Re: Wilderness: I Have formatted README.EXT into an HTML Document

Charles E. Thornton wrote:

> I have taken README.EXT (English Version Only) and have reformatted
> it as a html document.

How did you do the conversion?  I’m attaching a slight rewrite of the
README.EXT to use docutils syntax so that we can easily produce many
formats (although I’ve since given up on docutils as I prefer using TeX
directly).  My version is attached.  Should be easier to keep one file
in sync than two, so perhaps README.EXT can be updated to use the
docutils format, or perhaps asciidoc which the git people are using for
their documentation?

        nikolai

-- 
Nikolai Weibull: now available free of charge at http://bitwi.se/!
Born in Chicago, IL USA; currently residing in Gothenburg, Sweden.
main(){printf(&linux["\021%six\012\0"],(linux)["have"]+"fun"-97);}

--envbJBWh7q8WU6mo
Content-Type: text/plain; charset=us-ascii
Content-Disposition: attachment; filename="on-writing-ruby-extensions.txt"

==========================
On Writing Ruby Extensions
==========================



This document explains how to make extension libraries for Ruby.



:Author: Nikolai Weibull
:Date: 2004-11-19
:Revision: 1.0



.. contents:: Table of Contents




Basic Knowledge
===============


In C, variables have types and data do not have types.  In contrast, Ruby
variables do not have a static type, and data themselves have types, so data
will need to be converted between the languages.

Data in Ruby are represented by C type ``VALUE``.  Each ``VALUE`` data has its
data-type.

To retrieve C data from a ``VALUE``, you need to:

  1.  Identify the VALUE's data type
  2.  Convert the VALUE into C data

Converting to the wrong data type may cause serious problems.


Data-types
----------

The Ruby interpreter has the following data types:

==============  =========================
C               Ruby
==============  =========================
``T_NIL``       nil
``T_OBJECT``    ordinary object
``T_CLASS``     class
``T_MODULE``    module
``T_FLOAT``     floating point number
``T_STRING``    string
``T_REGEXP``    regular expression
``T_ARRAY``     array
``T_FIXNUM``    Fixnum(31bit integer)
``T_HASH``      associative array
``T_STRUCT``    (Ruby) structure
``T_BIGNUM``    multi precision integer
``T_FILE``      IO
``T_TRUE``      true
``T_FALSE``     false
``T_DATA``      data
``T_SYMBOL``    symbol
==============  =========================

In addition, there are several other types used internally::

  T_ICLASS
  T_MATCH
  T_UNDEF
  T_VARMAP
  T_SCOPE
  T_NODE

Most of the types are represented by C structures.


Check Data Type of the ``VALUE``
--------------------------------

The macro ``TYPE()`` defined in ``ruby.h`` shows the data type of the
``VALUE``.  ``TYPE()`` returns the constant number ``T_``\ *XXX* described above.
To handle data types, your code will look something like this::

  switch (TYPE(obj)) {
    case T_FIXNUM:
      /* process Fixnum */
      break;
    case T_STRING:
      /* process String */
      break;
    case T_ARRAY:
      /* process Array */
      break;
    default:
      /* raise exception */
      rb_raise(rb_eTypeError, "not valid value");
      break;
  }

There is the data-type check function ::

  void Check_Type(VALUE value, int type)

which raises an exception if the ``VALUE`` does not have the type specified.

There are also faster check macros for ``Fixnum``\ s and ``nil``::

  FIXNUM_P(obj)
  NIL_P(obj)


Convert ``VALUE`` Into C Data
-----------------------------

The data for type ``T_NIL``, ``T_FALSE``, ``T_TRUE`` are ``nil``, ``true``,
``false`` respectively.  They are singletons for the data type.

The ``T_FIXNUM`` data is a 31bit length fixed integer (63bit length on some
machines), which can be convert to a C integer by using the ``FIX2INT()``
macro.  There is also ``NUM2INT()`` which converts any Ruby numbers into C
integers.  The ``NUM2INT()`` macro includes a type check, so an exception will
be raised if the conversion failed.  ``NUM2DBL()`` can be used to retrieve the
double float value in same way.

