GlueGen Manual

Table of Contents

Chapter 1 - Introduction Chapter 2 - Using GlueGen Chapter 3 - Configuration File Examples

Chapter 1 - Introduction


GlueGen is a tool which automatically generates the Java and JNI code necessary to call C libraries. It reads as input ANSI C header files and separate configuration files which provide control over many aspects of the glue code generation. GlueGen uses a complete ANSI C parser and an internal representation (IR) capable of representing all C types to represent the APIs for which it generates interfaces. It has the ability to perform significant transformations on the IR before glue code emission. GlueGen is currently powerful enough to bind even low-level APIs such as the Java Native Interface (JNI) and the AWT Native Interface (JAWT) back up to the Java programming language.

GlueGen is currently used to generate the JOGL interface to the OpenGL 3D graphics API and the JOAL interface to the OpenAL audio library. In the case of JOGL, GlueGen is used not only to bind OpenGL to Java, but also the low-level windowing system APIs on the Windows, X11 and Mac OS X platforms. The implementation of the JOGL library is thereby written in the Java programming language rather than in C, which has offered considerable advantages during the development of the library.

GlueGen is designed in modular form and can be extended to alter the glue code emission style or to generate interface code for other languages than Java.

This manual describes how to use GlueGen to bind new C libraries to the Java programming language.

Structure of the Generated Glue Code

GlueGen supports two basic styles of glue code generation: everything in one class, or a separate interface and implementing class. The first mode, "AllStatic", exposes the underlying C functions as a set of static Java methods in a concrete class. This is a straightforward binding mechanism, but has the disadvantage of tying users to a concrete class (which may or may not be a problem) and makes it more difficult to support certain kinds of call-through-function-pointer semantics required by certain C APIs. The second mode, "InterfaceAndImpl", exposes the C functions as methods in an interface and emits the implementation of that interface into a separate class and package. The implementing class is not intended to be in the public API; this more strongly separates the user from the implementation of the API. Additionally, because it is necessary to hold an instance of the implementing class in order to access the underlying C routines, it is easier to support situations where call-through-function-pointer semantics must be followed, in particular where those function pointers might change from instance to instance.

The generated glue code follows some basic rules in binding C APIs to Java:
  • C primitive types are exposed as the corresponding Java primitive type.
  • Pointers to typed C primitives (int*, float*) are bound to java.nio Buffer subclasses (IntBuffer, FloatBuffer) and optionally to Java arrays (int[], float[]).
    • If a C function takes such a pointer as an outgoing argument, two method overloadings will generally be produced; one which accepts a Buffer, and one which accepts a primitive array plus an integer offset argument. The variant taking a Buffer may accept either a "direct" NIO Buffer or a non-direct one (wrapping a Java array). The exception is when such a routine is specified by the NioDirectOnly directive to keep a persistent pointer to the passed storage, in which case only the Buffer variant will be generated, and will only accept a direct Buffer as argument.
    • If a C function returns such a pointer as its result, it will be exposed as the corresponding Buffer type. In this case it is also typically necessary to specify to GlueGen via the ReturnValueCapacity directive the number of addressable elements in the resulting array.
  • Pointers to void* are bound to java.nio.Buffer.
    • By default any C function accepting a void* argument will allow either a direct or non-direct java.nio Buffer to be passed as argument. If the NioDirectOnly directive is specified, however, only a direct Buffer will be accepted.
    • Similar rules for void* return values apply to those for pointers to typed primitives.
  • To avoid an explosion in the number of generated methods, if a particular API accepts more than one typed primitive pointer argument, only two overloadings continue to be produced: one accepting all arrays as arguments and one accepting all Buffers as arguments. When calling the variant accepting Buffers, all of the Buffers passed in a particular call must be either direct or non-direct. Mixing of direct and non-direct Buffers in a given function call is not supported.
  • When a java.nio Buffer is passed from Java to C, the position of the Buffer is taken into account. The resulting pointer passed to C is equal to the base address of the Buffer plus the position scaled appropriately for the size of the primitive elements in the Buffer. This feature is called "auto-slicing", as it mimics the behavior of calling Buffer.slice() without the overhead of explicit object creation.
  • Pointers to constant char* may be bound to java.lang.String using the ArgumentIsString or ReturnsString directives.
  • #define statements in header files mapping names to constant values are exposed as public static final constant values in either the generated interface or AllStatic class.
  • C structs encountered during the glue code generation process and referenced by the C functions are exposed as Java classes of the same name (typically the name to which the struct is typedefed). Each primitive field in the struct is exposed as two methods; a getter, which accepts no arguments, and a setter, which accepts as argument a primitive value of the type of the field. Static factory methods are exposed allowing allocation of these structs from Java code. The backing storage for these Java classes is a direct java.nio Buffer. GlueGen fully supports returning of pointers to C structs up to Java.

Unique Features

GlueGen contains several unique features making it both a powerful and easy-to-use tool.
  • C structs are exposed as Java classes. The generated code for these classes supports both 32-bit and 64-bit platforms.
  • C structs containing function pointers are exposed as Java classes with methods. This makes it easy to interact with low-level C APIs such as the AWT Native Interface (JAWT) from the Java programming language level.
    • In this context, GlueGen automatically detects which argument to the various function pointers indicates the "this" pointer, hiding it at the Java level and passing it automatically.
    • GlueGen offers automatic handling of JNI-specific data types such as JNIEnv* and jobject. The tool understands that the JNIEnv* argument is implicit and that jobject maps to java.lang.Object at the Java programming language level. While this is most useful when binding JDK-internal APIs such as the JAWT to Java, there may be other JNI libraries which expose C functions taking these data types, and GlueGen can very easily bind to them.

Background and Design Principles

This section provides motivation for the design of the GlueGen tool and is not necessary to understand how to use the tool.

There are many tools available for assisting in the autogeneration of foreign function interfaces for various high-level languages. Only a few examples include Header2Scheme, an early tool allowing binding of a limited subset of C++ to the Scheme programming language; SWIG, a tool released at roughly the same time as Header2Scheme which by now supports binding C and C++ libraries to a variety of scripting languages; JNIWrapper, a commercial tool automating the binding of C APIs to Java; and NoodleGlue, a recently-released tool automating the binding of C++ APIs to Java. Other language-specific tools such as Perl's XS, Boost.Python and many others exist.

GlueGen was designed with a few key principles in mind. The most fundamental was to support binding of the lowest-level APIs on a given platform up to the Java programming language. The intended goal, in the context of the JOGL project, was to allow subsets of the Win32 and X11 APIs to be exposed to Java, and to use those APIs to write the behind-the-scenes OpenGL context creation and management code in Java instead of C. This informed several other design goals:

  • Avoid touching the C headers as much as possible. This makes it easier to upgrade to a more recent version of the C API just by copying in a new set of headers.
  • Avoid touching the generated glue code completely.
  • Avoid having to hand-write a lot of generated glue code. Instead, handle many complex constructs automatically and provide sufficient control over the glue code generation to avoid having to handwrite certain native methods where one or two lines of tweaking would suffice.
  • Support all C constructs in the parser and intermediate representation. The rationale is that it is acceptable to cut corners in the number of constructs supported in the Java binding, but not whether the tool can internally represent it in its C type system. This design goal implies starting with complete a ANSI C parser coupled with a complete C type system.
  • As the tool is targetting the Java programming language, build the tool in the Java programming language.
In order to make the problem more tractable, support for binding C++ to the Java programming language was not considered. C++ adds many constructs over ANSI C which make it much more difficult to reason about and to find a useful subset to support binding to Java. Additionally, it seems that there are relatively few C++-specific libraries in general use which could be usefully bound to Java, although this may be a matter of opinion.

