“Compilable” CORBA IDL for
Draft CORBA 3.1
Document number ptc/2003-01-10
Frequently Asked Questions
The ZIP file associated with this document contains the IDL and PIDL from
the CORBA Core Chapters 1 through 16 and 21 through 24 as found in the draft
of to be formally published CORBA 3.1 specification which essentially consists
of the formally published CORBA 3.0 as in formal/02-06-01
+ changes from CORBA Core 2002 RTF as reflected in ptc/2003-01-09
Table of Contents
What is “Compilable CORBA IDL”??
“Compilable CORBA IDL” is a package of files that
faithfully represents the eighteen IDL modules in the CORBA 3 document formal/02-06-01
+ changes from CORBA Core 2002 RTF as relected in ptc/2003-01-09
Who should use this IDL? How should it
If you are an application writer, you may need up
to four of the nine modules defined in this package. The other five are internal
to the ORB and are used to define protocol interoperability, a subject best
left to the trolls who live under the (interoperability) bridge. If you are
an ORB provider, you and your fellow trolls need all of it.
If you are writing an application, you may need operations
and data definitions from the CORBA, DynamicAny, BiDirPolicy,
or PortableServer modules. You may need their
operations and data definitions, but you may not need the IDL itself. For
three of the four, DynamicAny, BiDirPolicy,
and PortableServer, you will not use the IDL
directly. That is, you won’t include their files into your own IDL file
in order to use their data definitions or interfaces in your own IDL declarations.
You will use language specific header files in your application programs
for all four of these modules. Your ORB provider will have crafted those language
header files using the IDL declarations in the modules. In a sense, the ORB
provider has already compiled these four modules (perhaps by hand), producing
stub-equivalents in those header files. You need to know what the IDL is
for all four so that you can use your language’s mapping of the IDL, but
you don’t use the IDL directly for three of them.
is different. It contains definitions of data types and interfaces that
you can use in your own IDL files. OMG has specified that the name of the
file containing the CORBA
Your ORB provider makes this file available in the system library.
If you are using a pre-3.0 IDL compiler, by using a #include directive to bring it into your IDL file,
you can use its data types and inherit from its interfaces. The ORB provider
uses the definitions in the CORBA document to populate that orb.idl file. This package simply
formalizes the somewhat informal description or the CORBA module in the CORBA document for the benefit
of ORB providers so they can make it available to you. That means that you
don’t have to extract the IDL from this package to use it yourself. Indeed,
you shouldn’t use any file other than that supplied by your ORB provider because
that file also contains any special restrictions and extensions that your
provider has documented.
The IDL files are setup to be optionally compiled by compilers that do
not understand the import , typeid and typeprefix
directives. If you have such a compiler, define the macro _PRE_3_0_COMPILER_ to cause the
compiler to use the #include directives
equivalent to the import directives.
If you are using a #include directives
to bring in definitions into your IDL file, and are using the file FT.idl, make sure that you let
your IDL compiler know which directory the CORBA Services related IDL files
are located using the -I
option of the IDL compiler. The CORBA Services related IDL files that FT.idl depends on are not included
in this package.
Some pr-3.0 IDL compilers may not be abe to handle local interfaces. If you are using such a define
the macro _NO_LOCAL_
before attempting to compile any of the files. This will cause the macro
"#define local " to be inseted at the right
places. If you find a 3.0 or later compiler that cannot handle local interfaces, complain to the purveyor of said
compiler, since it is not conformant with CORBA 3.0 or later.
If you are using a 3.0 or later IDL compiler you can simply use the import directive of the compiler to import entire
By default the Interface Repository Code for the Component Repository will
not be compiled. In order to include this code in the compilation define the
before compiling <orb.idl>.
So, if you’re an application developer, you’ll only
use one of the nine IDL modules directly. You won’t even take that one IDL
module from this package. Application developers don’t need this package
except for its documentation value. ORB providers use it only as a summary
of what’s already in the CORBA document.
Before compiling GIOP.idI
make sure that you have set the macro MAX_GIOP_VERSION_NUMBER to the
Maximum GIOP Minor Version that you would like to compile the file for. The
default value that this is set to is the current maximum GIOP minor version
number, which is 4.
Is it really the same as in the CORBA document?