To get ``char*`` from a ``VALUE``, version 1.7 recommend to use new macros
``StringValue()`` and ``StringValuePtr()``.  ``StringValue(var)`` replaces
``var``'s value to the result of ``var.to_str()``.  ``StringValuePtr(var)``
does same replacement and returns ``char*`` representation of ``var``.  These
macros will skip the replacement if ``var`` is a ``String``.  Notice that the
macros requires to take only lvalue as their argument, to change the value of
var in the replacement. 

In version 1.6 or earlier, ``STR2CSTR()`` was used to do same thing but now it
is obsoleted in version 1.7 because of ``STR2CSTR()`` has a risk of dangling
pointer problem in ``to_str()`` impliclit conversion.

Other data types have corresponding C structures, e.g. ``struct RArray`` for
``T_ARRAY`` etc.  The ``VALUE`` of the type which has corresponding structure
can be cast to retrieve the pointer to the ``struct``.  The casting macro will
be of the form ``R``\ *XXX* for each data type; for instance, ``RARRAY(obj)``.
See ``<ruby.h>``.

For example, ``RSTRING(size)->len`` is the way to get the size of the Ruby
String object.  The allocated region can be accessed by ``RSTRING(str)->ptr``.
For arrays, use ``RARRAY(ary)->len`` and ``RARRAY(ary)->ptr`` respectively.

.. note:
  Do not change the value of the structure directly, unless you are responsible
  for the result.  This ends up being the cause of interesting bugs.


Convert C Data Into ``VALUE``
-----------------------------

To convert C data to Ruby values:

``FIXNUM``
  left shift 1 bit, and turn on LSB.

Other pointer values
  cast to ``VALUE``.

You can determine whether a ``VALUE`` is pointer or not by checking its LSB.  

.. note:
  Ruby does not allow arbitrary pointer values to be a ``VALUE``.  They should
  be pointers to the structures which Ruby knows about.  The known structures
  are defined in ``<ruby.h>``.

To convert C numbers to Ruby values, use these macros:

``INT2FIX()``
  for integers within 31bits.
``INT2NUM()``
  for arbitrary sized integer.

``INT2NUM()`` converts an integer into a ``Bignum`` if it is out of the
``FIXNUM`` range, but is a bit slower.


Manipulating Ruby Data
----------------------

As I already mentioned, it is not recommended to modify an object's internal
structure.  To manipulate objects, use the functions supplied by the Ruby
interpreter. Some (not all) of the useful functions are listed below.

String Functions
~~~~~~~~~~~~~~~~

``rb_str_new(const char *ptr, long len)``
  Creates a new Ruby string.

``rb_str_new2(const char *ptr)``
  Creates a new Ruby string from a C string.  This is equivalent to
  ``rb_str_new(ptr, strlen(ptr))``.

``rb_tainted_str_new(const char *ptr, long len)``
  Creates a new tainted Ruby string.  Strings from external data sources should
  be tainted.

``rb_tainted_str_new2(const char *ptr)``
  Creates a new tainted Ruby string from a C string.

``rb_str_cat(VALUE str, const char *ptr, long len)``
  Appends len bytes of data from ptr to the Ruby string.

Array Functions
~~~~~~~~~~~~~~~

``rb_ary_new()``
  Creates an array with no elements.

``rb_ary_new2(long len)``
  Creates an array with no elements, allocating internal buffer for ``len``
  elements.

``rb_ary_new3(long n, ...)``
  Creates an ``n``\ -element array from the arguments.

``rb_ary_new4(long n, VALUE *elts)``
  Creates an ``n``\ -element array from a C array.

``rb_ary_push(VALUE ary, VALUE val)``
  Push ``val``  onto the array.

``rb_ary_pop(VALUE ary)``
  Pop top value off of the array.

``rb_ary_shift(VALUE ary)``
  Shift the first value off of the array.

``rb_ary_unshift(VALUE ary, VALUE val)``
  Unshift ``val`` at the beginning of the array.



