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diff --git a/doc/manual/index.md b/doc/manual/index.md new file mode 100644 index 0000000..760d31b --- /dev/null +++ b/doc/manual/index.md @@ -0,0 +1,1855 @@ +<!--- +We convert markdown using pandoc using `gfm` (GitHub-Flavored Markdown) as source format +and `html5+smart` with a custom template as the target. + +Recipe: +``` + ~/pandoc-buttondown-cgit/pandoc_md2html_local.sh index.md > index.html +``` + +Git repos: +- https://jausoft.com/cgit/users/sgothel/pandoc-buttondown-cgit.git/about/ +- https://github.com/sgothel/pandoc-buttondown-cgit +--> + +<style> +table, th, td { + border: 1px solid black; +} +</style> + +# GlueGen Manual + +*Disclaimer: This documented shall be synchronized with source code, +especially the configuration options.* + +Please also consider reading [GlueGen Native Data & Function +Mapping](../GlueGen_Mapping.html) for details on native data and +function mappings. + +## References + +- [GlueGen Git Repo](https://jogamp.org/cgit/gluegen.git/about/) +- [GlueGen Java™ API-Doc](https://jogamp.org/deployment/jogamp-next/javadoc/gluegen/javadoc/) +* [GlueGen Native Data & Function Mapping for Java™](../GlueGen_Mapping.html) +- [GlueGen Project Page](https://jogamp.org/gluegen/www/) +- [How To Build](../HowToBuild.html) + +## <span id="Chapter1">Chapter 1 - Introduction</span> + +### <span id="SecIntroduction">Introduction</span> + +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. + +### <span id="SecStructure">Structure of the Generated Glue Code</span> + +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](#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](#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](#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](#ArgumentIsString) or + [ReturnsString](#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. + +### <span id="SecUnique">Unique Features</span> + +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. + +### <span id="SecBackground">Background and Design Principles</span> + +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](http://alumni.media.mit.edu/~kbrussel/Header2Scheme/), +an early tool allowing binding of a limited subset of C++ to the Scheme +programming language; [SWIG](http://www.swig.org/), 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](http://www.jniwrapper.com/), a commercial tool automating +the binding of C APIs to Java; and +[NoodleGlue](http://web.archive.org/web/20070419183658/http://www.noodleglue.org/noodleglue/noodleglue.html), +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. + +## <span id="Chapter2">Chapter 2 - Using GlueGen</span> + +### <span id="SecAcquiring">Acquiring and Building GlueGen</span> + +The source code for GlueGen may be obtained by cloning the Git +repository: + + $git clone --recursive git://jogamp.org/srv/scm/gluegen.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. + +#### <span id="SecCommon">Common Build Problems</span> + +**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. + +### <span id="SecBasic">Basic Operation</span> + +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: + +- -I*dir* (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. +- -E*emitterClassName* (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. +- -C*cfgFile* 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. + +### <span id="SecAnt">Running GlueGen as an Ant Task</span> + +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}" /> + </path> + + +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" + classname="com.jogamp.gluegen.ant.GlueGenTask" + 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]" + emitter="com.jogamp.gluegen.JavaEmitter"> + <classpath refid="gluegen.classpath" /> + </gluegen> + + +Please see the [JOGL](http://jogamp.org/jogl/) and +[JOAL](http://jogamp.org/joal/) build.xml files for concrete, though +non-trivial, examples of how to invoke GlueGen via Ant. + +### <span id="SecJCPP">JCPP</span> + +GlueGen contains and uses the [C preprocessor +JCPP](https://jogamp.org/cgit/jcpp.git/about/), see [original +homepage](https://www.anarres.org/projects/jcpp/). + +Constant values intended for use by end users are defined in many C +libraries' headers using `#define`s rather than constant int +declarations. If the header would be processed by a full C preprocessor, +the `#define` statement's macro name become unavailable for processing +by the glue code generator. Using JCPP allows us to utilize the +`#define` macro names and values. + +JCPP is largely an invisible part of the glue code generation process. +If GlueGen's output is not as expected and there is heavy use of the C +preprocessor in the header, run JCPP against the header directly (JCPP +takes simply the -I and filename arguments accepted by GlueGen) and +examine the output. + +### <span id="SecStub">Stub Headers</span> + +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; + #else + typedef int ptrdiff_t; + #endif + + +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](#Sec32)). 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 contained [C +preprocessor](#SecJCPP). 