The purpose of OMG IDL mapping is to act as translator between platforms and languages.
CORBA is independent of the programming language used to construct clients or implementations. In order to use the ORB, it is necessary for programmers to know how to access ORB functionality from their programming languages. It translates different IDL constructs to a specific programming language. This chapter describes the mapping of OMG IDL constructs to the Erlang programming language.
A complete language mapping will allow the programmer to have access to all ORB functionality in a way that is convenient for a specified programming language.
All mapping must define the following elements:
The use of some names is strongly discouraged due to ambiguities. However, the use of some names is prohibited when using the Erlang mapping , as they are strictly reserved for IC.
IC reserves all identifiers starting with
OE_
and oe_
for internal use.
Note also, that an identifier in IDL can contain alphabetic, digits and underscore characters, but the first character must be alphabetic.
Using underscores in IDL names can lead to ambiguities due to the name mapping described above. It is advisable to avoid the use of underscores in identifiers.
Refer to the IC documentation for further details.
The OMG IDL mapping is strongly typed and (even if you have a good knowledge of CORBA types), it is essential to read carefully the following mapping to Erlang types.
The mapping of basic types is straightforward. Note that the OMG IDL double type is mapped to an Erlang float which does not support the full double value range.
OMG IDL type | Erlang type | Note |
float | Erlang float | |
double | Erlang float | value range not supported |
short | Erlang integer | |
unsigned short | Erlang integer | |
long | Erlang integer | |
long long | Erlang integer | |
unsigned long | Erlang integer | |
unsigned long long | Erlang integer | |
char | Erlang integer | |
wchar | Erlang integer | |
boolean | Erlang atoms true or false | |
octet | Erlang integer | |
any | Erlang record #any{typecode, value} | |
long double | Not supported | |
Object | Orber object reference | |
void | Erlang atom ok |
The any
value is written as a record with the field typecode which
contains the Type Code representation,
see also the Type Code table,
and the value field itself.
Functions with return type void
will return the atom ok
.
Constructed types all have native mappings as shown in the table below.
string | Erlang string |
wstring | Erlang list of Integers |
struct | Erlang record |
union | Erlang record |
enum | Erlang atom |
sequence | Erlang list |
array | Erlang tuple |
Below are examples of values of constructed types.
Type | IDL code | Erlang code |
string |
typedef string S; void op(in S a); |
ok = op(Obj, "Hello World"), |
struct |
struct S {long a; short b;}; void op(in S a); |
ok = op(Obj, #'S'{a=300, b=127}), |
union |
union S switch(long) { case 1: long a;}; void op(in S a); |
ok = op(Obj, #'S'{label=1, value=66}), |
enum |
enum S {one, two}; void op(in S a); |
ok = op(Obj, one), |
sequence |
typedef sequence <long, 3> S; void op(in S a); |
ok = op(Obj, [1, 2, 3]), |
array |
typedef string S[2]; void op(in S a); |
ok = op(Obj, {"one", "two"}), |
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Every field in, for example, a struct must be initiated. Otherwise
it will be set to the atom |
Constants are generated as Erlang functions, and are accessed by a single function call. The functions are put in the file corresponding to the scope where they are defined. There is no need for an object to be started to access a constant.
Example:
// IDL module M { const long c1 = 99; };
Would result in the following conceptual code:
-module('M'). -export([c1/0]). c1() -> 99.
Objects are accessed by object references. An object reference is an opaque Erlang term created and maintained by the ORB.
Objects are implemented by providing implementations for all operations and attributes of the Object, see operation implementation.
A function call will invoke an operation. The first
parameter of the function should be the object reference and then
all in
and inout
parameters follow in the same
order as specified in the IDL specification. The result will be a return value unless the function has inout
or
out
parameters specified; in which case, a tuple of the
return value, followed by the parameters will be returned.
Example:
// IDL interface i1 { long op1(in short a); long op2(in char c, inout string s, out long count); void op3(out long Id); };
Is used in Erlang as :
%% Erlang f() -> ... Obj = ... %% get object reference R1 = i1:op1(Obj, 55), {R2, S, Count} = i1:op2(Obj, $a, "hello"), {ok, Id} = i1:op3(Obj), ...