To repeat the answer to the first question, this package
“faithfully represents” the IDL in the CORBA document. That is a very carefully
chosen phrase. If you compare the CORBA module in this package with the IDL
in the CORBA 3.1 document line-by-line, there are differences.
Most of the IDL in the document is presented pedagogically
rather than as a file intended to be compiled. This is especially true for
the CORBA module, which is quite large and which is explained in several
describing the CORBA::Object interface
in Section 4.3, the statement “typedef string InterfaceId”
appears in the IDL. When describing the Interface Repository, the same typedef is repeated in Section 10.5.1. It’s repeated
because it’s easier to understand the data types being used when the definition
is close at hand. However, if you were to blindly extract the IDL fragments
and try to compile them exactly as given, an IDL compiler would complain about
multiple definitions of the InterfaceId
identifier. (The elves tried it.)
The IDL in the compilable package deletes this repeated definition, and another
like it (Flags), leaving only one of each.
are “forward references” to the same interfaces in multiple chapters. IDL
compilers complain about this. (Again,
ask the elves or try it yourself.) You could attempt to avoid forward
references by arranging the interface definitions in a different order.
However, Professor Somebody’s Syntax Graph Theorem says it’s not always possible
to avoid a forward reference; that’s why it’s a part of IDL. A stylistic
choice was made for the compilable IDL to list all interfaces at the
beginning of the CORBA module as forward
references. This avoids the need for any deep analysis about which forward
references are needed and, as a by-product, helps document the CORBA module.
IDL for the CORBA module (the declaration
of NamedValue) is listed in two chapters,
Chapter 4 (as kind of a forward reference needed by CORBA::Object) and the Chapter 7 (where it is explained
in association with the CORBA::Request
object). Because of ordering constraints, the full declaration was moved
from the later chapter’s file (CORBA_Request.idl) to the
earlier chapter’s file (CORBA_Object.idl); since
both are CORBA module, this is a valid change.
IDL for TypeCode has been separated out from the IDL for InterfaceRepository
into a separate file TypeCode.idl, and it has been noted that TypeCode is
now specified in Chapter 4.
All the IDL in the package is the same as that
listed in the CORBA text, but in different relative locations than in the
What is in the package?
The base CORBA 3 document (excluding the components
extension chapters) specifies seventeen modules. This package contains
the IDL that makes up those seventeen modules. There are more than seventeen
IDL files in the package because the definitions of the modules are spread
across several chapters. Some modules are defined entirely within one chapter
and are listed in this package in one file. Pieces of the IOP module appear in two chapters and pieces of the CORBA module appear in about ten chapters. This package
keeps the chapter structure of the IDL definitions relatively intact by putting
each chapter’s contribution to a module into a separate file and then using #include on that file. If a chapter defines multiple
interfaces, they are usually put into a separate file. There are 36 IDL files.
This separation of a module into multiple files is
strictly for convenience of reference. It is likely (but not necessary) that
an ORB provider would physically merge the files. Note that the CORBA IDL
Style Guide only partly applies to files that do not contain a full module
definition. For example, file names can’t be taken from the module name (since
only one file can have the module name). File names are part of the CORBA
standard, but only for the first level files, those that contain a module
What happened to PIDL? This all looks
In truth, PIDL is not really a different language
from IDL, at least in the way it’s currently used in OMG. PIDL is not formally
specified but, in practice, it follows the same syntax as IDL. It’s simply
easier that way for the people inventing the CORBA specifications. When
the CORBA document designates something as PIDL, it means that the normal
language mapping rules don’t necessarily apply. The language mapping may
have to do something special for that construct; on the other hand, the language
mapping might not do something different than usual, in which case it’s useful
to have PIDL be the same as IDL.
Will the package really compile? I
thought you couldn’t compile PIDL.