Extending Ruby with C
=====================


Adding new Features to Ruby
----------------------------

You can add new features (classes, methods, etc.) to the Ruby interpreter.
Ruby provides APIs for defining the following things:

  * Classes, Modules
  * Methods, Singleton Methods
  * Constants

Class/Module Definition
~~~~~~~~~~~~~~~~~~~~~~~

To define a class or module, use the functions below::

  VALUE rb_define_class(const char *name, VALUE super)
  VALUE rb_define_module(const char *name)

These functions return the newly created class or module.  You may want to save
this reference into a variable to use later.

To define nested classes or modules, use the functions below::

  VALUE rb_define_class_under(VALUE outer, const char *name, VALUE super)
  VALUE rb_define_module_under(VALUE outer, const char *name)

Method/Singleton Method Definition
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

To define methods or singleton methods, use these functions::

  void rb_define_method(VALUE klass, const char *name, 
                        VALUE (*func)(), int argc)

  void rb_define_singleton_method(VALUE object, const char *name, 
                                  VALUE (*func)(), int argc)

The ``argc`` represents the number of the arguments to the C function, which
must be less than 17.  But I believe you don't need that much.  ``:-)``

If ``argc`` is negative, it specifies the calling sequence, not number of the
arguments.  

If ``argc`` is -1, the function will be called as ::

  VALUE func(int argc, VALUE *argv, VALUE obj)

where ``argc`` is the actual number of arguments, ``argv`` is the C array of
the arguments, and ``obj`` is the receiver.

If ``argc`` is -2, the arguments are passed in a Ruby array. The function will
be called like ::

  VALUE func(VALUE obj, VALUE args)

where ``obj`` is the receiver, and ``args`` is the Ruby array containing actual
arguments.

There are two more functions to define methods.  One is to define private
methods::

  void rb_define_private_method(VALUE klass, const char *name, 
                                VALUE (*func)(), int argc)

The other is to define module functions, which are private AND singleton
methods of the module.  For example, ``sqrt`` is the module function defined in
``Math`` module.  It can be call in the form like ::

  Math.sqrt(4)

or ::

  include Math
  sqrt(4)

To define module functions, use ::

  void rb_define_module_function(VALUE module, const char *name, 
                                 VALUE (*func)(), int argc)

Oh, in addition, function-like methods, which are private methods defined in
the ``Kernel`` module, can be defined using ::

  void rb_define_global_function(const char *name, VALUE (*func)(), int argc)

Furthermore, to define an method alias, use ::

  void rb_define_alias(VALUE module, const char* new, const char* old);

Constant Definitions
~~~~~~~~~~~~~~~~~~~~

We have 2 functions to define constants::

  void rb_define_const(VALUE klass, const char *name, VALUE val)
  void rb_define_global_const(const char *name, VALUE val)

The former is to define a constant under specified class/module.  The latter is
to define a global constant.


Use Ruby Features from C
------------------------

There are several ways to invoke Ruby's features from C code.

Evaluate Ruby Programs in a String
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The easiest way to use Ruby's functionality from a C program is to
evaluate the string as Ruby program.  This function will do the job::

  VALUE rb_eval_string(const char *str)

Evaluation is done under the current context, thus current local variables
of the innermost method (which is defined by Ruby) can be accessed.

``ID`` or ``Symbol``
~~~~~~~~~~~~~~~~~~~~

You can invoke methods directly, without parsing the string.  First I need to
explain about symbols (whose data type is ``ID``).  ``ID`` is the integer
number to represent Ruby's identifiers such as variable names.  It can be
accessed from Ruby in the form ::

  :identifier

You can get the symbol value from a string within C code by using ::

  rb_intern(const char *name)

Invoke Ruby Method from C
~~~~~~~~~~~~~~~~~~~~~~~~~

To invoke methods directly, you can use the function below ::

  VALUE rb_funcall(VALUE recv, ID mid, int argc, ...)

This function invokes a method on the ``recv``, with the method name
specified by the symbol ``mid``.

Accessing the Variables and Constants
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

You can access class variables and instance variables using access functions.
Also, global variables can be shared between both environments.  There's no way
to access Ruby's local variables.

The functions to access/modify instance variables are below::

  VALUE rb_ivar_get(VALUE obj, ID id)
  VALUE rb_ivar_set(VALUE obj, ID id, VALUE val)

``id`` must be the symbol, which can be retrieved by ``rb_intern()``.

To access the constants of the class/module::

  VALUE rb_const_get(VALUE obj, ID id)

See `Constant Definitions`_ for defining new constant.