Note how the inout
parameter is passed and
returned. There is no way to use a single occurrence of a
variable for this in Erlang. Also note, that you must return ok
,
Orber's representation of the IDL-type void
, must be returned by
op3
. These operations can be implemented in the call-back module as:
op1(State, AShort) -> {reply, ALong, State}. op2(State, AShort, AString) -> {reply, {ALong, "MyString", ALong}, State}. op3(State) -> {reply, {ok, AId}, State}.
The operations may require more arguments (depends on IC options used). For more information, see Stubs/Skeletons and Module_Interface.
A standard Erlang gen_server
behavior is used for
object implementation. The gen_server
state is then
used as the object internal state. Implementation of the object
function is achieved by implementing its methods and attribute operations.
These functions will usually have the internal state as their first parameter,
followed by any in
and inout
parameters.
Do not confuse the object internal state with its object reference. The object internal state is an Erlang term which has a format defined by the user.
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It is is not always the case that the internal state will be the first parameter, as stubs can use their own object reference as the first parameter (see the IC documentation). |
The special function init/1
is called at object start
time and is expected to return the tuple {ok,
InitialInternalState}
.
Exceptions are handled as Erlang catch and throws. Exceptions are translated to messages over an IIOP bridge but converted back to a throw on the receiving side. Object implementations that invoke operations on other objects must be aware of the possibility of a non-local return. This includes invocation of ORB and IFR services. See also the Exceptions section.
Exception parameters are mapped as an Erlang record and accessed as such.
An object implementation that raises an exception will use the
corba:raise/1
function, passing the exception record as
parameter.
Attributes are accessed through their access functions. An
attribute implicitly defines the _get
and _set
operations. The _get
operation is defined as a read-only
attribute. These operations are handled in the same way as
normal operations.
As mentioned in a previous section, struct
, union
and
exception
types yield to record definitions and access code
for that record.
For struct
, union
, exception
, array
and
sequence
types, a special file is generated that holds access
functions for TypeCode
, Identity
and Name
.
These functions are put in the file corresponding to the scope where
they are defined :
For example:
// IDL module m { struct s { long x; long y; }; };
Would result in the following code on file m_s.erl
:
-module(m_s). -include("m.hrl"). -export([tc/0,id/0,name/0]). %% returns type code tc() -> {tk_struct,"IDL:m/s:1.0","s",[{"x",tk_long},{"y",tk_long}]}. %% returns id id() -> "IDL:m/s:1.0". %% returns name name() -> m_s.
Type Codes are used in any
values. The table below
corresponds to the table on page 12-11 in the OMG CORBA
specification.
Type Code | Example |
tk_null | |
tk_void | |
tk_short | |
tk_long | |
tk_longlong | |
tk_ushort | |
tk_ulong | |
tk_ulonglong | |
tk_float | |
tk_double | |
tk_boolean | |
tk_char | |
tk_wchar | |
tk_octet | |
tk_any | |
tk_TypeCode | |
tk_Principal | |
{tk_objref, IFRId, Name} | {tk_objref, "IDL:M1\I1:1.0", "I1"} |
{tk_struct, IFRId, Name, [{ElemName, ElemTC}]} | {tk_struct, "IDL:M1\S1:1.0", "S1", [{"a", tk_long}, {"b", tk_char}]} |
{tk_union, IFRId, Name, DiscrTC, DefaultNr, [{Label,
ElemName, ElemTC}]} Note: DefaultNr tells which of tuples in the case list that is default, or -1 if no default |
{tk_union, "IDL:U1:1.0", "U1", tk_long, 1, [{1, "a", tk_long}, {default, "b", tk_char}]} |
{tk_enum, IFRId, Name, [ElemName]} | {tk_enum, "IDL:E1:1.0", "E1", ["a1", "a2"]} |
{tk_string, Length} | {tk_string, 5} |
{tk_wstring, Length} | {tk_wstring, 7} |
{tk_sequence, ElemTC, Length} | {tk_sequence, tk_long, 4} |
{tk_array, ElemTC, Length} | {tk_array, tk_char, 9} |
{tk_alias, IFRId, Name, TC} | {tk_alias, "IDL:T1:1.0", "T1", tk_short} |
{tk_except, IFRId, Name, [{ElemName, ElemTC}]} | {tk_except, "IDL:Exc1:1.0", "Exc1", [{"a", tk_long}, {"b", {tk_string, 0}}]} |
Various scopes exist in OMG IDL. Modules, interfaces and types define scopes. However, Erlang has only two levels of scope; module and function:
An Erlang module, corresponding to an IDL global name, is derived by converting occurencies of "::" to underscore, and eliminating the leading "::".
