The OncRPC (alias SunRPC) standard consists of two parts, namely the external data representation (XDR) and the RPC protocol. They are defined in RFC 1831 and RFC 1832.
In this document we describe how the various parts of XDR and RPC are mapped to the Objective Caml language.
The transformation of binary XDR messages to O'Caml values is done in several steps, corresponding to several ways of representing the values:
string values.Netxdr.xdr_type_term.
The message is represented as structured Netxdr.xdr_value
term. For example, an XDR struct with two components a and b
with integer values 1 and 2 is represented as
XV_struct [ "a", XV_int r1; "b" XV_int r2 ]r1 = Netnumber.int4_of_int 1 and r2 = Netnumber.int4_of_int 2. There
are sometimes several ways of representing a value on term level.struct example
would use the type type name = { a : int; b : int }ocamlrpcgen.ocamlrpcgen can be invoked on an input file name.x with
different switches to create three modules: the type mapper
Name_aux, the RPC client Name_clnt and the RPC server Name_srv.
The type mapper module mainly contains the necessary definitions to
convert values between the representation levels.
In particular, the type mapper module contains for every XDR type t several definitions:
Netxdr.xdr_type_term.
This definition is named xdrt_t. The type term is required to
convert a binary message to a value on term level. The conversion
functions to do so are available in the Netxdr module._of_t that turns a fully-mapped
value into a term value represented as Netxdr.xdr_value._to_t that turns a term value
to a fully-mapped value.
The following table shows:
x would be declared in the XDR fileXDR declaration for x |
Term-level mapping | Full mapping | Comment |
void |
Netxdr.X_void |
unit (in some contexts omitted) |
- |
int x |
Netxdr.X_int |
Netnumber.int4, int32, int64, or int [1] |
32 bit signed integer |
_abstract int x |
Netxdr.X_int |
Netnumber.int4 [6] |
32 bit signed integer |
_int32 int x |
Netxdr.X_int |
int32 |
32 bit signed integer |
_int64 int x |
Netxdr.X_int |
int64 |
32 bit signed integer |
_unboxed int x |
Netxdr.X_int |
int [2] |
32 bit signed integer |
unsigned int x |
Netxdr.X_uint |
Netnumber.uint4, int32, int64, or int [1] |
32 bit unsigned integer |
unsigned _abstract int x |
Netxdr.X_uint |
Netnumber.uint4 [6] |
32 bit unsigned integer |
unsigned _int32 int x |
Netxdr.X_uint |
int32 [4] |
32 bit unsigned integer |
unsigned _int64 int x |
Netxdr.X_uint |
int64 |
32 bit unsigned integer |
unsigned _unboxed int x |
Netxdr.X_uint |
int [2] |
32 bit unsigned integer |
hyper x |
Netxdr.X_hyper |
Netnumber.int8, int64, or int [1] |
64 bit signed integer |
_abstract hyper x |
Netxdr.X_hyper |
Netnumber.int8 [6] |
64 bit signed integer |
_int64 hyper x |
Netxdr.X_hyper |
int64 |
64 bit signed integer |
_unboxed hyper x |
Netxdr.X_hyper |
int [3] |
64 bit signed integer |
unsigned hyper x |
Netxdr.X_uhyper |
Netnumber.int8, int64, or int [1] |
64 bit unsigned integer |
unsigned _abstract hyper x |
Netxdr.X_uhyper |
Netnumber.uint8 [6] |
64 bit unsigned integer |
unsigned _int64 hyper x |
Netxdr.X_uhyper |
int64 [4] |
64 bit unsigned integer |
unsigned _unboxed hyper x |
Netxdr.X_uhyper |
int [3] |
64 bit unsigned integer |
bool x |
Netxdr.x_bool
| bool |
Boolean type |
float x |
Netxdr.X_float |
float [5] |
32 bit IEEE float |
double x |
Netxdr.X_double |
float [5] |
64 bit IEEE float |
opaque x[n] |
Netxdr.X_opaque_fixed |
string |
Opaque data with exactly n bytes.
The length n is dynamically checked |
opaque x<n> |
Netxdr.X_opaque |
string |
Opaque data with up to n bytes.
The length n is dynamically checked |
string x<n> |
Netxdr.X_string |
string |
String consisting of up to n bytes.
The length n is dynamically checked |
_managed string x<n> |
Netxdr.X_mstring |
Netxdr_mstring.mstring |
Alternative runtime representation. See Netxdr_mstring. |
Footnotes:
ocamlrpcgen one of the other
options for the whole XDR fileNetnumber.Cannot_represent
is raised.Netnumber.Cannot_represent
is raised.float type is a 64 bit IEEE floating point number.Netnumber is an extended version of the older
(and now removed) Rtypes definition, and contains wrapper types for all
XDR number types.
