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Module Xdr

module Xdr: sig .. end
External Data Representation

This module supports the "external data representation", or XDR for short. XDR is a means to pack structured values as strings and it serves to transport such values across character streams even between computers with different architectures.

XDR values must be formed according to an XDR type. Such types are usually written in a notation that is close to the C notation of structured types. There are some important details where XDR is better than C:

  • direct support for strings
  • arrays may have fixed or variable length
  • unions must have a discriminator
  • no pointers. The notation *t is allowed, but means something different, namely "t option" in O'Caml notation.
  • recursive types are possible and behave like recursive types in O'Caml. For example,
         struct *list {
           int value;
           next list;
    is a list of integer values and equivalent to
         type intlist = intlist_el option
          and intlist_el = { value : int; next : intlist }
See RFC 1014 for details about XDR.

This module defines:

  • XDR types
  • XDR type terms
  • XDR type systems
  • XDR type term systems
In "type terms" you can see the components from which the type has been formed, while a "type" is an opaque representation that has been checked and for that some preprocessing has been done.

A "type system" is a collection of several types that have names and that can refer to previously defined types (i.e. a sequence of "typedef"s). As with simple types, there is an extensive and an opaque representation.

A typical way of using this module is to define an "XDR type term system" by simply writing an O'Caml expression. After that, this system is validated and you get the "type system". From now on, you can refer to the types defined in the system by name and get the corresponding "XDR types". Once you have an XDR type you can use it to pack or unpack an XDR value.

Terms that describe possible XDR types:

  • X_int: integer (32 bit)
  • X_uint: unsigned integer
  • X_hyper: hyper (64 bit signed integer)
  • X_uhyper: unsigned hyper
  • X_enum [x1,i1; ...]: enum { x1 = i1, ... }
  • X_float: float (32 bit fp number)
  • X_double: double (64 bit fp number)
  • X_opaque_fixed n: opaque[n]
  • X_opaque n: opaque<n>
  • X_string n: string<n>
  • X_array_fixed (t,n): t[n]
  • X_array (t,n): t<n>
  • X_struct [x1,t1;...]: struct { t1 x1; ...}
  • X_union_over_int ([n1,t1;...], None): union switch(int) {case n1: t1; ...}
  • X_union_over_int ([n1,t1;...], Some t): union switch(int) {case n1: t1; ...; default t}
  • X_union_over_uint ([n1,t1;...], None): union switch(unsigned int) {case n1: t1; ...}
  • X_union_over_uint ([n1,t1;...], Some t): union switch(unsigned int) {case n1: t1; ...; default t}
  • X_union_over_enum (e, [n1,t1;...], None): union switch(e) {case n1:t1; ...} where e is an enumeration type
  • X_union_over_enum (e, [n1,t1;...], Some t): union switch(e) {case n1:t1; ...; default t} where e is an enumeration type
  • X_void: void
The X_type constructor is only useful for types interpreted relative to a type system. Then it refers to a named type in this system.

The X_param constructor includes a reference to an arbitrary type which must only be given while packing or unpacking values. (A "lazy" type reference.)

Example how to define a recursive type:

X_rec ("a", X_array ( X_struct ["value", X_int; "next", X_refer "a"], 1))

