Unless changed on the command line (options -int and -hyper), the default mapping is _abstract:

  1. int: means _abstract int unless changed on the command line by the -int option
  2. unsigned int: means unsigned _abstract int unless changed on the command line by the -int option
  3. hyper: means _abstract hyper unless changed on the command line by the -hyper option
  4. unsigned hyper: means unsigned _abstract hyper unless changed on the command line by the -hyper option

Floating-point types

The XDR types single and double are supported and both mapped to the O'Caml type float. The XDR type quadruple is not supported.

Note that the support for single is very bad; the conversion function doing the necessary bit shifting is written in O'Caml itself (instead of letting the CPU doing the dirty work).

The code for double assumes that the CPU represents floating-point numbers according to the IEEE standards.

String and Opaque types

Strings and opaque values are mapped to O'Caml strings. If strings have a fixed length or a maximum length, this constraint is checked when the conversion is performed.

Array types

Arrays are mapped to O'Caml arrays. If arrays have a fixed length or a maximum length, this constraint is checked when the conversion is performed.

Record types (struct)

Structs are mapped to O'Caml records.

Enumerated types (enum)

Enumerated types are mapped to Rtypes.int4 (always, regardless of what the -int option specifies). The enumerated constants are mapped to let-bound values of the same name. Example: The XDR definition


enum e {
  A = 1;
  B = 2;
}

generates the following lines of code in the auxiliary module:


type e = Rtypes.int4;;
val a : Rtypes.int4;;
val b : Rtypes.int4;;

However, when the XDR conversion is performed, it is checked whether values of enumerators are contained in the set of allowed values.

The special enumerator bool is mapped to the O'Caml type bool.

Union types discriminated by enumerations

Often, XDR unions are discriminated by enumerations, so this case is handled specially. For every case of the enumerator, a polymorphic variant is generated that contains the selected arm of the union. Example:


enum e {
  A = 1;
  B = 2;
  C = 3;
  D = 4;
}

union u (e discr) {
  case A: 
    int x;
  case B:
    hyper y;
  default:
    string z;
}

This is mapped to the O'Caml type definitions:


type e = Rtypes.int4;;
type u =
  [ `a of Rtypes.int4
  | `b of Rtypes.int8
  | `c of string
  | `d of string
  ]

Note that the identifiers of the components (discr, x, y, z) have vanished; they are simply not necessary in a sound typing environment. Also note that the default case has been "expanded"; because the cases of the enumerator are known it is possible to determine the missing cases meant by "default" and to define these cases explicitly.

Union types discriminated by integers

If the discriminant has integer type, a different mapping scheme is used. For every case occuring in the union definition a separate polymorphic variant is defined; if necessary, an extra default variant is added. Example:


union u (int discr) {
  case -1: 
    int x;
  case 1:
    hyper y;
  default:
    string z;
}

This is mapped to the O'Caml type definition:


type u = 
  [ `__1 of Rtypes.int4
  | `_1  of Rtypes.int8
  | `default of (Rtypes.int4 * string)
  ]

Note that positive cases get variant tags of the form "_n" and that negative cases get variant tags of the form "__n". The default case is mapped to the tag `default with two arguments: First the value of the discriminant, second the value of the default component.

This type of mapping is not recommended, and only provided for completeness.

The special "option" union type

The XDR * type is mapped to the O'Caml option type. Example:


typedef string *s;

is mapped to


type s = string option

Recursive types

Recursive types are fully supported. Unlike in the C language, you can recursively refer to types defined before or after the current type definition. Example:


typedef intlistbody *intlist;   /* Forward reference */
typedef struct {
  int value;
  intlist next;
} intlistbody;

This is mapped to:


type intlist = intlistbody option
and intlistbody = 
  { value : Rtypes.int4;
    next : intlist;
  }

However, it is not checked whether there is a finite fixpoint of the recursion. The O'Caml compiler will do this check anyway, so it not really needed within ocamlrpcgen.


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