Information Sharing Between Ruby and C
======================================


Ruby Constants that can be Accessed from C
------------------------------------------

The following Ruby constants can be referred from C::

  Qtrue
  Qfalse

which are the boolean values.  ``Qfalse`` is ``false`` in C also (i.e., it's
0).

::

  Qnil

Ruby ``nil`` in C scope.


Global Variables Shared Between C and Ruby
------------------------------------------

Information can be shared between the two environments using shared global
variables.  To define them, you can use functions listed below::

  void rb_define_variable(const char *name, VALUE *var)

This function defines the variable which is shared by both environments.  The
value of the global variable pointed to by ``var`` can be accessed through
Ruby's global variable named ``name``.

You can define read-only (from Ruby, of course) variables using the function
below. ::

  void rb_define_readonly_variable(const char *name, VALUE *var)

You can defined hooked variables.  The accessor functions (getter and setter)
are called on access to the hooked variables. ::

  void rb_define_hooked_variable(constchar *name, VALUE *var,
                                 VALUE (*getter)(), void (*setter)())

If you need to supply either setter or getter, just supply ``NULL`` for the
hook you don't need.  If both hooks are ``NULL``,
``rb_define_hooked_variable()`` works just like ``rb_define_variable()``. ::

  void rb_define_virtual_variable(const char *name,
                                  VALUE (*getter)(), void (*setter)())

This function defines a Ruby global variable without a corresponding C
variable.  The value of the variable will be set/get only by hooks.

The prototypes of the getter and setter functions are as follows::

  (*getter)(ID id, void *data, struct global_entry* entry);
  (*setter)(VALUE val, ID id, void *data, struct global_entry* entry);


Encapsulate C Data into Ruby Object
-----------------------------------

To wrap and objectify a C pointer as a Ruby object (so called ``DATA``), use
``Data_Wrap_Struct()``. ::

  Data_Wrap_Struct(klass, mark, free, ptr)

``Data_Wrap_Struct()`` returns a created ``DATA`` object.  The ``klass`` argument
is the class for the ``DATA`` object.  The ``mark`` argument is the function to mark
Ruby objects pointed by this data.  The ``free`` argument is the function to free
the pointer allocation.  If this is -1, the pointer will be just freed.  The
functions ``mark`` and ``free`` will be called from garbage collector.

You can allocate and wrap the structure in one step. ::

  Data_Make_Struct(klass, type, mark, free, sval)

This macro returns an allocated ``Data`` object, wrapping the pointer to the
structure, which is also allocated.  This macro works like ::

  (sval = ALLOC(type), Data_Wrap_Struct(klass, mark, free, sval))

Arguments ``klass``, ``mark``, and ``free`` work like their counterparts in
``Data_Wrap_Struct()``.  A pointer to the allocated structure will be assigned
to ``sval``, which should be a pointer of the type specified.

To retrieve the C pointer from the ``Data`` object, use the macro
``Data_Get_Struct()``. ::

  Data_Get_Struct(obj, type, sval)

A pointer to the structure will be assigned to the variable ``sval``.

See the example below for details. 



Example - Creating a DBM Extension
==================================

OK, here's the example of making an extension library.  This is the extension
to access DBMs.  The full source is included in the ``ext/`` directory in the
Ruby's source tree.


Make the Directory
------------------

::

  % mkdir ext/dbm

Make a directory for the extension library under ext directory.


Create ``MANIFEST`` File
------------------------

::

  % cd ext/dbm
  % touch MANIFEST

There should be ``MANIFEST`` file in the directory for the extension
library.  Make an empty file for now.

Design the Library
------------------

You need to design the library features, before making it.

Write C Code
------------

You need to write C code for your extension library.  If your library has
only one source file, choosing *LIBRARY*\ ``.c`` as a file name is preferred.
On the other hand, in case your library has multiple source files, avoid
choosing *LIBRARY*\ ``.c`` for a file name.  It may conflict with an
intermediate file *LIBRARY*\ ``.o`` on some platforms.

Ruby will execute the initializing function named ``Init_``\ *LIBRARY* in the
library.  For example, ``Init_``\ *dbm*\ ``()`` will be executed when loading
the library.