type xdr_type_term =

| X_int
| X_uint
| X_hyper
| X_uhyper
| X_enum of (string * Rtypes.int4) list
| X_float
| X_double
| X_opaque_fixed of Rtypes.uint4
| X_opaque of Rtypes.uint4
| X_string of Rtypes.uint4
| X_array_fixed of xdr_type_term * Rtypes.uint4
| X_array of xdr_type_term * Rtypes.uint4
| X_struct of (string * xdr_type_term) list
| X_union_over_int of (Rtypes.int4 * xdr_type_term) list * xdr_type_term option
| X_union_over_uint of (Rtypes.uint4 * xdr_type_term) list * xdr_type_term option
| X_union_over_enum of xdr_type_term * (string * xdr_type_term) list
* xdr_type_term option
| X_void
| X_type of string
| X_param of string
| X_rec of (string * xdr_type_term)
| X_refer of string
type xdr_type 
This is the validated version of xdr_type_term. Note that it does not contain X_type constructors, i.e. is completely expanded. It is allowed that an xdr_type contains X_param constructors (parameters). The set of occurring parameters can be determined very quickly for an xdr_type.
type xdr_type_term_system = (string * xdr_type_term) list 
Bind names to types. In a correct system you can only refer to previously defined types, i.e. the type called n must be defined in the list before it can be used via X_type n. It is possible to use this module without the means of type systems, but often code is more readable if types are defined in an environment allowing bindings to names.
type xdr_type_system 
A validated type system.
val x_bool : xdr_type_term
Common abbreviation for boolean types. Has values xv_true and xv_false.
val x_optional : xdr_type_term -> xdr_type_term
Common abbreviation for optional types. Has values xv_none and xv_some v.
val x_opaque_max : xdr_type_term
Common abbreviation for opaque data of arbitrary length
val x_string_max : xdr_type_term
Common abbreviation for strings of arbitrary length
val x_array_max : xdr_type_term -> xdr_type_term
Common abbreviation for arrays of arbitrary length

Values possible for XDR types. This is straight-forward, except the "_fast" variants:

type xdr_value =
| XV_int of Rtypes.int4
| XV_uint of Rtypes.uint4
| XV_hyper of Rtypes.int8
| XV_uhyper of Rtypes.uint8
| XV_enum of string
| XV_float of Rtypes.fp4
| XV_double of Rtypes.fp8
| XV_opaque of string
| XV_string of string
| XV_array of xdr_value array
| XV_struct of (string * xdr_value) list
| XV_union_over_int of (Rtypes.int4 * xdr_value)
| XV_union_over_uint of (Rtypes.uint4 * xdr_value)
| XV_union_over_enum of (string * xdr_value)
| XV_void
| XV_enum_fast of int (*The integer is the _position_ in the X_enum list, sorted by enum values (ascending). For example, if we have X_enum [ "A", 4; "B", 2; "C", 6 ] the element "B" has the position 0, because 2 is the lowest number*)
| XV_struct_fast of xdr_value array (*The array elements are in the same order as declared in X_struct*)
| XV_union_over_enum_fast of (int * xdr_value) (*The integer is the _position_ in the X_enum list. "position" means the same as for XV_enum_fast*)
| XV_array_of_string_fast of string array (*To be used with an X_array or X_array_fixed with an inner type of X_string*)
val xv_true : xdr_value
val xv_false : xdr_value
See x_bool
val xv_none : xdr_value
val xv_some : xdr_value -> xdr_value
See x_optional
exception Dest_failure
raised if the dest_* function are applied to non-matching xdr_value
val dest_xv_int : xdr_value -> Rtypes.int4
val dest_xv_uint : xdr_value -> Rtypes.uint4
val dest_xv_hyper : xdr_value -> Rtypes.int8
val dest_xv_uhyper : xdr_value -> Rtypes.uint8
val dest_xv_enum : xdr_value -> string
val dest_xv_enum_fast : xdr_value -> int
val dest_xv_float : xdr_value -> Rtypes.fp4
val dest_xv_double : xdr_value -> Rtypes.fp8
val dest_xv_opaque : xdr_value -> string
val dest_xv_string : xdr_value -> string
val dest_xv_array : xdr_value -> xdr_value array
val dest_xv_array_of_string_fast : xdr_value -> string array
val dest_xv_struct : xdr_value -> (string * xdr_value) list
val dest_xv_struct_fast : xdr_value -> xdr_value array
val dest_xv_union_over_int : xdr_value -> Rtypes.int4 * xdr_value
val dest_xv_union_over_uint : xdr_value -> Rtypes.uint4 * xdr_value
val dest_xv_union_over_enum : xdr_value -> string * xdr_value
val dest_xv_union_over_enum_fast : xdr_value -> int * xdr_value
val dest_xv_void : xdr_value -> unit
val map_xv_enum_fast : xdr_type -> xdr_value -> int32
Works for both XV_enum and XV_enum_fast
val map_xv_struct_fast : xdr_type -> xdr_value -> xdr_value array
Works for both XV_struct and XV_struct_fast
val map_xv_union_over_enum_fast : xdr_type -> xdr_value -> int * int32 * xdr_value
Works for both XV_union_over_enum and XV_union_over_enum_fast. Returns the triple (k,i,x):
  • k: Position of the selected value in the T_enum array
  • i: value of the enum
  • x: selected arm of the union