GlueGen was designed with the Java programming language in mind, but is not necessarily restricted to generating glue code for the Java language. The tool is divided into separate parse and code generation phases, and the internal representation is fairly easy to iterate over. The core driver of GlueGen may therefore be useful in producing other tools which autogenerate foreign function interfaces to C libraries for other languages.

Chapter 2 - Using GlueGen

Acquiring and Building GlueGen

The source code for GlueGen may be obtained by cloning the Git repository:
    $git clone --recursive git:// gluegen
To build GlueGen, cd into the gluegen/make folder and invoke ant.
    $ant clean all test
Ant 1.8 or later and a Java 6 compatible JDK is required.

Common Build Problems

CharScanner; panic: ClassNotFoundException: com.jogamp.gluegen.cgram.CToken
This occurs because ANTLR was dropped into the Extensions directory of the JRE/JDK. On Windows and Linux, delete any ANTLR jars from jre/lib/ext, and on Mac OS X, delete them from /Library/Java/Extensions. Use the antlr.jar property in the build.xml to point to a JRE-external location of this jar file.

Basic Operation

GlueGen can be run either as an executable jar file (java -jar gluegen.jar; note that antlr.jar must be in the same directory as gluegen.jar in order for this invocation to work) or from within Ant as described in the following section. When run from the command line, GlueGen accepts four kinds of command-line arguments:

  • -Idir (optional) adds dir to the include path. Similarly to a C compiler or preprocessor, GlueGen scans a set of directories to locate header files it encounters in #include directives. Unlike most C preprocessors, however, GlueGen has no default include path, so it is typically necessary to supply at least one -I option on the command line in order to handle any #include directives in the file being parsed.
  • -EemitterClassName (optional) uses emitterClassName as the fully-qualified name of the emitter class which will be used by GlueGen to generate the glue code. The emitter class must implement the com.jogamp.gluegen.GlueEmitter interface. If this option is not specified, a com.jogamp.gluegen.JavaEmitter will be used by default.
  • -CcfgFile adds cfgFile to the list of configuration files used to set up the chosen emitter. This is the means by which a large number of options are passed in to the GlueGen tool and to the emitter in particular. Configuration files are discussed more in the following section.
  • [ filename | - ] selects the file or standard input from which GlueGen should read the C header file for which glue code should be generated. This must be the last command-line argument, and only one filename argument is supported. To cause multiple header files to be parsed, write a small .c file #including the multiple headers and point GlueGen at the .c file.

Running GlueGen as an Ant Task

GlueGen can also be invoked as a subtask within Ant. In order to do so, a path element should be defined as follows:

    <path id="gluegen.classpath">
        <pathelement location="${gluegen.jar}" />
        <pathelement location="${antlr.jar}" />
where the gluegen.jar and antlr.jar properties point to the respective jar files. A taskdef defining the GlueGen task should then be specified as follows:
<taskdef name="gluegen"
    classpathref="gluegen.classpath" />
At this point GlueGen may be invoked as follows:
<gluegen src="[header to parse]" 
         config="[configuration file]"
         includeRefid="[dirset for include path]"
    <classpath refid="gluegen.classpath" />
Please see the JOGL and JOAL build.xml files for concrete, though non-trivial, examples of how to invoke GlueGen via Ant.


GlueGen contains and uses a minimal C preprocessor called the "Pseudo C Pre-Processor", or PCPP. A slightly specialized C preprocessor is required for correct glue code generation with most libraries. Constant values intended for use by end users are defined in many C libraries' headers using #defines rather than constant int declarations, and if the header is processed by a full C preprocessor then the #define statements will be stripped become unavailable for processing by the glue code generator.

PCPP is largely an invisible part of the glue code generation process; however, it has certain limitations which make it difficult to parse certain header files. First, it does not support macro evaluation in any form, so if a header relies on macro evaluation in order to generate code, PCPP will fail. It is possible that PCPP may fail silently in this situation, causing GlueGen to simply not produce code for the associated constructs. If GlueGen's output is not as expected and there is heavy use of the C preprocessor in the header, run PCPP against the header directly (PCPP takes simply the -I and filename arguments accepted by GlueGen) and examine the output.

Second, PCPP contains only limited support for #if clauses. Generally speaking, its handling of #if defined(foo) || defined(bar) constructs is limited to approximately what is required to handle the OpenGL header files. If the header being parsed relies on moderately complicated expressions being evaluated by the C preprocessor, check the output from PCPP and ensure it is as expected.

Contributions to PCPP would be especially welcome. It would be very desirable to turn it into a full-blown C preprocessor with simply the option of passing through #define statements unchanged.

Error Reporting

Error reporting by GlueGen's parser is currently less than ideal. Because PCPP makes #include directives disappear completely with respect to the C parser (it appears that the #line directives it emits are not being consumed properly -- an area which needs more investigation), the line numbers reported in parse failures are incorrect in all but the simplest cases. This makes it difficult to determine in exactly what header file and on exactly what construct the C parser failed.

Fortunately, there is a relatively simple workaround. PCPP can be run with all of the same -I arguments passed to GlueGen and the result piped to a new .c file. GlueGen can then be invoked on that .c file (now containing no #include directives) and the line numbers on any parse failures will be correct.

Stub Headers

As much as is possible, GlueGen is intended to operate on unmodified C header files, so that it is easy to upgrade the given C API being bound to Java simply by dropping in a new set of header files. However, most C headers contain references to standard headers like stdio.h, and if this header is parsed by GlueGen, the tool will automatically attempt to generate Java entry points for such routines as fread and fwrite, among others. It is impractical to exclude these APIs on a case by case basis. Therefore, the suggested technique to avoid polluting the binding with these APIs is to "stub out" the headers.

GlueGen searches the include path for headers in the order the include directories were specified to the tool. Placing another directory in front of the one in which the bulk of the headers are found allows, for example, an alternative stdio.h to be inserted which contains few or no declarations but which satisfies the need of the dependent header to find such a file.

GlueGen uses a complete ANSI and GNU C parser written by John Mitchell and Monty Zukowski from the set of grammars available for the ANTLR tool by Terrence Parr. As a complete C parser, this grammar requires all data types encountered during the parse to be fully defined. Often a particular header will be included by another one in order to pick up data type declarations rather than API declarations. Stubbing out the header with a smaller one providing a "fake" type declaration is a useful technique for avoiding the binding of unnecessary APIs during the glue code process.

Here's an example from the JOGL glue code generation process. The glext.h header defining OpenGL extensions references stddef.h in order to pick up the ptrdiff_t data type. We choose to not include the real stddef.h but instead to swap in a stub header. The contents of this header are therefore as follows:

    #if defined(_WIN64)
        typedef __int64 ptrdiff_t;
    #elif defined(__ia64__) || defined(__x86_64__)
        typedef long int ptrdiff_t;
        typedef int ptrdiff_t;

This causes the ptrdiff_t data type to be defined appropriately for the current architecture. It will be referenced during the glue code generation and cause a Java value of the appropriate type (int or long) to be used to represent it.

