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Chapter 5  The IDL compiler

omniORB’s IDL compiler is called omniidl. It consists of a generic front-end parser written in C++, and a number of back-ends written in Python. omniidl is very strict about IDL validity, so you may find that it reports errors in IDL which compiles fine with other IDL compilers.

The general form of an omniidl command line is:

omniidl [options] -b<back-end> [back-end options] <file 1> <file 2>

5.1  Common options

The following options are common to all back-ends:

-bback-endRun the specified back-end. For the C++ ORB, use -bcxx.
-Dname[=value]Define name for the preprocessor.
-UnameUndefine name for the preprocessor.
-IdirInclude dir in the preprocessor search path.
-EOnly run the preprocessor, sending its output to stdout.
-YcmdUse cmd as the preprocessor, rather than the normal C preprocessor.
-NDo not run the preprocessor.
-TUse a temporary file, not a pipe, for preprocessor output.
-Wparg[,arg…]Send arguments to the preprocessor.
-Wbarg[,arg…]Send arguments to the back-end.
-nfDo not warn about unresolved forward declarations.
-kKeep comments after declarations, to be used by some back-ends.
-KKeep comments before declarations, to be used by some back-ends.
-CdirChange directory to dir before writing output files.
-dDump the parsed IDL then exit, without running a back-end.
-pdirUse dir as a path to find omniidl back-ends.
-VPrint version information then exit.
-uPrint usage information.
-vVerbose: trace compilation stages.

Most of these options are self explanatory, but some are not so obvious.

5.1.1  Preprocessor interactions

IDL is processed by the C preprocessor before omniidl parses it. omniidl always uses the GNU C preprocessor (which it builds with the name omnicpp). The -D, -U, and -I options are just sent to the preprocessor. Note that the current directory is not on the include search path by default—use ‘-I.’ for that. The -Y option can be used to specify a different preprocessor to omnicpp. Beware that line directives inserted by other preprocessors are likely to confuse omniidl.  Windows 9x

The output from the C preprocessor is normally fed to the omniidl parser through a pipe. On some Windows 98 machines (but not all!) the pipe does not work, and the preprocessor output is echoed to the screen. When this happens, the omniidl parser sees an empty file, and produces useless stub files with strange long names. To avoid the problem, use the ‘-T’ option to create a temporary file between the two stages.

5.1.2  Forward-declared interfaces

If you have an IDL file like:

interface I; interface J { attribute I the_I; };

then omniidl will normally issue a warning:

  test.idl:1: Warning: Forward declared interface `I' was never
  fully defined

It is illegal to declare such IDL in isolation, but it is valid to define interface I in a separate file. If you have a lot of IDL with this sort of construct, you will drown under the warning messages. Use the -nf option to suppress them.


By default, omniidl discards comments in the input IDL. However, with the -k and -K options, it preserves the comments for use by the back-ends. The C++ back-end ignores this information, but it is relatively easy to write new back-ends which do make use of comments.

The two different options relate to how comments are attached to declarations within the IDL. Given IDL like:

interface I { void op1(); // A comment void op2(); };

the -k flag will attach the comment to op1(); the -K flag will attach it to op2().

5.2  C++ back-end options

When you specify the C++ back-end (with -bcxx), the following -Wb options are available. Note that the -Wb options must be specified after the -bcxx option, so omniidl knows which back-end to give the arguments to.

-Wbh=suffixUse suffix for generated header files. Default ‘.hh’.
-Wbs=suffixUse suffix for generated stub files. Default ‘’
-Wbd=suffixUse suffix for generated dynamic files. Default ‘’
-WbaGenerate stubs for TypeCode and Any.
-WbinlineOutput stubs for #included IDL files in line with the main file.
-WbtpGenerate ‘tie’ implementation skeletons.
-WbtfGenerate flattened ‘tie’ implementation skeletons.
-Wbsplice-modulesSplice together multiply-opened modules into one.
-WbexampleGenerate example implementation code.
-WbFGenerate code fragments (for experts only).
-WbBOAGenerate BOA compatible skeletons.
-WboldGenerate old CORBA 2.1 signatures for skeletons.
-Wbold_prefixMap C++ reserved words with prefix ‘_’ rather than ‘_cxx_’.
-Wbkeep_inc_pathPreserve IDL ‘#include’ paths in generated ‘#include’ directives.
-Wbuse_quotesUse quotes in ‘#include’ directives (e.g. "foo" rather than <foo>.)

Again, most of these are self-explanatory.

5.2.1  Stub / skeleton files

By default, omniidl separates the normal stub and skeleton file (the file) from the ‘dynamic’ stubs (the file), so applications that do not need support for Any and TypeCode for a particular IDL file do not waste space with unnecessary definitions. It is possible to output both the normal stubs and the dynamic stubs to a single file, by simply specifying the same extension for both files. This command places both the normal stubs and the dynamic stubs in

omniidl -bcxx -Wba a.idl

5.2.2  Module splicing

On ancient C++ compilers without namespace support, IDL modules map to C++ classes, and so cannot be reopened. For some IDL, it is possible to ‘splice’ reopened modules on to the first occurrence of the module, so all module definitions are in a single class. It is possible in this sort of situation:

module M1 { interface I {}; }; module M2 { interface J { attribute M1::I ok; }; }; module M1 { interface K { attribute I still_ok; }; };

but not if there are cross-module dependencies:

module M1 { interface I {}; }; module M2 { interface J { attribute M1::I ok; }; }; module M1 { interface K { attribute M2::J oh_dear; }; };

In both of these cases, the -Wbsplice-modules option causes omniidl to put all of the definitions for module M1 into a single C++ class. For the first case, this will work fine. For the second case, class M1::K will contain a reference to M2::J, which has not yet been defined; the C++ compiler will complain.

5.2.3  Flattened tie classes

Another problem with mapping IDL modules to C++ classes arises with tie templates. The C++ mapping says that for the interface M::I, the C++ tie template class should be named POA_M::I_tie. However, since template classes cannot be declared inside other classes, this naming scheme cannot be used with compilers without namespace support.

The standard solution is to produce ‘flattened’ tie class names, using the -Wbtf command line argument. With that flag, the template class is declared at global scope with the name POA_M_I_tie. i.e. all occurrences of ‘::’ are replaced by ‘_’.

5.2.4  Generating example implementations

If you use the -Wbexample flag, omniidl will generate an example implementation file as well as the stubs and skeletons. For IDL file foo.idl, the example code is written to The example file contains class and method declarations for the operations of all interfaces in the IDL file, along with a main() function which creates an instance of each object. You still have to fill in the operation implementations, of course.

5.3  Examples

Generate the C++ headers and stubs for a file a.idl:

omniidl -bcxx a.idl

Generate with Any support:

omniidl -bcxx -Wba a.idl

As above, but also generate Python stubs (assuming omniORBpy is installed):

omniidl -bcxx -Wba -bpython a.idl

Just check the IDL files for validity, generating no output:

omniidl a.idl b.idl

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