Here's the example of an initializing function::

  Init_dbm()
  {
      /* define DBM class */
      cDBM = rb_define_class("DBM", rb_cObject);
      /* DBM includes Enumerate module */
      rb_include_module(cDBM, rb_mEnumerable);

      /* DBM has class method open(): arguments are received as C array */
      rb_define_singleton_method(cDBM, "open", fdbm_s_open, -1);

      /* DBM instance method close(): no args */
      rb_define_method(cDBM, "close", fdbm_close, 0);
      /* DBM instance method []: 1 argument */
      rb_define_method(cDBM, "[]", fdbm_fetch, 1);
                  :

      /* ID for a instance variable to store DBM data */
      id_dbm = rb_intern("dbm");
  }

The dbm extension wraps the dbmdata ``struct`` in the C environment using
``Data_Make_Struct``::

  struct dbmdata {
      int  di_size;
      DBM *di_dbm;
  };


  obj = Data_Make_Struct(klass, struct dbmdata, 0, free_dbm, dbmp);

This code wraps the ``dbmdata`` structure into a Ruby object.  We avoid
wrapping ``DBM*`` directly, because we want to cache size information.

To retrieve the ``dbmdata`` structure from a Ruby object, we define the
following macro::

  #define GetDBM(obj, dbmp) {\
      Data_Get_Struct(obj, struct dbmdata, dbmp);\
      if (dbmp->di_dbm == 0) closed_dbm();\
  }

This sort of complicated macro does the retrieving and close checking for the
DBM.

There are three kinds of way to receive method arguments.  First, methods with
a fixed number of arguments receive arguments like this::

  static VALUE
  fdbm_delete(obj, keystr)
      VALUE obj, keystr;
  {
          :
  }

The first argument of the C function is the self, the rest are the arguments to
the method.

Second, methods with an arbitrary number of arguments receive arguments like
this::

  static VALUE
  fdbm_s_open(argc, argv, klass)
      int argc;
      VALUE *argv;
      VALUE klass;
  {
          :
      if (rb_scan_args(argc, argv, "11", &file, &vmode) == 1) {
          mode = 0666;            /* default value */
      }
          :
  }

The first argument is the number of method arguments, the second argument is
the C array of the method arguments, and the third argument is the receiver of
the method.

You can use the function ``rb_scan_args()`` to check and retrieve the
arguments.  For example, ``"11"`` means that the method requires at least one
argument, and at most receives two arguments.

Methods with an arbitrary number of arguments can receive arguments by Ruby's
array, like this::

  static VALUE
  fdbm_indexes(obj, args)
      VALUE obj, args;
  {
          :
  }

The first argument is the receiver, the second one is the Ruby array
which contains the arguments to the method.

.. note::

  GC should know about global variables which refer to Ruby's objects, but are
  not exported to the Ruby world.  You need to protect them by ::

    void rb_global_variable(VALUE *var)

Prepare ``extconf.rb``
----------------------

If the file named ``extconf.rb`` exists, it will be executed to generate
``Makefile``.  If not, the compilation scheme will try to generate ``Makefile``
anyway.

``extconf.rb`` is the file for check compilation conditions etc.  You need to
put ::

  require 'mkmf'

at the top of the file.  You can use the functions below to check various
conditions.

=========================== =================================================
Function                    Check
=========================== =================================================
``have_library(lib, func)`` check whether library containing function exists.
``have_func(func, header)`` check whether function exists
``have_header(header)``     check whether header file exists
``create_makefile(target)`` generate Makefile
=========================== =================================================

The value of the variables below will affect the Makefile.

============  =============================================================
Variable      Effect
============  =============================================================
``$CFLAGS``   included in ``CFLAGS`` ``make(1)`` variable (such as ``-I``)
``$LDFLAGS``  included in ``LDFLAGS`` ``make(1)`` variable (such as ``-L``)
============  =============================================================

If a compilation condition is not fulfilled, you should not call
``create_makefile``.  The Makefile will not generated, compilation will not be
done.


Prepare ``depend`` (optional)
-----------------------------

If the file named ``depend`` exists, ``Makefile`` will include that file to
check dependencies.  You can make this file by invoking ::

  % gcc -MM *.c > depend

It's no harm.  Prepare it.