This is not the best example because it involves a data type which changes size between 32- and 64-bit platforms, and there are otner considerations to take into account in these situations (see the section 32- and 64-bit considerations). Here's another example, again from the JOGL source tree. JOGL binds the AWT Native Interface, or JAWT, up to the Java programming language so that the low-level code which binds OpenGL contexts to Windows device contexts may be written in Java. The JDK's jawt_md.h on the Windows platform includes windows.h to pick up the definitions of data types such as HWND (window handle) and HDC (handle to device context). However, it is undesirable to try to parse the real windows.h just to pick up these typedefs; not only does this header contain thousands of unneeded APIs, but it also uses certain macro constructs not supported by GlueGen's minimal C preprocessor. To avoid these problems, a "stub" windows.h header is placed in GlueGen's include path containing only the necessary typedefs:

    typedef struct _handle*     HANDLE;
    typedef HANDLE              HDC;
    typedef HANDLE              HWND;

Note that it is essential that the type being specified to GlueGen is compatible at least in semantics with the real definition of the HANDLE typedef in the real windows.h, so that during compilation of GlueGen's autogenerated C code, when the real windows.h is referenced by the C compiler, the autogenerated code will compile correctly.

This example is not really complete as it also requires consideration of the size of data types on 32- and 64-bit platforms as well as a discussion of how certain opaque data types are described to GlueGen and exposed in its autogenerated APIs. Nonetheless, it illustrates at a basic level why using a stub header is necessary and useful in certain situations.

32- and 64-bit Considerations

When binding C functions to the Java programming language, it is important that the resulting Java code support execution on a 64-bit platform if the associated native methods are compiled appropriately. In other words, the public Java API should not change if the underlying C data types change to another data model such as LP64 (in which longs and pointers become 64-bit).

GlueGen internally maintains two descriptions of the underlying C data model: one for 32-bit architectures and one for 64-bit architectures. These machine descriptions are used when deciding the mapping between integral C types such as int and long and the corresponding Java types, as well as when laying out C structs for access by the Java language. For each autogenerated C struct accessor, both a 32-bit and 64-bit variant are generated behind the scenes, ensuring that the resulting Java code will run correctly on both 32-bit and 64-bit architectures.

When generating the main class containing the bulk of the method bindings, GlueGen uses the 64-bit machine description to map C data types to Java data types. This ensures that the resulting code will run properly on 64-bit platforms. Note that it also generally means that C longs will be mapped to Java longs, since an LP64 data model is assumed.

If Opaque directives are used to cause a given C integer or pointer data type to be mapped directly to a Java primitive type, care should be taken to make sure that the Java primitive type is wide enough to hold all of the data even on 64-bit platforms. Even if the data type is defined in the header file as being only a 32-bit C integer, if there is a chance that on a 64-bit platform the same header may define the data type as a 64-bit C integer or long, the Opaque directive should map the C type to a Java long.

Opaque Directives

Complex header files may contain declarations for certain data types that are either too complex for GlueGen to handle or unnecessarily complex from the standpoint of glue code generation. In these situations a stub header may be used to declare a suitably compatible typedef for the data type. An Opaque directive can be used to map the resulting typedef to a Java primitive type if it is undesirable to expose it as a full-blown Java wrapper class.

GlueGen hashes all typedefs internally down to their underlying primitive type. (This is probably not really correct according to the C type system, but is correct enough from a glue code generation standpoint, where if the types are compatible they are considered equivalent.) This means that if the parser encounters

    typedef void* LPVOID;

then an Opaque directive stating

    Opaque long LPVOID

will cause all void* or LPVOID arguments in the API to be mapped to Java longs, which is almost never desirable. Unfortunately, it is not currently possible to distinguish between the LPVOID typedef and the underlying void* data type in this situation.

A similar problem occurs for other data types for which Opaque directives may be desired. For example, a Windows HANDLE equates to a typedef to void*, but performing this typedef in a stub header and then adding the Opaque directive

    Opaque long HANDLE

will cause all void* arguments to be exposed as Java longs instead of Buffers, which is again undesirable. Attempting to work around the problem by typedef'ing HANDLE to an integral type, as in:

    typedef long HANDLE;

may itself have problems, because GlueGen will assume the two integral types are compatible and not perform any intermediate casts between HANDLE and jlong in the autogenerated C code. (When casting between a pointer type and a JNI integral type such as jlong in C code, GlueGen automatically inserts casts to convert the pointer first to an "intptr_t" and then to the appropriate JNI type, in order to silence compiler warnings and/or errors.)

What is desired is to produce a new type name distinct from all others but still compatible with the pointer semantics of the original type. Then an Opaque directive can be used to map the new type name to, for example, a Java long.

To implement this in the context of the HANDLE example, the following typedef may be inserted into the stub header:

    typedef struct _handle*     HANDLE;

This uses a pointer to an anonymous struct name to produce a new pointer type. This is legal ANSI C and is supported by GlueGen's parser without having seen a declaration for "struct _handle". Subsequently, an Opaque directive can be used to map the HANDLE data type to a Java long:

    Opaque long HANDLE

Now HANDLEs are exposed to Java as longs as desired. A similar technique is used to expose XIDs on the X11 platform as Java longs.

Argument Name Substitution

Certain configuration file directives allow the insertion of Java or C code at various places in the generated glue code, to both eliminate the need to hand-edit the generated glue code as well as to minimize the hand-writing of glue code, which sidesteps the GlueGen process. In some situations the inserted code may reference incoming arguments to compute some value or perform some operation. Examples of directives supporting this substitution include ReturnValueCapacity and ReturnedArrayLength.

The expressions in these directives may contain Java MessageFormat expressions like {0} which refer to the incoming argument names to the function. {0} refers to the first incoming argument.

Strongly-typed C primitive pointers such as int*, which ordinarily expand to overloaded Java methods taking e.g. int[] as well as IntBuffer, present a problem. The expansion to int[] arr also generates an int arr_offset argument to be able to pass a pointer into the middle of the array down to C. To allow the same MessageFormat expression to be used for both cases, the subsitution that occurs when such a primitive array is referenced is the string arr, arr_offset; in other words, the subtituted string contains a comma. This construct may be used in the following way: the code being manually inserted may itself contain a method call taking e.g. {3} (the incoming argument index of the primitive array or buffer). The user should supply two overloaded versions of this method, one taking a strongly-typed Buffer and one taking e.g. an int[] arr and int arr_offset argument. The implementation of RangeChecks for primitive arrays and strongly-typed buffers uses this construct.

It should be noted that in the autogenerated C code the offset argument is expressed in bytes while at the Java level it is expressed in elements. Most uses of GlueGen will probably not have to refer to the primitive array arguments in C code so this slight confusion should be minor.

Configuration File Directives

In addition to the C headers, GlueGen requires a certain amount of metadata in the form of configuration files in order to produce its glue code. There are three basic reasons for this: first, GlueGen must be informed into which Java classes the C methods are to be bound; second, there are many configuration options for the generated glue code, and passing them all on the command line is infeasible; and third, there are ambiguities in many constructs in the C programming language which must be resolved before a Java binding can be produced.

The contents of the configuration file are dependent on the class of emitter specified to GlueGen. Currently there are three built-in emitter classes: JavaEmitter, which produces a basic, static Java binding of C functions; ProcAddressEmitter, which extends JavaEmitter by calling the underlying C functions through function pointers, resulting in more dynamic behavior and supporting C APIs with optional functionality; and GLEmitter, which specializes ProcAddressEmitter to support some OpenGL-specific constructs. The GLEmitter will be ignored in this manual as it is specialized for JOGL and provides very little additional functionality beyond the ProcAddressEmitter. The JavaEmitter and ProcAddressEmitter support many options in their configuration files. As the ProcAddressEmitter is a subclass of JavaEmitter, all of the constructs in the JavaEmitter's configuration files are also legal in the ProcAddressEmitter's configuration files.