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](#Sec32) as well +as a discussion of how certain [opaque data types](#SecOpaque) 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. + +### <span id="Sec32">32- and 64-bit Considerations</span> + +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 +`long`s will be mapped to Java `long`s, since an LP64 data model is +assumed. + +If [Opaque directives](#SecOpaque) 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. + +### <span id="SecOpaque">Opaque Directives</span> + +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](#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. + +### <span id="SecSubstitution">Argument Name Substitution</span> + +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](#ReturnValueCapacity) +and [ReturnedArrayLength](#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 `RangeCheck`s 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. + +### <span id="SecConfiguration">Configuration File Directives</span> + +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](#Chapter3) may be more helpful in seeing exactly how to +structure a configuration file for proper glue code generation. + +#### <span id="SecJavaEmitter">JavaEmitter Configuration</span> + +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](#Package), +[ImplPackage](#ImplPackage), [JavaClass](#JavaClass), +[ImplJavaClass](#ImplJavaClass), and [Style](#Style). Other directives +such as [NioDirectOnly](#NioDirectOnly) are required in some +circumstances for the glue code to be correct, and some such as +[ReturnedArrayLength](#ReturnedArrayLength), +[ReturnValueCapacity](#ReturnValueCapacity), and +[ReturnValueLength](#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. + +**<span id="AccessControl">AccessControl</span>** +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](#CustomJavaCode) to write the exposed API. In +this case is most useful in conjunction with +[RenameJavaMethod](#RenameJavaMethod). + +**<span id="ArgumentIsString">ArgumentIsString</span>** +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`. + +**<span id="ArgumentIsPascalString">ArgumentIsPascalString</span>** +Syntax: +`ArgumentIsPascalString [function name] [indice-tuples...]`, +with each tuple being the argument-index for the '`int length`' and the +'`char* value`' argument with index 0 for the the first argument +(optional) For a C function with one or more outgoing '`int length`' and +'`char* value`' (or compatible data type) arguments, indicates that +those arguments are semantically non-null-terminated Pascal strings +rather than null-terminated C strings or 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`. + +**<span id="ClassJavadoc">ClassJavadoc</span>** +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. + +**<span id="CustomCCode">CustomCCode</span>** +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](#CustomJavaCode) directive is omitted. + +**<span id="CustomJavaCode">CustomJavaCode</span>** +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](#Style)), or any of +the Java classes corresponding to referenced C structs in the parsed +headers. This usage is somewhat verbose, and the [IncludeAs](#IncludeAs) +directive provides a more concise way of including large bodies of Java +code into the generated code. + +**<span id="CustomJNICode">CustomJNICode</span>** +Syntax: `CustomJNICode [class name] [code...]` +(optional) Causes the specified line of C code to be emitted into the +generated JNI code related of specified Java class. Can be used to emit +JNI code related of any generated class: the public interface, the +implementing class, the sole concrete class (in the case of the +AllStatic [Style](#Style)), or any of the Java classes corresponding to +referenced C structs in the parsed headers. This usage is somewhat +verbose, and the [IncludeAs](#IncludeAs) directive provides a more +concise way of including large bodies of C code into the generated code. + +**<span id="EmitStruct">EmitStruct</span>** +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. + +**<span id="GlueGenRuntimePackage">GlueGenRuntimePackage</span>** +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. + +**<span id="ExtendedInterfaceSymbolsIgnore">ExtendedInterfaceSymbolsIgnore</span>** +Syntax: `ExtendedInterfaceSymbolsIgnore [Java file]` +(optional) Causes all autogenerated Java interface ignore all symbols +from interface declared inside named Java source file. +This directive can be used with [Extends](#Extends) directive. +Cf here for more information : +[GlueGen_Mapping](../GlueGen_Mapping.html#oo-style-example) + +**<span id="ExtendedInterfaceSymbolsOnly">ExtendedInterfaceSymbolsOnly</span>** +Syntax: `ExtendedInterfaceSymbolsOnly [Java file]` +(optional) Causes all autogenerated Java interface generate only symbols +from interface declared inside named Java source file. +This directive can be used with [Extends](#Extends) directive. +Cf here for more information : +[GlueGen_Mapping](../GlueGen_Mapping.html#oo-style-example) + +**<span id="ExtendedImplementationSymbolsIgnore">ExtendedImplementationSymbolsIgnore</span>** +Syntax: `ExtendedImplementationSymbolsIgnore [Java file]` +(optional) Causes