The "pointer type" *t is considered as an option type in XDR
corresponding to option in O'Caml, i.e. a variant with the two
cases that an argument is missing or present. Option types are
written
t *varname
On term level, the missing argument value is represented as
Netxdr.xv_none. The present argument value is represented as
Netxdr.xv_some v when v is the mapped argument value. Actually,
xv_none and xv_some construct XDR terms that are unions over
the boolean enumeration as discriminator.
On the fully-mapped level, the option type is mapped to
t' optiont' is the mapped argument type.
In XDR arrays are formed over an element type. Furthermore, there may be the size constraint that exactly or at most n elements are contained in the array. If the size constraint is missing, the array may have arbitrary many elements. However, due to the binary representation, the number is actually limited to 2 ^ 32 - 1.
XDR declaration for x |
Term-level mapping |
Full mapping |
Comment |
t x[n] |
Netxdr.X_array |
t' array |
Arrays with exactly n elements |
t x<n> |
Netxdr.X_array |
t' array |
Arrays with up to n elements |
t x<> |
Netxdr.X_array |
t' array |
Arrays with any number of elements |
The size constraints are dynamically checked in all cases when RPC message are analyzed or created.
Structs are products with named components, like record types in O'Caml. The components have, in addition to their name, a fixed order, because the order of the components determines the order in the binary message format. That means that the components can be accessed by two methods: by name and by index.
Struct are written as
struct {
t0 varname0;
t1 varname1;
...
}
For example, struct { int a; hyper b } means a struct with two
components. At position 0 we find "a", and at position 1 we find "b".
Of course, this type is different from struct { hyper b; int a }
because the order of the components is essential.
On term level, there are two ways of representing structs Netxdr.X_struct: one
identifies components by name, one by position. The latter is also
called the "fast" representation (and the one used by ocamlrpcgen).
In the "by name" case, the struct value is represented as
Netxdr.XV_struct components where components is an association
list [(c0_name, c0_val); (c1_name, c1_val); ...] where
cK_name are the names of the components and cK_val their
actual values as terms. The order of the components can be arbitrary.
In the "by position" case, the struct value is represented as
Netxdr.XV_struct_fast components where components is an
array of terms such that components.(k) is the term value of
the k-th component.
On the fully-mapped level, the struct is mapped to an O'Caml record. The order of the components remains the same, but the names of the components may be modified. First, the names are modified such that they are valid component names in O'Caml by ensuring that the first letter is lowercase. Second, the names may be changed because several structs use the same component names which is not possible in O'Caml. Thus, the generated O'Caml record type look like
{
mutable varname0' : t0';
mutable varname1' : t1';
...
}
where varnameK' is the component name after the mentioned renaming
and tK' is the mapped component type, both for position K.
Since Ocamlnet-3.6.7 the keyword _equals is understood and
generates an equality constraint, e.g.
struct _equals "M.t" { int a; hyper b }, meaning that this OCaml
record type is the same as the one defined as M.t.
Ocamlnet-3.6.7 also introduces the name mapping directives for struct fields:
_lowercase: the XDR name is lowercased_uppercase: the XDR name is uppercased_capitalize: the XDR name is capitalized_prefix "p": this prefix is prepended to the XDR namestruct _lowercase _prefix "my_" { int A; hyper B }
would generate a record type with fields my_a and my_b.
Since Ocamlnet-3.6.7 it is possible to select an alternate mapping
to OCaml tuples by specifying the _tuple keyword (e.g.
struct _tuple { int a; hyper b } ).
In XDR it is possible to define enumerations which are considered as
subtypes of int. These consist of a list of integers with associated
symbolic names. In the .x file this is written as
enum {
Name0 = Int0,
Name1 = Int1,
...
}
where NameK are identifiers and IntK are literal numbers.
In this section we only consider the case that the enumerations are not used as discriminator for a union. (See below for the other case.)
On term level, there are again two representations. One uses the names to identify one of the enumerated values, and the other uses a positional method.
In the "by name" case, the value named NameK is represented as
Netxdr.XV_enum "NameK", i.e. the name is the argument of XV_enum.
In the "by position" case, the value named NameK is represented as
Netxdr.XV_enum_fast K, i.e. the position in the enum declaration is
the argument of XV_enum.
On the fully-mapped level, the enumerated value named NameK is
represented as O'Caml value of type Netnumber.int4 whose value is
IntK, i.e. the number associated with the name. In the type mapper
file generated by ocamlrpcgen there are additional definitions
for every enum. In particular, there is a constant whose name
is NameK (after makeing the name OCamlish) and whose value is
IntK.
Ocamlnet-3.6.7 also introduces the name mapping directives for enum constants:
_lowercase: the XDR name is lowercased_uppercase: the XDR name is uppercased_capitalize: the XDR name is capitalized_prefix "p": this prefix is prepended to the XDR nameenum _lowercase _prefix "my_" { A=0, B=1 }
would generate constants my_a and my_b (with the values
0 and 1, resp.).