The configuration files have a very simple line-by-line structure, and are parsed by a very rudimentary, hand-written parser. Each non-whitespace and non-comment line (note: comment lines begin with '#') contains a directive like Package, Style or JavaClass followed by arguments to that directive. There are a certain set of directives that are required for any code generation; others are optional and their omission results in some default behavior. Directives are case-insensitive.

The following is an exhaustive list of the options currently supported by each of these emitters' configuration files. It is difficult to see exactly how to use the tool based simply on these descriptions, so the examples may be more helpful in seeing exactly how to structure a configuration file for proper glue code generation.

JavaEmitter Configuration

Note that only a very few of the following directives are specified as being "required" rather than "optional"; these indicate the minimal directives needed for a valid configuration file to begin to get glue code to be produced. In general, these are Package, ImplPackage, JavaClass, ImplJavaClass, and Style. Other directives such as NioDirectOnly are required in some circumstances for the glue code to be correct, and some such as ReturnedArrayLength, ReturnValueCapacity, and ReturnValueLength should be specified in some situations in order for certain return values to be useful at the Java level.

The following directives are specified in alphabetical order, although this is not necessarily the best semantic order.

Syntax: AccessControl [method name] [ PUBLIC | PROTECTED | PRIVATE | PACKAGE_PRIVATE ]
(optional) Controls the access control of a certain Java method corresponding to a C function. The access control of all APIs defaults to public. This is useful when using the C binding of a particular function only as one implementation strategy of the real public API and using CustomJavaCode to write the exposed API. In this case is most useful in conjunction with RenameJavaMethod.
Syntax: ArgumentIsString [function name] [indices...] where the first argument index is 0
(optional) For a C function with one or more outgoing char* (or compatible data type) arguments, indicates that those arguments are semantically null-terminated C strings rather than arbitrary arrays of bytes. The generated glue code will be modified to emit those arguments as java.lang.String objects rather than byte[] or ByteBuffer.
Syntax: ClassJavadoc [class name] [code...]
(optional) Causes the specified line of code to be emitted in the appropriate place in the generated code to become the per-class Javadoc for the specified class. By default GlueGen produces no Javadoc for its generated classes, so this is the mechanism by which a user can emit Javadoc for these classes. The specified Javadoc undergoes no transformation by GlueGen, so the initial /** and trailing */ must be included in the correct place. Each line of Javadoc is emitted in the order encountered during parsing of the configuration files.
Syntax: CustomCCode [code...]
(optional) Causes the specified line of C code to be emitted into the generated native code for the implementing class. Currently there is no way (and no real need) to be able to emit custom C code into any other generated .c file, so the class name in the CustomJavaCode directive is omitted.
Syntax: CustomJavaCode [class name] [code...]
(optional) Causes the specified line of Java code to be emitted into the specified generated Java class. Can be used to emit code into any generated class: the public interface, the implementing class, the sole concrete class (in the case of the AllStatic Style), or any of the Java classes corresponding to referenced C structs in the parsed headers. This usage is somewhat verbose, and the IncludeAs directive provides a more concise way of including large bodies of Java code into the generated code.
Syntax: EmitStruct [C struct type name]
(optional) Forces a Java class to be emitted for the specified C struct. Normally only those structs referenced directly by the parsed C APIs have corresponding Java classes emitted.
Syntax: GlueGenRuntimePackage [package name, like com.jogamp.gluegen.runtime]
(optional) Changes the package in which the generated glue code expects to find its run-time helper classes (like Buffers, CPU, StructAccessor). Defaults to com.jogamp.gluegen.runtime (no quotes). This is useful if you want to bundle the runtime classes in your application without the possibility of interfering with other versions elsewhere in the system.
Syntax: Extends [Java interface name] [interface name to extend]
(optional) Causes the specified autogenerated Java interface to declare that it extends another one. This directive may only be applied to autogenerated interfaces, not concrete classes. For concrete classes, use the Implements directive.
Syntax: HierarchicalNativeOutput true
(optional) If "true", makes subdirectories for the generated native code matching the package names of the associated classes. This is typically not needed (or desired, as it complicates the compilation process for this native code) and defaults to false.
Syntax: Ignore [regexp]
(optional) Ignores one or more functions or data types matching the regexp argument which are encountered during parsing of the C headers. By default GlueGen will emit all encountered C functions as well as Java classes corresponding to all C structs referenced by those functions. Related directives are IgnoreNot, Unignore and EmitStruct.
Syntax: IgnoreField [struct type name] [field name]
(optional) Causes the specified field of the specified struct type to be ignored during code generation, typically because it is too complex for GlueGen to handle.
Syntax: see Ignore. (optional) Similar to the Ignore directive, but evaluates the negation of the passed regexp when deciding whether to ignore the given function or data type. The Unignore mechanism may be used with IgnoreNot as well. NOTE: the IgnoreNot mechanism may ultimately turn out to be superfluous; the authors do not have sufficient experience with regular expressions to know whether general negation of a regexp is possible. Feedback in this area would be appreciated.
Syntax: Implements [Java class name] [interface name to implement]
(optional) Causes the specified autogenerated Java concrete class to declare that it implements the specified interface. This directive may only be applied to autogenerated concrete classes, not interfaces. For interfaces, use the Extends directive.
Syntax: ImplJavaClass [class name]
(optional) Specifies the name of the typically non-public, implementation Java class which contains the concrete Java and native methods for the glue code. If the emission style is AllStatic, there is no distinction between the public and implementation class and ImplJavaClass should not be specified. Otherwise, if the ImplJavaClass is unspecified, it defaults to the JavaClass name plus "Impl". (If both are unspecified in this configuration, an error is reported.) See also JavaClass.
Syntax: ImplPackage [package name]
(optional) Specifies the package name into which the implementing class containing the concrete Java and native methods will be emitted, assuming an emission style of InterfaceAndImpl or ImplOnly. If AllStatic, there is no separate implementing class from the public interface. If the emission style is not AllStatic and the ImplPackage is not specified, it defaults to the Package plus ".impl". See also Package.
Syntax: Import [package name] (no trailing semicolon)
(optional) Adds an import statement at the top of each generated Java source file.
Syntax: Include [filename]
(optional) Causes another configuration file to be read at the current point in parsing the current configuration file. The filename argument may be either absolute or relative; in the latter case it is specified relative to the location of the current configuration file.
Syntax: IncludeAs [prefix tokens] [filename]
(optional) Similar to the Include directive, but prepends the specified prefix tokens on to every line of the file to be read. The last token parsed is the name of the file to be read. This allows, for example, CustomJavaCode to be stored as Java source rather than in the configuration file; in this example the configuration file might contain IncludeAs CustomJavaCode MyClass
Syntax: JavaClass [class name]
(optional / required) Specifies the name of the public, non-implementation Java class or interface into which the glue code will be generated. If the emission style is not ImplOnly, the JavaClass directive is required. See also ImplJavaClass.
Syntax: JavaEpilogue [C function name] [code...]
(optional) Adds the specified code as an epilogue in the Java method for the specified C function; this code is run after the underlying C function has been called via the native method but before any result is returned. As in the ReturnedArrayLength and other directives, argument name substitution is performed on MessageFormat expressions in the specified code. See also JavaPrologue.
Syntax: JavaOutputDir [directory name]
(optional) Specifies the root directory into which the emitted Java code will be produced. Subdirectories for the packages of the associated Java classes will be automatically created. If unspecified, defaults to the current working directory.
Syntax: JavaPrologue [C function name] [code...]
(optional) Adds the specified code as a prologue in the Java method for the specified C function; this code is run before the underlying C function is called via the native method. As in the ReturnedArrayLength and other directives, argument name substitution is performed on MessageFormat expressions in the specified code. See also JavaEpilogue.
Syntax: ManuallyImplement [function name]
(optional) Indicates to GlueGen to not produce a method into the implementing class for the specified C function; the user must provide one via the CustomJavaCode directive. If the emission style is InterfaceAndImpl or InterfaceOnly, a public method will still be generated for the specified function.
Syntax: NativeOutputDir [directory name]