Put File Names into ``MANIFEST`` (optional)
-------------------------------------------

::

  % find * -type f -print > MANIFEST
  % vi MANIFEST

Append file names into ``MANIFEST``.  The compilation scheme requires
``MANIFEST`` only to exist, but it's better to take this step in order to
distinguish which files are required.


Generate ``Makefile``
---------------------

Try generating the Makefile by running ::

  % ruby extconf.rb

You don't need this step if you put the extension library under the ``ext``
directory of the ruby source tree.  In that case, compilation of the
interpreter will do this step for you.

``make(1)``
-----------

Type ::

  % make

to compile your extension.  You don't need this step either if you have put the
extension library under the ``ext`` directory of the ruby source tree.

Debug
-----

You may need to ``rb_debug`` the extension.  Extensions can be linked
statically by the adding directory name in the ``ext/Setup`` file so that you
can inspect the extension with the debugger.


Done, now you Have the Extension Library
----------------------------------------

You can do anything you want with your library.  The author of Ruby will not
claim any restrictions on your code depending on the Ruby API.  Feel free to
use, modify, distribute or sell your program.



Appendix A. Ruby Source Files Overview
======================================


Ruby Language Core
  ``class.c``
  ``error.c``
  ``eval.c``
  ``gc.c``
  ``object.c``
  ``parse.y``
  ``variable.c``

Utility Functions
  ``dln.c``
  ``regex.c``
  ``st.c``
  ``util.c``

Ruby Interpreter Implementation
  ``dmyext.c``
  ``inits.c``
  ``main.c``
  ``ruby.c``
  ``version.c``

Class Library
  ``array.c``
  ``bignum.c``
  ``compar.c``
  ``dir.c``
  ``enum.c``
  ``file.c``
  ``hash.c``
  ``io.c``
  ``marshal.c``
  ``math.c``
  ``numeric.c``
  ``pack.c``
  ``prec.c``
  ``process.c``
  ``random.c``
  ``range.c``
  ``re.c``
  ``signal.c``
  ``sprintf.c``
  ``string.c``
  ``struct.c``
  ``time.c``



Appendix B. Ruby Extension API Reference
========================================


Types
-----

``VALUE``
  The type for the Ruby object.  Actual structures are defined in ``ruby.h``,
  such as ``struct RString``, etc.  To refer the values in structures, use
  casting macros like ``RSTRING(obj)``.


Variables and Constants
-----------------------

``Qnil``
  const: ``nil`` object

``Qtrue``
  const: ``true`` object (default true value)

``Qfalse``
  const: ``false`` object


C Pointer Wrapping
------------------

``Data_Wrap_Struct(VALUE klass, void (*mark)(), void (*free)(), void *sval)``
  Wrap a C pointer into a Ruby object.  If object has references to other Ruby
  objects, they should be marked by using the ``mark`` function during the GC
  process.  Otherwise, ``mark`` should be ``NULL``.  When this object is no
  longer referred by anywhere, the pointer will be discarded by ``free``
  function.

``Data_Make_Struct(klass, type, mark, free, sval)``
  This macro allocates memory using ``malloc()``, assigns it to the variable
  ``sval``, and returns the ``DATA`` encapsulating the pointer to memory
  region.

``Data_Get_Struct(data, type, sval)``
  This macro retrieves the pointer value from ``DATA``, and assigns it to the
  variable ``sval``. 


Checking Data Types
-------------------

``TYPE(value)``
``FIXNUM_P(value)``
``NIL_P(value)``
``void Check_Type(VALUE value, int type)``
``void Check_SafeStr(VALUE value)``


Data Type Conversion
--------------------

``FIX2INT(value)``
``INT2FIX(i)``
``NUM2INT(value)``
``INT2NUM(i)``
``NUM2DBL(value)``
``rb_float_new(f)``
``STR2CSTR(value)``
``rb_str_new2(s)``

Defining Class/Module
---------------------

``VALUE rb_define_class(const char *name, VALUE super)``
  Defines a new Ruby class as a subclass of ``super``.

``VALUE rb_define_class_under(VALUE module, const char *name, VALUE super)``
  Creates a new Ruby class as a subclass of ``super``, under the module's
  namespace.

``VALUE rb_define_module(const char *name)``
  Defines a new Ruby module.

``VALUE rb_define_module_under(VALUE module, const char *name, VALUE super)``
  Defines a new Ruby module under the module's namespace.