For example, an operation op1
defined in interface I1
which is defined in module
M1
would be written in IDL as M1::I1::op1
and as
'M1_I1':op1
in Erlang, where op1
is the function
name and 'M1_I1'
is the name of the Erlang module.
Several files can be generated for each scope.
.erl
) is generated
for top level scope as well as the Erlang header file.
.hrl
) will be generated for
each scope. The header file will contain record definitions
for all struct
, union
and exception
types in that scope.
.erl
).
That Erlang file will contain constant functions for
that scope.
Modules that contain no constant definitions are considered
empty and no code will be produced for them, but only for
their included modules/interfaces.
.erl
),
this code will contain all operation stub code and implementation
functions.
struct
,
union
and exception
(these are the types that
will be represented in Erlang as records).
This file will contain special access functions for that record.
.hrl
) and one Erlang source file (.erl
).
These files are named as the IDL file, prefixed with oe_
.
Example:
// IDL, in the file "spec.idl" module m { struct s { long x; long y; }; interface i { void foo( in s a, out short b ); }; };
This will produce the following files:
oe_spec.hrl
and
oe_spec.erl
for the top scope level.
m.hrl
for the module m
.
m_i.hrl
and m_i.erl
for the interface i
.
m_s.erl
for the structure s
in module m
.
This is a small example of a simple stack. There are two operations on the stack, push and pop. The example shows all generated files as well as conceptual usage of a stack object.
// The source IDL file interface stack { exception overflow {}; void push(in long val); long pop() raises (overflow); };
When this file is compiled it produces four files, two for the top scope and two for the stack interface scope. The generated Erlang code for the stack object server is shown below:
-module(stack). -export([push/2, pop/1]). init(Env) -> stack_impl:init(Env). %% This is the stub code used by clients push(THIS, Val) -> corba:call(THIS, push, [Val]). pop(THIS) -> corba:call(THIS, pop, []). %% gen_server handle_calls handle_call({THIS, push, [Val]}, From, State) -> case catch stack_impl:push(State, Val) of {'EXCEPTION', E} -> {reply, {'EXCEPTION', E}, State}; {reply, Reply, NewState} -> {reply, Reply, NewState} end; handle_call({THIS, pop, []}, From, State) -> case catch stack_impl:pop(State) of {'EXCEPTION, E} -> {reply, {'EXCEPTION', E}, State}; {reply, Reply, NewState} -> {reply, Reply, NewState} end.
The Erlang code has been simplified but is conceptually
correct. The generated stack
module is the Erlang
representation of the stack interface. Note that the variable
THIS
is the object reference and the variable
State
is the internal state of the object.
So far the example only deals with interfaces and call chains. It is now time to implement the stack. The example represents the stack as a simple list. The push operation then is just to add a value on to the front of the list and the pop operation is then to return the head of the list.
In this simple representation the internal state of the object
becomes just a list. The initial value for the state is the
empty list as shown in the init/1
function below.
The implementation is put into a file called
stack_impl.erl
.
-module(stack_impl). -include("stack.hrl"). -export([push/2, pop/1, init/1]). init(_) -> {ok, []}. push(Stack, Val) -> {reply, ok, [Val | Stack]}. pop([Val | Stack]) -> {reply, Val, Stack}; pop([]) -> corba:raise(#stack_overflow{}).
The stack object can be accessed client code. This example
shows a typical add
function from a calculator class:
-module(calc_impl). -export([add/1]). add({Stack, Memory}) -> Sum = stack:pop(Stack)+stack:pop(Stack), stack:push(Stack, Sum), {ok, {Stack, Memory}}.
Note that the Stack
variable above is an object reference
and not the internal state of the stack.