(optional) Specifies the root directory into which the emitted JNI code will be produced. If unspecified, defaults to the current working directory. See also HierarchicalNativeOutput.
Syntax: NioDirectOnly [function name]
(required when necessary) When passing a pointer down to a C API, it is semantically undefined whether the underlying C code expects to treat that pointer as a persistent pointer, living past the point of return of the function call, or whether the pointer is used only during the duration of the function call. For APIs taking C primitive pointers such as void*, float*, etc., GlueGen will typically generate up to two overloaded Java methods, one taking a Buffer or Buffer subclass such as FloatBuffer, and one taking a primitive array such as float[]. (In the case of void* outgoing arguments, GlueGen produces only one variant taking a Buffer.) Normally the generated glue code accepts either a "direct" or non-"direct" buffer (according to the New I/O APIs) as argument. However, if the semantics of the C function are that it either expects to hold on to this pointer past the point of the function call, or if it can block while holding on to the pointer, the NioDirectOnly directive must be specified for this C function in order for the generated glue code to be correct. Failing to observe this requirement may cause JVM hangs or crashes.
Syntax: Opaque [Java primitive data type] [C data type]
(optional) Causes a particular C data type to be exposed in opaque form as a Java primitive type. This is most useful for certain pointer types for which it is not desired to generate full Java classes but instead expose them to Java as e.g. longs. It is also useful for forcing certain integral C data types to be exposed as e.g. long to Java to ensure 64-bit cleanliness of the generated glue code. See the examples. The C data type may be a multiple-level pointer type; for example Opaque long void**. Note that it is not currently supported to make a given data type opaque for just a few functions; the Opaque directive currently applies to all C functions in the headers being parsed. This means that sweeping Opaque declarations like Opaque long void* will likely have unforseen and undesirable consequences.
Syntax: Package [package name] (no trailing semicolon)
(optional / required) Specifies the package into which the public interface or class for the autogenerated glue code will be generated. Required whenever the emission style is not ImplOnly. See also ImplPackage.
Syntax: RangeCheck [C function name] [argument number] [expression]
(optional) Causes a range check to be performed on the specified array or Buffer argument of the specified autogenerated Java method. This range check ensures, for example, that a certain number of elements are remaining in the passed Buffer, knowing that the underlying C API will access no more than that number of elements. For range checks that should be expressed in terms of a number of bytes rather than a number of elements, see the RangeCheckBytes directive. As in the ReturnedArrayLength and other directives, argument name substitution is performed on MessageFormat expressions.
Syntax: RangeCheckBytes [C function name] [argument number] [expression]
(optional) Same as the RangeCheck directive, but the specified expression is treated as a minimum number of bytes remaining rather than a minimum number of elements remaining. This directive may not be used with primitive arrays.
Syntax: RenameJavaMethod [from name] [to name]
(optional) Causes the specified C function to be emitted under a different name in the Java binding. This is most useful in conjunction with the AccessControl directive when the C function being bound to Java is only one potential implementation of the public API, or when a considerable amount of Java-side custom code is desired to wrap the underlying C native method entry point.
Syntax: RenameJavaType [from name] [to name]
(optional) Causes the specified C struct to be exposed as a Java class under a different name. This only applies to autogenerated classes corresponding to C structs encountered during glue code generation; full control is provided over the name of the top-level classes associated with the set of C functions via the JavaClass and ImplJavaClass directives.
Syntax: ReturnedArrayLength [C function name] [expression] where expression is a legal Java expression with MessageFormat specifiers such as "{0}". These specifiers will be replaced in the generated glue code with the incoming argument names where the first argument to the method is numbered 0. See the section on argument name substitution.
(optional) For a function returning a compound C pointer type such as an XVisualInfo*, indicates that the returned pointer is to be treated as an array and specifies the length of the returned array as a function of the arguments passed to the function. Note that this directive differs subtly from ReturnValueCapacity and ReturnValueLength. It is also sometimes most useful in conjunction with the TemporaryCVariableDeclaration and TemporaryCVariableAssignment directives.
Syntax: ReturnsString [function name]
(optional) Indicates that the specified C function which returns a char* or compatible type actually returns a null-terminated C string which should be exposed as a java.lang.String. NOTE: currently does not properly handle the case where this storage needs to be freed by the end user. In these situations the data should be returned as a direct ByteBuffer, the ByteBuffer converted to a String using custom Java code, and the ByteBuffer freed manually using another function bound to Java.
Syntax: ReturnValueCapacity [C function name] [expression]
(optional) Specifies the capacity of a java.nio Buffer or subclass wrapping a C primitive pointer such as char* or float* being returned from a C function. Typically necessary in order to properly use such pointer return results from Java. As in the ReturnedArrayLength directive, argument name substitution is performed on MessageFormat expressions.
Syntax: ReturnValueLength [C function name] [expression]
(optional) Specifies the length of a returned array of pointers, typically to C structs, from a C function. This differs from the ReturnedArrayLength directive in the pointer indirection to the array elements. The ReturnedArrayLength directive handles slicing up of a linear array of structs, while the ReturnValueLength directive handles boxing of individual elements of the array (which are pointers) in to the Java class which wraps that C struct type. See the examples for a concrete example of usage. As in the ReturnedArrayLength directive, argument name substitution is performed on MessageFormat expressions.
Syntax: RuntimeExceptionType [class name]
(optional) Specifies the class name of the exception type which should be thrown when run-time related exceptions occur in the generated glue code, for example if a non-direct Buffer is passed to a method for which NioDirectOnly was specified. Defaults to RuntimeException.
Syntax: StructPackage [C struct type name] [package name]. Package name contains no trailing semicolon.
(optional) Indicates that the specified Java class corresponding to the specified C struct should be placed in the specified package. By default, these autogenerated Java classes corresponding to C structs are placed in the main package (that defined by PackageName).
Syntax: Style [ AllStatic | InterfaceAndImpl |InterfaceOnly | ImplOnly ]
(optional) Defines how the Java API for the parsed C headers is structured. If AllStatic, one concrete Java class will be generated containing static methods corresponding to the C entry points. If InterfaceAndImpl, a public Java interface will be generated into the Package with non-static methods corresponding to the C functions, and an "implementation" concrete Java class implementing this interface will be generated into the ImplPackage. If InterfaceOnly, the InterfaceAndImpl code generation style will be followed, but only the interface will be generated. If ImplOnly, the InterfaceAndImpl code generation style will be followed, but only the concrete implementing class will be generated. The latter two options are useful when generating a public API in which certain operations are unimplemented on certain platforms; platform-specific implementation classes can be generated which implement or leave unimplemented various parts of the API.
Syntax: TemporaryCVariableAssignment [C function name][code...]
(optional) Inserts a C variable assignment declared using the TemporaryCVariableDeclaration directive in to the body of a particular autogenerated native method. The assignment is performed immediately after the call to the underlying C function completes. This is typically used in conjunction with the ReturnValueCapacity or ReturnValueLength directives to capture the size of a returned C buffer or array of pointers. See the examples for a concrete example of usage of this directive. Note that unlike, for example, the ReturnedArrayLength directive, no substitution is performed on the supplied code, so the user must typically have previously looked at the generated code and seen what work needed to be done and variables needed to be examined at exactly that line.
Syntax: TemporaryCVariableDeclaration [C function name] [code...]
(optional) Inserts a C variable declaration in to the body of a particular autogenerated native method. This is typically used in conjunction with the TemporaryCVariableAssignment and ReturnValueCapacity or ReturnValueLength directives to capture the size of a returned C buffer or array of pointers. See the examples for a concrete example of usage of this directive.
Syntax: Unignore [regexp]
(optional) Removes a previously-defined Ignore directive. This is useful when one configuration file includes another and wishes to disable some of the Ignores previously specified.
Syntax: Unimplemented [regexp]
(optional) Causes the binding for the functions matching the passed regexp to have bodies generated which throw the stated RuntimeExceptionType indicating that this function is unimplemented. This is most useful when an API contains certain functions that are not supported on all platforms and there are multiple implementing classes being generated, one per platform.