In XDR a union must always have disriminator. This can be an int, an
unsigned int, or an enumeration. The latter case is described in the
next section. In the integer case, the union declaration enumerates
a number of arms and a default arm:
union switch (d varname) {
case Int0:
t0 varname0;
case Int1:
t1 varname1;
...
default:
tD varnameD;
}
Here, d is either int or unsigned int.
On term level, this is represented as Netxdr.XV_union_over_int(n,v) for
the int case or Netxdr.XV_union_over_uint(n,v) for the unsigned int case.
The number n is the selected arm of the union (it is not indicated
whether the arm is one of the declared arms or the default arm).
The value v is the mapped value of the arm.
On the fully-mapped level, the union is mapped to a polymorphic variant that corresponds to the original union declaration:
[ `_Int0 of t0'
| `_Int1 of t1'
...
| `default of tD'
]
The labels of the variants are derived from the decimal literals of
the numbers IntK associated with the arms. For example, the
union
union switch (int d) {
case -1:
hyper a;
case 0:
bool b;
default:
string s<>;
}
is mapped to
[ `__1 of int64 | `_0 of bool | `default of Netnumber.int4 * string ] Note that the default case consists of the value of the discriminant on the left and the value of the union on the right.
If an arm is simply void, the corresponding variant will not have
an argument.
If the discriminator is an enumeration, different O'Caml types are used, as a much nicer mapping is possible.
As for integer-discriminated unions, the arms are enumerated. The default arm, however, is now optional. The whole construct looks like:
enum e {
Name0 = Int0,
Name1 = Int1,
...
}
union switch (e varname) {
case Name0:
t0 varname0;
case Name1:
t1 varname1;
...
default: /* optional! */
tD varnameD;
}
On the term level, there are again two different ways of representing a union value, namely by referring to the arm symbolically or by position.
In the first case, the value is represented as
Netxdr.XV_union_over_enum(n,v) where n is the string name of the
value of the discriminator (i.e. "NameK"), and v is the mapped
value of the selected arm.
In the second case, the value is represented as
Netxdr.XV_union_over_enum_fast(K,v) where K is the position of
the value of the discriminator in the enumeration, and v is the mapped
value of the selected arm.
On the fully-mapped level, the union is again mapped to a polymorphic variant:
[ `Name0 of t0'
| `Name1 of t1'
| ...
]
Every label of an enumerated value is turned into the label of the variant. The argument is the mapped value of the corresponding arm. Note that default values do not occur in this representation as such.
For example, the union
enum e {
CASEA = 5,
CASEB = 42,
CASEC = 7,
CASED = 81
}
union switch (e d) {
case CASEB:
int b;
case CASEC:
void;
default:
hyper ad;
}
is mapped to the O'Caml type (the tags are all lowercase by default):
[ 'casea of int64 (* expanded default case *)
| `caseb of int32
| `casec
| `cased of int64 (* expanded default case *)
]
If an arm is simply void like for CASEC, the corresponding variant will
not have an argument like `casec.
Ocamlnet-3.6.7 also introduces the name mapping directives for union tags:
_lowercase: the XDR name is lowercased_uppercase: the XDR name is uppercased_capitalize: the XDR name is capitalized_prefix "p": this prefix is prepended to the XDR name
union _capitalize switch (e d) {
case CASEB:
int b;
case CASEC:
void;
default:
hyper ad;
}
is mapped to
[ 'Casea of int64
| `Caseb of int32
| `Casec
| `Cased of int64
]
If there are name mapping directives in the definition of the enumeration
e, these directives will be ignored. Only the directives in the union
definition are used for generating the OCaml tag names.
In an .x file one can declare programs. A program consists of a number
of program versions, and every version consists of a number of
procedures. Every procedure takes a (possibly empty) list of arguments
and yields exactly one result (which may be void, however). This
is written as:
/* type definitions come first */
...
/* Now the programs: */
program P1 {
version V1 {
r1 name1(arg11, arg12, ...) = L1;
r2 name2(arg21, arg22, ...) = L2;
...
} = M1;
version V2 {
...
} = M2;
...
} = N1;
program P2 {
...
} = N2;
...
Here, P1, P2, ..., V1, V2, ...,name1, name2, ... are identifiers. r1, r2, arg11, ... are type expressions. N1, N2, ..., M1, M2, ..., L1, L2, ... are unsigned numbers.
Programs are dynamically represented using the Rpc_program module.
Every Rpc_program.t value contains the full signature of
exactly one version of one program.
In the generated type mapper module, the definitions for the programs
are available as constants program_P'V where P is the name of
the program and V is the version of the program.
To write
Rpc_client as basisRpc_clientocamlrpcgen generates an enhanced client module containing
procedure stubs. These stubs are on the fully-mapped level.To write
Rpc_server as basisRpc_serverocamlrpcgen generates an enhanced server module containing a
converter to/from the fully-mapped level.