exception Xdr_format of string
Format error found during unpacking a string
exception Xdr_format_message_too_long of xdr_value
The message is too long and cannot be packed into a string

You must use these two functions to obtain validated types and type systems. They fail with "validate_xdr_type" resp. "validate_xdr_type_system" if the parameters are incorrect.
val validate_xdr_type : xdr_type_term -> xdr_type
val validate_xdr_type_system : xdr_type_term_system -> xdr_type_system

Get the unvalidated version back:

  • Note that X_type constructions are always resolved

val xdr_type_term : xdr_type -> xdr_type_term
val xdr_type_term_system : xdr_type_system -> xdr_type_term_system

You can expand any type term relative to a (validated) type system. For example: expanded_xdr_type sys1 (X_type "xy") extracts the type called "xy" defined in sys1. Expansion removes all X_type constructions in a type term.
val expanded_xdr_type : xdr_type_system -> xdr_type_term -> xdr_type
val expanded_xdr_type_term : xdr_type_term_system -> xdr_type_term -> xdr_type_term
val are_compatible : xdr_type -> xdr_type -> bool
are_compatible: currently not implemented

Is the value properly formed with respect to this type? The third argument of this function is a list of parameter instances. Note that all parameters must be instantiated to compare a value with a type and that the parameters instances are not allowed to have parameters themselves.
val value_matches_type : xdr_value -> xdr_type -> (string * xdr_type) list -> bool

pack_xdr_value v t p print: Serialize v into a string conforming to the XDR standard where v matches t. In p the parameter instances are given. All parameters must be given, the parameters must not contain parameters themselves. The fourth argument, print, is a function which is evaluated for the pieces of the resultant string. You can use pack_xdr_value_as_string to get the whole string at once.

unpack_xdr_value s t p: Unserialize a string to a value matching t. If this operation fails you get an Xdr_format exception explaining what the reason for the failure is. Mostly the cause for failures is that t isn't the type of the value. Note that there are some implementation restrictions limiting the number of elements in array, strings and opaque fields. If you get such an error this normally still means that the value is not of the expected type, because these limits have no practical meaning (they are still higher than the usable address space).

val pack_xdr_value : xdr_value ->
xdr_type -> (string * xdr_type) list -> (string -> unit) -> unit
val pack_xdr_value_as_string : ?rm:bool ->
xdr_value -> xdr_type -> (string * xdr_type) list -> string
rm: If true, four null bytes are prepended to the string for the record mark
val unpack_xdr_value : ?pos:int ->
?len:int ->
?fast:bool ->
?prefix:bool ->
string -> xdr_type -> (string * xdr_type) list -> xdr_value
val unpack_xdr_value_l : ?pos:int ->
?len:int ->
?fast:bool ->
?prefix:bool ->
string -> xdr_type -> (string * xdr_type) list -> xdr_value * int
fast: whether to prefer the new "fast" values (default: false)

prefix: whether it is ok that the string is longer than the message (default: false)

The variant unpack_xdr_value_l returns not only the decoded value, but also the actual length in bytes.

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