ProcAddressEmitter Configuration

The ProcAddressEmitter is a subclass of the core JavaEmitter which knows how to call C functions through function pointers. In particular, the ProcAddressEmitter detects certain constructs in C header files which imply that the APIs are intended to be called through function pointers, and generates the glue code appropriately to support that.

The ProcAddressEmitter detects pairs of functions and function pointer typedefs in a set of header files. If it finds a matching pair, it converts the glue code emission style for that API to look for the function to call in an autogenerated table called a ProcAddressTable rather than linking the autogenerated JNI code directly to the function. It then changes the calling convention of the underlying native method to pass the function pointer from Java down to C, where the call-through-function-pointer is performed.

The ProcAddressEmitter discovers the function and function pointer pairs by being informed of the mapping between their names by the user. In the OpenGL and OpenAL libraries, there are fairly simple mappings between the functions and function pointers. For example, in the OpenGL glext.h header file, one may find the following pair:

    GLAPI void APIENTRY glFogCoordf (GLfloat);
    typedef void (APIENTRYP PFNGLFOGCOORDFPROC) (GLfloat coord);

Therefore the mapping rule between the function name and the function pointer typedef for the OpenGL extension header file is "PFN + Uppercase(funcname) + PROC". Similarly, in the OpenAL 1.1 header files, one may find the following pair:

    AL_API void AL_APIENTRY alEnable( ALenum capability );
    typedef void           (AL_APIENTRY *LPALENABLE)( ALenum capability );

Therefore the mapping rule between the function name and the function pointer typedef for the OpenAL header files is "LP + Uppercase(funcname)".

These are the two principal function pointer-based APIs toward which the GlueGen tool has currently been applied. It may turn out to be that this simple mapping heuristic is insufficient, in which case it will need to be extended in a future version of the GlueGen tool.

Note that it is currently the case that in order for the ProcAddressEmitter to notice that a given function should be called through a function pointer, it must see both the function prototype as well as the function pointer typedef. Some headers, in particular the OpenAL headers, have their #ifdefs structured in such a way that either the declaration or the typedef is visible, but not both simultaneously. Because the PCPP C preprocessor GlueGen uses obeys #ifdefs, it is in a situation like this that the headers would have to be modified to allow GlueGen to see both declarations.

The following directives are specified in alphabetical order, although this is not necessarily the best semantic order. The ProcAddressEmitter also accepts all of the directives supported by the JavaEmitter. The required directives are GetProcAddressTableExpr and ProcAddressNameExpr.

Syntax: EmitProcAddressTable [true | false]
(optional) Indicates whether to emit the ProcAddressTable during glue code generation. Defaults to false.
Syntax: ForceProcAddressGen [function name]
(optional) Indicates that a ProcAddressTable entry should be produced for the specified function even though it does not have an associated function pointer typedef in the header. This directive does not currently cause the autogenerated Java and C code to change to call-through-function-pointer style, which should probably be considered a bug. (FIXME)
Syntax: GetProcAddressTableExpr [expression]
(required) Defines the Java code snippet used by the generated glue code to fetch the ProcAddressTable containing the function pointers for the current API. It is up to the user to decide where to store the ProcAddressTable. Common places for it include in an instance field of the implementing class, in an associated object with which there is a one-to-one mapping, or in a static field of another class accessed by a static method. In the JOGL project, for example, each GLImpl instance has an associated GLContext in an instance field called "_context", so the associated directive is GetProcAddressTableExpr _context.getGLProcAddressTable(). In the JOAL project, the ProcAddressTables are currently held in a separate class accessed via static methods, so one of the associated directives is GetProcAddressTableExpr ALProcAddressLookup.getALCProcAddressTable().
Syntax: ProcAddressNameExpr [expression]
(required) Defines the mapping from function name to function pointer typedef to be able to properly identify this function as needing call-through-function-pointer semantics. The supplied expression uses a set of simple commands to describe certain operations on the function name:
  • $UpperCase(arg) converts the argument to uppercase. "UpperCase" is case-insensitive.
  • $LowerCase(arg) converts the argument to lowercase. "LowerCase" is case-insensitive.
  • {0} represents the name of the function.
  • Any other string represents a constant string.
  • Concatenation is implicit.
The corresponding ProcAddressNameExpr for the OpenGL extension functions as described at the start of this section is PFN $UPPERCASE({0}) PROC. The ProcAddressNameExpr for the OpenAL functions as described at the start of this section is LP $UPPERCASE({0}).
Syntax: ProcAddressTableClassName [class name]
(optional) Specifies the class name into which the table containing the function pointers will be emitted. Defaults to "ProcAddressTable".
Syntax: ProcAddressTablePackage [package name] (no trailing semicolon)
(optional) Specifies the package into which to produce the ProcAddressTable for the current set of APIs. Defaults to the implementation package specified by the ImplPackage directive.
Syntax: SkipProcAddressGen [function name]
(optional) Indicates that the default behavior of call-through-function-pointer should be skipped for this function despite the fact that it has an associated function pointer typedef in the header.

Chapter 3 - Configuration File Examples

Simplest possible example


This example shows the simplest possible usage of GlueGen; a single routine taking as arguments and returning only primitive types. The signature of the C function we are interested in binding is

    int one_plus(int a);

To bind this function to Java, we only need a configuration file with very basic settings, indicating the style of glue code emission, the package and class into which the glue code will be generated, and the output directories for the Java and native code. The contents of the configuration file are as follows:

    Package testfunction
    Style AllStatic
    JavaClass TestFunction
    JavaOutputDir   gensrc/java
    NativeOutputDir gensrc/native

GlueGen can then be invoked with approximately the following command line:

    java -cp gluegen.jar:antlr.jar com.jogamp.gluegen.GlueGen \
        -I. -Ecom.jogamp.gluegen.JavaEmitter -Cfunction.cfg function.h

The resulting Java and native code needs to be compiled, and the application needs to load the native library for the Java binding before attempting to invoke the native method by calling System.load() or System.loadLibrary().

Arrays and buffers


This example shows how C primitive arrays are bound to Java. The header file contains three functions to bind:

    float process_data(float* data, int n);
    void set_global_data(float* data);
    float process_global_data(int n);

The semantics of process_data are that it takes in a pointer to a set of primitive float values and the number of elements in the array and performs some operation on them, returning a floating-point value as the result. Afterward the passed data is no longer referenced.

set_global_data, on the other hand, takes a pointer to the data and stores it persistently in the C code. process_global_data then accepts as argument the number of elements to process from the previously-set global data, performs this processing and returns a result. The global data may be accessed again afterward. As an example, these kinds of semantics are used in certain places in the OpenGL API.