all autogenerated Java classes ignore all symbols from +interface or classe declared inside named Java source file. +This directive can be used with [ParentClass](#ParentClass) directive. +Cf here for more information : +[GlueGen_Mapping](../GlueGen_Mapping.html#oo-style-example) + +**<span id="ExtendedImplementationSymbolsOnly">ExtendedImplementationSymbolsOnly</span>** +Syntax: `ExtendedImplementationSymbolsOnly [Java file]` +(optional) Causes all autogenerated Java classes generate only symbols +from interface or classe declared inside named Java source file. +This directive can be used with [ParentClass](#ParentClass) directive. +Cf here for more information : +[GlueGen_Mapping](../GlueGen_Mapping.html#oo-style-example) + +**<span id="ExtendedIntfAndImplSymbolsIgnore">ExtendedIntfAndImplSymbolsIgnore</span>** +Syntax: `ExtendedIntfAndImplSymbolsIgnore [Java file]` +(optional) Causes all autogenerated Java interface and classes ignore +all symbols from interface or classe declared inside named Java source +file. +This directive can be used with [Extends](#Extends) or +[ParentClass](#ParentClass) directives. +Cf here for more information : +[GlueGen_Mapping](../GlueGen_Mapping.html#oo-style-example) + +**<span id="ExtendedIntfAndImplSymbolsOnly">ExtendedIntfAndImplSymbolsOnly</span>** +Syntax: `ExtendedIntfAndImplSymbolsOnly [Java file]` +(optional) Causes all autogenerated Java interface and classes generate +only symbols from interface or classe declared inside named Java source +file. +This directive can be used with [Extends](#Extends) or +[ParentClass](#ParentClass) directives. +Cf here for more information : +[GlueGen_Mapping](../GlueGen_Mapping.html#oo-style-example) + +**<span id="Extends">Extends</span>** +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 [Implements](#Implements) directive or [ParentClass](#ParentClass) +directive. + +**<span id="HierarchicalNativeOutput">HierarchicalNativeOutput</span>** +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. + +**<span id="Ignore">Ignore</span>** +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](#IgnoreNot), [Unignore](#Unignore) +and [EmitStruct](#EmitStruct). + +**<span id="IgnoreField">IgnoreField</span>** +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. + +**<span id="IgnoreNot">IgnoreNot</span>** +Syntax: see [Ignore](#Ignore). (optional) Similar to the +[Ignore](#Ignore) directive, but evaluates the negation of the passed +regexp when deciding whether to ignore the given function or data type. +The [Unignore](#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. + +**<span id="Implements">Implements</span>** +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](#Extends) directive. + +**<span id="ImplJavaClass">ImplJavaClass</span>** +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](#JavaClass). + +**<span id="ImplPackage">ImplPackage</span>** +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](#Package). + +**<span id="Import">Import</span>** +Syntax: `Import [package name]` (no trailing semicolon) +(optional) Adds an import statement at the top of each generated Java +source file. + +**<span id="Include">Include</span>** +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. + +**<span id="IncludeAs">IncludeAs</span>** +Syntax: `IncludeAs [prefix tokens] [filename]` +(optional) Similar to the [Include](#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](#CustomJavaCode) to be stored as Java source +rather than in the configuration file; in this example the configuration +file might contain +`IncludeAs CustomJavaCode MyClass MyClass-CustomJavaCode.java`. + +**<span id="JavaClass">JavaClass</span>** +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](#ImplJavaClass). + +**<span id="JavaEpilogue">JavaEpilogue</span>** +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](#ReturnedArrayLength) and +other directives, [argument name substitution](#SecSubstitution) is +performed on MessageFormat expressions in the specified code. See also +[JavaPrologue](#JavaPrologue). + +**<span id="JavaOutputDir">JavaOutputDir</span>** +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. + +**<span id="JavaPrologue">JavaPrologue</span>** +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](#ReturnedArrayLength) and other directives, +[argument name substitution](#SecSubstitution) is performed on +MessageFormat expressions in the specified code. See also +[JavaEpilogue](#JavaEpilogue). + +**<span id="ManuallyImplement">ManuallyImplement</span>** +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](#CustomJavaCode) directive. If the emission +style is InterfaceAndImpl or InterfaceOnly, a public method will still +be generated for the specified function. + +**<span id="MaxOneElement">MaxOneElement</span>** +Syntax: `MaxOneElement [function name]` +(optional) Indicates that the specified C function/attribute which +returns a single element instead a ByteBuffer if signature or compatible +type actually returns a pointer like int\* but isn't an array. +Cf here for more information : +[GlueGen_Mapping](../GlueGen_Mapping.html#gluegen-struct-settings) + +**<span id="NativeOutputDir">NativeOutputDir</span>** +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](#HierarchicalNativeOutput). + +**<span id="NioDirectOnly">NioDirectOnly</span>** +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. + +**<span id="Opaque">Opaque</span>** +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. `long`s. 