``void rb_include_module(VALUE klass, VALUE module)``
  Includes module into ``klass``.  If ``klass`` already includes it, just
  ignored.

``void rb_extend_object(VALUE object, VALUE module)``
  Extend the object with the module's attributes.


Defining Global Variables
-------------------------

``void rb_define_variable(const char *name, VALUE *var)``
  Defines a global variable which is shared between C and Ruby.  If ``name``
  contains a character which is not allowed to be part of the symbol, it can't
  be seen from Ruby programs.

``void rb_define_readonly_variable(const char *name, VALUE *var)``
  Defines a read-only global variable.  Works just like
  ``rb_define_variable()``, except defined variable is read-only.

``void rb_define_virtual_variable(const char *name, VALUE (*getter)(), VALUE (*setter)())``

Defines a virtual variable, whose behavior is defined by a pair of C functions.
The ``getter`` function is called when the variable is referred.  The
``setter`` function is called when the value is set to the variable.  The
prototype for getter/setter functions are::

  VALUE getter(ID id)
  void setter(VALUE val, ID id)

The getter function must return the value for the access.

``void rb_define_hooked_variable(const char *name, VALUE *var, VALUE (*getter)(), VALUE (*setter)())``

Defines hooked variable.  It's a virtual variable with a C variable.  
The getter is called as ::

  VALUE getter(ID id, VALUE *var)

returning a new value.  The setter is called as ::

  void setter(VALUE val, ID id, VALUE *var)

GC requires C global variables which hold Ruby values to be marked.

``void rb_global_variable(VALUE *var)``

Tells GC to protect these variables.


Constant Definition
-------------------

``void rb_define_const(VALUE klass, const char *name, VALUE val)``
  Defines a new constant under the class/module.

``void rb_define_global_const(const char *name, VALUE val)``
  Defines a global constant.  This is just the same as ::

     rb_define_const(cKernal, name, val)


Method Definition
-----------------

``rb_define_method(VALUE klass, const char *name, VALUE (*func)(), int argc)``
  Defines a method for the class.  ``func`` is the function pointer.  ``argc``
  is the number of arguments.  if ``argc`` is -1, the function will receive 3
  arguments: ``argc``, ``argv``, and ``self``.  if ``argc`` is -2, the function
  will receive 2 arguments, ``self`` and ``args``, where ``args`` is a Ruby
  array of the method arguments.

``rb_define_private_method(VALUE klass, const char *name, VALUE (*func)(), int argc)``
  Defines a private method for the class.  Arguments are same as
  ``rb_define_method()``.

``rb_define_singleton_method(VALUE klass, const char *name, VALUE (*func)(), int argc)``
  Defines a singleton method.  Arguments are same as ``rb_define_method()``.

``rb_scan_args(int argc, VALUE *argv, const char *fmt, ...)``
  Retrieve argument from ``argc`` and ``argv``.  The ``fmt`` is the format
  string for the arguments, such as ``"12"`` for 1 non-optional argument, 2
  optional arguments.  If ``*`` appears at the end of ``fmt``, it means the
  rest of the arguments are assigned to the corresponding variable, packed in
  an array.


Invoking Ruby Method
--------------------

``VALUE rb_funcall(VALUE recv, ID mid, int narg, ...)``
  Invokes a method.  To retrieve ``mid`` from a method name, use
  ``rb_intern()``.

``VALUE rb_funcall2(VALUE recv, ID mid, int argc, VALUE *argv)``
  Invokes a method, passing arguments by an array of values.

``VALUE rb_eval_string(const char *str)``
  Compiles and executes the string as a Ruby program.

``ID rb_intern(const char *name)``
  Returns ID corresponding to the name.

``char *rb_id2name(ID id)``
  Returns the name corresponding ID.

``char *rb_class2name(VALUE klass)``
  Returns the name of the class.

``int rb_respond_to(VALUE object, ID id)``
  Returns ``true`` if the object responds to the message specified by ``id``.


Instance Variables
------------------

``VALUE rb_iv_get(VALUE obj, const char *name)``
  Retrieve the value of the instance variable.  If the name is not prefixed by
  ``@``, that variable will be inaccessible from Ruby.