From a Java binding standpoint, process_data may accept data stored either inside the Java heap (in the form of a float[] or non-direct FloatBuffer) or outside the Java heap (in the form of a direct FloatBuffer), because it does not access the data after the function call has completed and therefore would not be affected if garbage collection moved the data after the function call was complete. However, set_global_data can cause the passed data to be accessed after the function call is complete, if process_global_data is called. Therefore the data passed to set_global_data may not reside in the Java garbage-collected heap, but must reside outside the heap in the form of a direct FloatBuffer.

It is straightforward to take into account these differences in semantics in the configuration file using the NioDirectOnly directive:

    # The semantics of set_global_data imply that
    # only direct Buffers are legal
    NioDirectOnly set_global_data

Note the differences in the generated Java-side overloadings for the two functions:

    public static void process_data(java.nio.FloatBuffer data, int n) {...}
    public static void process_data(float[] data, int data_offset, int n) {...}
    public static void set_global_data(java.nio.FloatBuffer data) {...}

No overloading is produced for set_global_data taking a float[], as it can not handle data residing in the Java heap. Further, the generated glue code will verify that any FloatBuffer passed to this routine is direct, throwing a RuntimeException if not. The type of the exception thrown in this and other cases may be changed with the RuntimeExceptionType directive.

String handling


This example shows how to pass and return C strings. The functions involved are a bit contrived, as nobody would ever need to bind the C library's string handling routines to Java, but they do illustrate situations in which Java strings might need to be passed to C and C strings returned to Java. As an example, both styles of function are present in the OpenGL and OpenAL APIs.

The included source code exposes two functions to Java:

    size_t strlen(const char* str);
    char*  strstr(const char* str1, const char* str2);

Note that we might just as easily parse the C standard library's string.h header file to pick up these function declarations. However for the purposes of this example it is easier to extract just the functions we need.

Note that the function.h header file contains a typedef for size_t. This is needed because GlueGen does not inherently know about this data type. An equivalent data type for the purposes of this example is int, so we choose to tell GlueGen to use that data type in place of size_t while generating glue code.

The following directive in the configuration file tells GlueGen that strlen takes a string as argument 0 (the first argument):

    ArgumentIsString strlen 0

The following directive tells GlueGen that strstr takes two strings as its arguments:

    ArgumentIsString strstr 0 1

Finally, the following directive tells GlueGen that strstr returns a string instead of an array of bytes:

    ReturnsString strstr

We also use the CustomCCode directive to cause the string.h header file to be #included in the generated glue code:

    CustomCCode /* Include string.h header */
    CustomCCode #include <string.h>

Now the bindings of these two functions to Java look as expected:

    public static native int strlen(java.lang.String str);
    public static native java.lang.String strstr(java.lang.String str1, java.lang.String str2);
Note that the ReturnsString directive does not currently correctly handle the case where the char* returned from C needs to be explicitly freed. As an example, a binding of the C function strdup using a ReturnsString directive would cause a C heap memory leak.

Memory allocation


This example shows how memory allocation is handled when binding C to Java. It gives the example of a custom memory allocator being bound to Java; this is a construct that at least at one point was present in OpenGL in the NV_vertex_array_range extension.

The two functions we are exposing to Java are as follows:

    void* custom_allocate(int num_bytes);
    void  custom_free(void* data);

The Java-side return type of custom_allocate will necessarily be a ByteBuffer, as that is the only useful way of interacting with arbitrary memory produced by C. The question is how to inform the glue code generator of the size of the returned sequence of memory. The semantics of custom_allocate are obvious to the programmer; the incoming num_bytes argument specifies the amount of returned memory. We tell GlueGen this fact using the ReturnValueCapacity directive:

    # The length of the returned ByteBuffer from custom_allocate is
    # specified as the argument
    ReturnValueCapacity custom_allocate {0}

Note that we name incoming argument 0 with the MessageFormat specifier "{0}" rather than the explicit name of the parameter ("num_bytes") for generality, in case the header file is changed later.

Because custom_free will only ever receive Buffers produced by custom_allocate, we use the NioDirectOnly directive to prevent accidental usage with the wrong kind of Buffer:

    # custom_free will only ever receive a direct Buffer
    NioDirectOnly custom_free

The generated Java APIs for these functions are as follows:

    public static java.nio.ByteBuffer custom_allocate(int num_bytes) {...}
    public static void custom_free(java.nio.Buffer data) {...}

Ingoing and outgoing structs


This example shows how GlueGen provides access to C structs and supports both passing them to and returning them from C functions. The header file defines a sample data structure that might describe the bit depth of a given screen:

    typedef struct {
        int redBits;
        int greenBits;
        int blueBits;
    } ScreenInfo;

Two functions are defined which take and return this data type:

    ScreenInfo* default_screen_depth();
    void set_screen_depth(ScreenInfo* info);

The semantics of default_screen_depth() are that it returns a pointer to some static storage which does not need to be freed, which describes the default screen depth. set_screen_depth() is a hypothetical function which would take a newly-allocated ScreenInfo and cause the primary display to switch to the specified bit depth.

The only additional information we need to tell GlueGen, beyond that in the header file, is how much storage is returned from default_screen_depth(). Note the semantic ambiguity, where it might return a pointer to a single ScreenInfo or a pointer to an array of ScreenInfos. We tell GlueGen that the return value is a single value with the ReturnValueCapacity directive, similarly to the memory allocation example above:

    # Tell GlueGen that default_screen_depth() returns a pointer to a
    # single ScreenInfo
    ReturnValueCapacity default_screen_depth sizeof(ScreenInfo)

Note that if default_screen_depth had returned newly-allocated storage, it would be up to the user to expose a free() function to Java and call it when necessary.

GlueGen automatically generates a Java-side ScreenInfo class which supports not only access to any such objects returned from C, but also allocation of new ScreenInfo structs which can be passed (persistently) down to C. The Java API for the ScreenInfo class looks like this:

    public abstract class ScreenInfo {
        public static ScreenInfo create();
        public abstract ScreenInfo redBits(int val);
        public abstract int redBits();

The create() method allocates a new ScreenInfo struct which may be passed, even persistently, out to C. Its C-heap storage will be automatically reclaimed when the Java-side ScreenInfo object is no longer reachable, as it is backed by a direct New I/O ByteBuffer. The fields of the struct are exposed as methods which supply both getters and setters.

Returned arrays of structs


This example, taken from JOGL's X11 binding, illustrates how to return an array of structs from C to Java. The XGetVisualInfo function from the X library has the following signature:

    XVisualInfo *XGetVisualInfo(
        Display*     display,
        long         vinfo_mask,
        XVisualInfo* vinfo_template,
        int*         nitems_return

Note that the XVisualInfo data structure itself contains many elements, including a pointer to the current visual. We use the following trick in the header file to cause GlueGen to treat the Display* in the above signature as well as the Visual* in the XVisualInfo as opaque pointers:

    typedef struct {}     Display;
    typedef struct {}     Visual;
    typedef unsigned long VisualID;

    typedef struct {
        Visual *visual;
        VisualID visualid;
        int screen;
        int depth;
        int c_class; /* C++ */
        unsigned long red_mask;
        unsigned long green_mask;
        unsigned long blue_mask;
        int colormap_size;
        int bits_per_rgb;
    } XVisualInfo;

XGetVisualInfo returns all of the available pixel formats in the form of XVisualInfos which match a given template. display is the current connection to the X server. vinfo_mask indicates which fields from the template to match against. vinfo_template is a partially filled-in XVisualInfo specifying the characteristics to match. nitems_return is a pointer to an integer indicating how many XVisualInfos were returned. The return value, rather than being a pointer to a single XVisualInfo, is a pointer to the start of an array of XVisualInfo data structures.

