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.
#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:
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:
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:
<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 and JOAL build.xml files for concrete,
though non-trivial, examples of how to invoke GlueGen via Ant.
Constant values intended for use by end users are defined in many C
libraries' headers using #defines 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.
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;
#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). 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. To avoid these
problems, a "stub" windows.h header is placed in GlueGen's
include path containing only the necessary typedefs:
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.
AccessControl
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.
ArgumentIsString
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.
ArgumentIsPascalString
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.
ClassJavadoc
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.
CustomCCode
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.
CustomJavaCode
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.
CustomJNICode
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), 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 C
code into the generated code.
EmitStruct
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.
GlueGenRuntimePackage
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.
ExtendedInterfaceSymbolsIgnore
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
directive.
Cf here for more information : GlueGen_Mapping
ExtendedInterfaceSymbolsOnly
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
directive.
Cf here for more information : GlueGen_Mapping
ExtendedImplementationSymbolsIgnore
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
directive.
Cf here for more information : GlueGen_Mapping
ExtendedImplementationSymbolsOnly
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
directive.
Cf here for more information : GlueGen_Mapping
ExtendedIntfAndImplSymbolsIgnore
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 or ParentClass directives.
Cf here for more information : GlueGen_Mapping
ExtendedIntfAndImplSymbolsOnly
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 or ParentClass directives.
Cf here for more information : GlueGen_Mapping
Extends
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 directive or ParentClass directive.
HierarchicalNativeOutput
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.
Ignore
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.
IgnoreField
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.
IgnoreNot
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.
Implements
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.
ImplJavaClass
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.
ImplPackage
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.
Import
Syntax: Import [package name] (no trailing semicolon)
(optional) Adds an import statement at the top of each generated Java
source file.
Include
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.
IncludeAs
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 MyClass-CustomJavaCode.java.
JavaClass
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.
JavaEpilogue
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.
JavaOutputDir
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.
JavaPrologue
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.
ManuallyImplement
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.
MaxOneElement
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
NativeOutputDir
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.
NioDirectOnly
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.
Opaque
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.
Package
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.
ParentClass
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 directive.
RangeCheck
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.
RangeCheckBytes
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.
RenameJavaMethod
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.
RenameJavaType
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.
ReturnedArrayLength
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.
ReturnsString
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.
ReturnsStringOnly
Syntax: ReturnsStringOnly [function name]
(optional) Like the ReturnsString
instruction, but without the classic getters and setters with
ByteBuffer.
Cf here for more information : GlueGen_Mapping
ReturnValueCapacity
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.
ReturnValueLength
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.
RuntimeExceptionType
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.
StructPackage
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).
Style
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.
TemporaryCVariableAssignment
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.
TemporaryCVariableDeclaration
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.
Unignore
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.
Unimplemented
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:
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:
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 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.
EmitProcAddressTable
Syntax: EmitProcAddressTable [true | false]
(optional) Indicates whether to emit the ProcAddressTable during glue
code generation. Defaults to false.
ForceProcAddressGen
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)
GetProcAddressTableExpr
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().
ProcAddressNameExpr
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}).
ProcAddressTableClassName
Syntax: ProcAddressTableClassName [class name]
(optional) Specifies the class name into which the table containing the
function pointers will be emitted. Defaults to "ProcAddressTable".
ProcAddressTablePackage
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.
SkipProcAddressGen
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.
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:
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().
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.
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:
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.
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:
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) {...}
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:
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.
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:
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:
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:
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.
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);
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.
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.