``VALUE rb_iv_set(VALUE obj, const char *name, VALUE val)``
  Sets the value of the instance variable.


Control Structure
-----------------

``VALUE rb_iterate(VALUE (*func1)(), void *arg1, VALUE (*func2)(), void *arg2)``
  Calls the function ``func1``, supplying ``func2`` as the block.  ``func1``
  will be called with the argument ``arg1``.  ``func2`` receives the value from
  yield as the first argument, ``arg2`` as the second argument.
 
``VALUE rb_yield(VALUE val)``
  Evaluates the block with value ``val``.

``VALUE rb_rescue(VALUE (*func1)(), void *arg1, VALUE (*func2)(), void *arg2)``
  Calls the function ``func1``, with ``arg1`` as the argument.  If an exception
  occurs during ``func1``, it calls ``func2`` with ``arg2`` as the argument.
  The return value of ``rb_rescue()`` is the return value from ``func1`` if no
  exception occurs, from ``func2`` otherwise.

``VALUE rb_ensure(VALUE (*func1)(), void *arg1, void (*func2)(), void *arg2)``
  Calls the function ``func1`` with ``arg1`` as the argument, then calls
  ``func2`` with ``arg2`` if execution terminated.  The return value from
  ``rb_ensure()`` is that of ``func1``.


Exceptions and Errors
---------------------

``void rb_warn(const char *fmt, ...)``
  Prints a warning message according to a ``printf``\ -like format.

``void rb_warning(const char *fmt, ...)``
  Prints a warning message according to a ``printf``\ -like format, if
  ``$VERBOSE`` is ``true``.

``void rb_raise(rb_eRuntimeError, const char *fmt, ...)``
  Raises ``RuntimeError``.  The ``fmt`` is a format string just like
  ``printf()``.

``void rb_raise(VALUE exception, const char *fmt, ...)``
  Raises a class exception.  The ``fmt`` is a format string just like
  ``printf()``.

``void rb_fatal(const char *fmt, ...)``
  Raises a fatal error, terminates the interpreter.  No exception handling will
  be done for fatal errors, but ensure blocks will be executed.

``void rb_bug(const char *fmt, ...)``
  Terminates the interpreter immediately.  This function should be called under
  the situation caused by the bug in the interpreter.  No exception handling
  nor ensure execution will be done.

Initialize and Starting the Interpreter
---------------------------------------

The embedding API functions are below (not needed for extension libraries).

``void ruby_init()``
  Initializes the interpreter.

``void ruby_options(int argc, char **argv)``
  Process command line arguments for the interpreter.

``void ruby_run()``
  Starts execution of the interpreter.

``void ruby_script(char *name)``
  Specifies the name of the script ($0).



Appendix C. Functions Available in ``extconf.rb``
=================================================


The following functions are available in ``extconf.rb``.

``have_library(lib, func)``
  Checks whether the library exists, containing the specified function.
  Returns true if the library exists.

``find_library(lib, func, path...)``
  Checks whether a library which contains the specified function exists in
  ``path``.  Returns true if the library exists.

``have_func(func, header)``
  Checks whether ``func`` exists with ``header``.  Returns true if the function
  exists.  To check functions in an additional library, you need to check that
  library first using ``have_library()``.

``have_header(header)``
  Checks whether ``header`` exists.  Returns true if the header file exists.

``create_makefile(target)``
  Generates the Makefile for the extension library.  If you don't invoke this
  method, the compilation will not be done.

``with_config(withval[, default=nil])``
  Parses the command line options and returns the value specified by
  ``--with-<withval>``.

``dir_config(target[, default_dir])``
  See below.

``dir_config(target[, default_include, default_lib])``
  Parses the command line options and adds the directories specified by
  ``--with-<target>-dir``, ``--with-<target>-include``, and/or
  ``--with-<target>-lib`` to ``$CFLAGS`` and/or ``$LDFLAGS``.
  ``--with-<target>-dir=/path`` is equivalent to
  ``--with-<target>-include=/path/include --with-<target>-lib=/path/lib``.
  Returns an array of the added directories (``[include_dir, lib_dir]``).



.. arch-tag: a88f96c7-556b-4505-a963-72e8cdbc8d0f
.. vim: set ft=rst et sw=2 sts=2:

--envbJBWh7q8WU6mo--