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](#Chapter3). 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. + +**<span id="Package">Package</span>** +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](#ImplPackage). + +**<span id="ParentClass">ParentClass</span>** +Syntax: `ParentClass [Java class name] [class name to extend] ` +(optional) Causes the specified autogenerated Java classe to declare +that it extends another one. This directive may only be applied to +autogenerated classes, not interface. For interfaces, use the +[Extends](#Extends) directive. + +**<span id="RangeCheck">RangeCheck</span>** +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](#RangeCheckBytes) directive. As +in the [ReturnedArrayLength](#ReturnedArrayLength) and other directives, +[argument name substitution](#SecSubstitution) is performed on +MessageFormat expressions. + +**<span id="RangeCheckBytes">RangeCheckBytes</span>** +Syntax: +`RangeCheckBytes [C function name] [argument number] [expression]` +(optional) Same as the [RangeCheck](#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. + +**<span id="RenameJavaMethod">RenameJavaMethod</span>** +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](#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. + +**<span id="RenameJavaType">RenameJavaType</span>** +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](#JavaClass) and +[ImplJavaClass](#ImplJavaClass) directives. + +**<span id="ReturnedArrayLength">ReturnedArrayLength</span>** +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](#SecSubstitution). +(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](#ReturnValueCapacity) and +ReturnValueLength. It is also sometimes most useful in conjunction with +the [TemporaryCVariableDeclaration](#TemporaryCVariableDeclaration) and +TemporaryCVariableAssignment directives. + +**<span id="ReturnsString">ReturnsString</span>** +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. + +**<span id="ReturnsStringOnly">ReturnsStringOnly</span>** +Syntax: `ReturnsStringOnly [function name]` +(optional) Like the [ReturnsString](#ReturnsString) instruction, but +without the classic getters and setters with ByteBuffer. +Cf here for more information : +[GlueGen_Mapping](../GlueGen_Mapping.html#gluegen-struct-settings) + +**<span id="ReturnValueCapacity">ReturnValueCapacity</span>** +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](#ReturnedArrayLength) directive, [argument name +substitution](#SecSubstitution) is performed on MessageFormat +expressions. + +**<span id="ReturnValueLength">ReturnValueLength</span>** +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](#ReturnedArrayLength) directive in the pointer +indirection to the array elements. The +[ReturnedArrayLength](#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](#Chapter3) for a concrete example of usage. As in the +[ReturnedArrayLength](#ReturnedArrayLength) directive, [argument name +substitution](#SecSubstitution) is performed on MessageFormat +expressions. + +**<span id="RuntimeExceptionType">RuntimeExceptionType</span>** +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](#NioDirectOnly) was specified. Defaults to +`RuntimeException`. + +**<span id="StructPackage">StructPackage</span>** +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](#PackageName)). + +**<span id="Style">Style</span>** +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](#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](#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. + +**<span id="TemporaryCVariableAssignment">TemporaryCVariableAssignment</span>** +Syntax: `TemporaryCVariableAssignment [C function name][code...]` +(optional) Inserts a C variable assignment declared using the +[TemporaryCVariableDeclaration](#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](#ReturnValueCapacity) or +[ReturnValueLength](#ReturnValueLength) directives to capture the size +of a returned C buffer or array of pointers. See the +[examples](#Chapter3) for a concrete example of usage of this directive. +Note that unlike, for example, the +[ReturnedArrayLength](#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. + +**<span id="TemporaryCVariableDeclaration">TemporaryCVariableDeclaration</span>** +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](#TemporaryCVariableAssignment) and +[ReturnValueCapacity](#ReturnValueCapacity) or +[ReturnValueLength](#ReturnValueLength) directives to capture the size +of a returned C buffer or array of pointers. See the +[examples](#Chapter3) for a concrete example of usage of this directive. + +**<span id="Unignore">Unignore</span>** +Syntax: `Unignore [regexp]` +(optional) Removes a previously-defined [Ignore](#Ignore) directive. +This is useful when one configuration file includes another and wishes +to disable some of the Ignores previously specified. + +**<span