There are two basic steps to being able to return this array properly to Java using GlueGen. The first is creating a direct ByteBuffer of the appropriate size in the autogenerated JNI code. The second is slicing up this ByteBuffer appropriately in order to return an XVisualInfo[] at the Java level.

In the autogenerated JNI code, after the call to XGetVisualInfo is made, the outgoing nitems_return value points to the number of elements in the returned array, which indicates the size of the direct ByteBuffer which would need to wrap these elements. However, if we look at the implementation of one of the generated glue code variants for this method (specifically, the one taking an int[] as the third argument), we can see a problem in trying to access this value in the C code:

        JNIEnv *env, jclass _unused, jobject arg0, jlong arg1, jobject arg2, jobject arg3, jint arg3_byte_offset) {
        int * _ptr3 = NULL;
        if (arg3 != NULL) {
            _ptr3 = (int *) (((char*) (*env)->GetPrimitiveArrayCritical(env, arg3, NULL)) + arg3_byte_offset);
        _res = XGetVisualInfo((Display *) _ptr0, (long) arg1, (XVisualInfo *) _ptr2, (int *) _ptr3);
        if (arg3 != NULL) {
            (*env)->ReleasePrimitiveArrayCritical(env, arg3, _ptr3, 0);
        if (_res == NULL) return NULL;
        return (*env)->NewDirectByteBuffer(env, _res,  ??? What to put here ???);

Note that at the point of the statement "What to put here?" the pointer to the storage of the int[], _ptr3, has already been released via ReleasePrimitiveArrayCritical. This means that it may not be referenced at the point needed in the code.

To solve this problem we use the TemporaryCVariableDeclaration and TemporaryCVariableAssignment directives. We want to declare a persistent integer variable down in the C code and assign the returned array length to that variable before the primitive array is released. While in order to do this we unfortunately need to know something about the structure of the autogenerated JNI code, at least we don't have to hand-edit it afterward. We add the following directives to the configuration file:

    # Get returned array's capacity from XGetVisualInfo to be correct
    TemporaryCVariableDeclaration XGetVisualInfo   int count;
    TemporaryCVariableAssignment  XGetVisualInfo   count = _ptr3[0];

Now in the autogenerated JNI code the variable "count" will contain the number of elements in the returned array. We can then reference this variable in a ReturnValueCapacity directive:

    ReturnValueCapacity XGetVisualInfo   count * sizeof(XVisualInfo)

At this point the XGetVisualInfo binding will return a Java-side XVisualInfo object whose backing ByteBuffer is the correct size. We now have to inform GlueGen that the underlying ByteBuffer represents not a single XGetVisualInfo struct, but an array of them, using the ReturnedArrayLength directive. This conversion is performed on the Java side of the autogenerated code. Here, the first element of either the passed IntBuffer or int[] contains the number of elements in the returned array. (Alternatively, we could examine the length of the ByteBuffer returned from C to Java and divide by XVisualInfo.size().) Because there are two overloadings produced by GlueGen for this method, if we reference the nitems_return argument in a ReturnedArrayLength directive, we need to handle not only the differing data types properly (IntBuffer vs. int[]), but also the fact that both the integer array and its offset value are substituted for any reference to the fourth argument.

To solve this problem, we define a pair of private helper functions whose purpose is to handle this overloading.

    CustomJavaCode TestFunction  private static int getFirstElement(IntBuffer buf) {
    CustomJavaCode TestFunction    return buf.get(buf.position());
    CustomJavaCode TestFunction  }
    CustomJavaCode TestFunction  private static int getFirstElement(int[] arr,
    CustomJavaCode TestFunction                                     int offset) {
    CustomJavaCode TestFunction    return arr[offset];
    CustomJavaCode TestFunction  }

Now we can simply write for the returned array length:

    ReturnedArrayLength XGetVisualInfo  getFirstElement({3})

That's all that is necessary. GlueGen will then produce the following Java-side overloadings for this function:

    public static XVisualInfo[] XGetVisualInfo(Display arg0,
                                               long arg1,
                                               XVisualInfo arg2,
                                               java.nio.IntBuffer arg3);
    public static XVisualInfo[] XGetVisualInfo(Display arg0,
                                              long arg1,
                                              XVisualInfo arg2,
                                              int[] arg3, int arg3_offset);

As it happens, we don't really need the Display and Visual data structures to be produced; they can be treated as longs on the Java side. Therefore we can add the following directives to the configuration file:

    # We don't need the Display and Visual data structures to be
    # explicitly exposed
    Opaque long Display *
    Opaque long Visual *
    # Ignore the empty Display and Visual data structures (though made
    # opaque, the references from XVisualInfo and elsewhere are still
    # traversed)
    Ignore Display
    Ignore Visual

The final generated Java API is the following:

    public static XVisualInfo[] XGetVisualInfo(long arg0,
                                               long arg1,
                                               XVisualInfo arg2,
                                               java.nio.IntBuffer arg3);
    public static XVisualInfo[] XGetVisualInfo(long arg0,
                                               long arg1,
                                               XVisualInfo arg2,
                                               int[] arg3, int arg3_offset);

Returned arrays of pointers


As with the example above, this example is taken from JOGL's X11 binding. Here we show how to expose to Java a C routine returning an array of pointers to a data structure.

The declaration of the function we are binding is as follows:

    typedef struct __GLXFBConfigRec *GLXFBConfig;

    GLXFBConfig *glXChooseFBConfig( Display *dpy, int screen,
                                    const int *attribList, int *nitems );

This function is used during allocation of a hardware-accelerated off-screen surface ("pbuffer") on X11 platforms; its exact meaning is not important. The semantics of the arguments and return value are as follows. As in the previous example, it accepts a connection to the current X display as one argument. The screen of this display is the second argument. The attribList is a zero-terminated list of integer attributes; because it is zero-terminated, the length of this list is not passed to the function. As in the previous example, the nitems argument points to an integer into which the number of returned GLXFBConfig objects is placed. The return value is an array of GLXFBConfig objects.

Because the GLXFBConfig data type is typedefed as a pointer to an opaque (undefined) struct, the construct GLXFBConfig* is implicitly a "pointer-to-pointer" type. GlueGen automatically assumes this is convertible to a Java-side array of accessors to structs. The only configuration necessary is to tell GlueGen the length of this array.

As in the previous example, we use the TemporaryCVariableDeclaration and TemporaryCVariableAssignment directives to capture the length of the returned array:

TemporaryCVariableDeclaration glXChooseFBConfig int count; TemporaryCVariableAssignment glXChooseFBConfig count = _ptr3[0];

The structure of the generated glue code for the return value is subtly different than in the previous example. The question in this case is not whether the return value is a pointer to a single object vs. a pointer to an array of objects; it is what the length of the returned array is, since we already know that the return type is pointer-to-pointer and is therefore an array. We use the ReturnValueLength directive for this case:

    ReturnValueLength glXChooseFBConfig   count
We add similar Opaque directives to the previous example to yield the resulting Java bindings for this function:
    public static GLXFBConfig[] glXChooseFBConfig(long dpy,
                                                  int screen,
                                                  java.nio.IntBuffer attribList,
                                                  java.nio.IntBuffer nitems);
    public static GLXFBConfig[] glXChooseFBConfig(long dpy,
                                                  int screen,
                                                  int[] attribList, int attribList_offset,
                                                  int[] nitems, int nitems_offset);
Note that because the GLXFBConfig data type is returned as an element of an array, we can not use the Opaque directive to erase this data type to long as we did with the Display data type.