Of the eighteen modules in the CORBA document, fifteen
compile without problems. The three remaining modules, CORBA, PortableServer
and Messaging, are the subjects of the
rest of this section. The discussion gets a little detailed, so feel free
to skip it, specially if you don't plan to try to run these modules through
an IDL compiler. Language purists will have fun and you know what they’re
But first, a reminder: only one module (the CORBA module subset in orb.idl)
ever really sees an IDL compiler, so only that module must compile
correctly from a strictly application viewpoint. Having the other seventeen
compile correctly is nice but, since applications never see them, whether
they compile or not doesn’t affect applications. What is more important
is that that the content of the modules is unambiguous so that their internal
use by an ORB provider will be correct. The easiest way of guaranteeing
this is by making sure the modules are syntactically correct, that is, that
The CORBA module
contains both PIDL and IDL definitions. The orb.idl file supplied in
this package contains only the IDL part because the orb.idl
file is visible to applications and an IDL compiler must be able to parse
it. Since it has only IDL, that subset of the CORBA module compiles just fine.
So what about the PIDL pieces? Even though PIDL is
not formally defined and it need not be compilable, this package verifies
that the PIDL parts of the CORBA module
are syntactically equivalent to valid IDL statements. (Note: this doesn’t
say that the PIDL can be used as IDL, only that it is syntactically well-formed
according to the IDL rules.) Showing this well-formedness is not difficult
because almost all OMG PIDL is equivalent to OMG IDL. It does take a trick
or two, however, because “almost all” isn’t “all”.
One other facet of IDL causes compilation problems
for conforming compilers. Both the CORBA, PortableServer and Messaging
modules use the native keyword. This keyword
was designed to be part of IDL as a notational convenience for OMG documents
that need a way of specifying that an entity needs special language mapping.
Languages must map every native type individually.
In one sense, the native keyword is a kind
of formalized PIDL. IDL compilers available to application programmers do
not usually allow applications to use the native
keyword to define a data type. If they did allow it, they wouldn’t know
what to generate for stubs and skeletons because the definition is supplied
by a language binding. As a result, most IDL compilers don’t accept native at all. Only an ORB provider deals with the native keyword; the provider must supply a language
mapping to them by hand. Since native is
part of the IDL in this package, it can only be compiled by tricking an IDL
Since native and
PIDL aren’t intended to be compiled, a language mapping specifies what the
PIDL corresponds to in a header file. An ORB provider supplies that language
header file with the language mapping results. Applications include that header
file in their language files and get the definitions they need.
To validate syntax correctness of the PIDL, the package
file pseudo_orb.idl includes
all the statements designated as PIDL. This file will compile – with the aid
of two tricks. The first trick is necessary because three of the objects defined
in PIDL have names that collide, intentionally, with IDL keywords: Object,ValueBase, and context. Feeding the PIDL definition for the similarly
named interfaces directly into an IDL compiler would be an act of madness.
An IDL compiler will reject each of the interfaces so defined. (The elves tried it and failed – they’ve never claimed
to be sane.)
The trick uses the IDL preprocessor to change what
the IDL compiler sees. For the conflict with the Object keyword, for example, the pseudo_orb.idl file has the statement,
“#define Object OObject”. That hides the keyword
conflict but still allows valid syntax checking. The other keywords are treated
the same way.
The second trick involves the native keyword, which, as explained above, is not
usually accepted by an IDL compiler. Since it appears in CORBA, PortableServer
and Messaging modules, they use the preprocessor
again to change what the compiler sees. They contain “#define native typedef long”. After preprocessing,
the compiler sees an innocuous type definition for the identifier being declared native. That is sufficient to allow the compiler to
continue checking syntax. Since these files don’t generate stubs, the trickery
doesn’t matter. You can enable this trick by setting defining the compiler
How is the package organized?
It is a .zip file containing 38 files,
37 of which are IDL and the other is this file. The .zip file has paths (directories)
so you can unzip one of two ways: into one directory or into four subdirectories.
The four subdirectories are organized as follows.
– Real IDL files for the CORBA module
– The “main” file; it has the module statement
and has #include statement to bring in
– PIDL files for the CORBA module
– The “main” file; it has the module statement
and has #include statement to bring in
– Files associated with protocol issues. All but IOP_DCE.idl are independent
modules; that is part of the IOP module.
– Files that don’t fit the above categories. All are independant modules
that use definitions from various other modules as identified by the import or #include
statements in each file.
Last revised: January 2003 (by Chief
Sidekick of King Elf, of Elf Enterprises, PLC:-))
based on the version in formal/00-04-01originally created (by King Elf, of Elf Enterprises, PLC)