id="Unimplemented">Unimplemented</span>** +Syntax: `Unimplemented [regexp]` +(optional) Causes the binding for the functions matching the passed +regexp to have bodies generated which throw the stated +[RuntimeExceptionType](#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. + +#### <span id="SecProcAddressEmitter">ProcAddressEmitter Configuration</span> + +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 [JCPP](#SecJCPP) 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](#GetProcAddressTableExpr) and +[ProcAddressNameExpr](#ProcAddressNameExpr). + +**<span id="EmitProcAddressTable">EmitProcAddressTable</span>** +Syntax: `EmitProcAddressTable [true | false]` +(optional) Indicates whether to emit the ProcAddressTable during glue +code generation. Defaults to false. + +**<span id="ForceProcAddressGen">ForceProcAddressGen</span>** +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) + +**<span id="GetProcAddressTableExpr">GetProcAddressTableExpr</span>** +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()`. + +**<span id="ProcAddressNameExpr">ProcAddressNameExpr</span>** +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})`. + +**<span id="ProcAddressTableClassName">ProcAddressTableClassName</span>** +Syntax: `ProcAddressTableClassName [class name]` +(optional) Specifies the class name into which the table containing the +function pointers will be emitted. Defaults to "ProcAddressTable". + +**<span id="ProcAddressTablePackage">ProcAddressTablePackage</span>** +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](#ImplPackage) +directive. + +**<span id="SkipProcAddressGen">SkipProcAddressGen</span>** +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. + +## <span id="Chapter3">Chapter 3 - Configuration File Examples</span> + +### <span id="SecSimplest">Simplest possible example</span> + +Files: + +- [function.c](example1/function.c) +- [function.h](example1/function.h) +- [function.cfg](example1/function.cfg) +- [gen.sh](example1/gen.sh) + +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()`. + +### <span id="SecArrays">Arrays and buffers</span> + +Files: + +- [function.c](example2/function.c) +- [function.h](example2/function.h) +- [function.cfg](example2/function.cfg) +- [gen.sh](example2/gen.sh) + +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](#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](#RuntimeExceptionType) directive. + +### <span id="SecString">String handling</span> + +Files: + +- [function.h](example3/function.h) +- [function.cfg](example3/function.cfg) +- [gen.sh](example3/gen.sh) + +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](example3/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](#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](#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. + +### <span id="SecMemory">Memory allocation</span> + +Files: + +- [function.c](example4/function.c) +- [function.h](example4/function.h) +- [function.cfg](example4/function.cfg) +- [gen.sh](example4/gen.sh) + +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](#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](#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) {...} + + +### <span id="SecStructs">Ingoing and outgoing structs</span> + +Files: + +- [function.c](example5/function.c) +- [function.h](example5/function.h) +- [function.cfg](example5/function.cfg) +- [gen.sh](example5/gen.sh) + +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 +`ScreenInfo`s. We tell GlueGen that the return value is a single value +with the [ReturnValueCapacity](#ReturnValueCapacity) directive, +similarly to the [memory allocation](#SecMemory) 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. + +### <span id="SecStructArrays">Returned arrays of structs</span> + +Files: + +- [function.h](example6/function.h) +- [function.cfg](example6/function.cfg) +- [gen.sh](example6/gen.sh) + +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 `XVisualInfo`s 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 `XVisualInfo`s 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: + + JNIEXPORT jobject JNICALL + Java_testfunction_TestFunction_XGetVisualInfo1__Ljava_nio_ByteBuffer_2JLjava_nio_ByteBuffer_2Ljava_lang_Object_2I( + 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](#TemporaryCVariableDeclaration) and +[TemporaryCVariableAssignment](#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](#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](#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](#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 `long`s 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); + + +### <span id="SecPointerArrays">Returned arrays of pointers</span> + +Files: + +- [function.h](example7/function.h) +- [function.cfg](example7/function.cfg) +- [gen.sh](example7/gen.sh) + +As with the [example above](#SecStructArrays), 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](#SecStructArrays), 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](#TemporaryCVariableDeclaration) and +[TemporaryCVariableAssignment](#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](#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. + |