=====================
Writing Bindable APIs
=====================

Things to do
------------

Type and function naming
~~~~~~~~~~~~~~~~~~~~~~~~

Follow the [GObject naming conventions](https://docs.gtk.org/gobject/concepts.html#conventions).
This allows the introspection tooling to group C functions together as methods
on types, and to group types and methods together into namespaces.

If these conventions are not followed, C functions may be incorrectly
introspected as functions rather than methods; methods may end up on the
incorrect class; class names may end up incorrectly including part of the
namespace in their name; amongst other problems.

Some of these problems can be worked around by passing additional arguments to
the introspection tooling to explicitly specify namespacing, but the tools work
better if C naming avoids that ambiguity.

Things to avoid
---------------

Structures with custom memory management
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Avoid creating C structures with custom memory management unless they are
registered as a `boxed type <https://docs.gtk.org/gobject/boxed.html>`__.
If you don't register them as a boxed type bindings will fall back to
simple memory copying, which might not be what you want.

Also consider using a full `GObject <https://docs.gtk.org/gobject/class.Object.html>`__
as that allows bindings to better integrate those objects with the binding
language, like for example preserve user defined state across language
boundaries.

Example to avoid:

.. code-block:: c

    struct _GstMiniObject {
      GTypeInstance instance;
      /*< public >*/ /* with COW */
      gint refcount;
      guint flags;


Functionality only accessible through a C macro or inline function
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The scanner does not support C macros as API. Solution - add a function
accessor rather than a macro. This also has the side effect of making
debugging in C code easier.

Example:

.. code-block:: c

    #define GTK_WIDGET_FLAGS(wid)             (GTK_OBJECT_FLAGS (wid))

    GtkWidgetFlags gtk_widget_get_flags (GtkWidget *widget); /* Actually, see http://bugzilla.gnome.org/show_bug.cgi?id=69872 */

Likewise, inline functions cannot be loaded from a dynamic library. Make sure to
provide a non-inline equivalent.


Direct C structure access for objects
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Having GObjects also have fields can be difficult to bind. Create accessor
functions.

Example:

.. code-block:: c

    struct _SoupMessage {
            GObject parent;

            /*< public >*/
            const char         *method;

            guint               status_code;
    ...
    }

    const char * soup_message_get_method (SoupMessage *message);  /*  Or use a GObject property */


va_list
~~~~~~~

Using varargs can be convenient for C, but they are difficult to bind.
Solution: Keep the C function for the convenience of C programmers, but add an
another function which takes an array (either zero terminated or with a length
parameter).

**Good** example: 

.. code-block:: c

    GtkListStore *gtk_list_store_new              (gint          n_columns,
                                                   ...);
    GtkListStore *gtk_list_store_newv             (gint          n_columns,
                                                   GType        *types);

You can also expose the array variant under the name of the varargs variant
using the ``rename-to`` annotation:
``gtk_list_store_newv: (rename-to gtk_list_store_new)``

Also consider using C99's compound literals and designated initializers to avoid
``va_list`` even in the C API, which is more type-safe.


Multiple out parameters
~~~~~~~~~~~~~~~~~~~~~~~

Multiple out parameters are supported by introspection, but not all languages
have an obvious mapping for multiple out values. A boxed structure could serve
as an alternative.

Example to think about (here, there could be a boxed ``struct GtkCoordinate {
gint x; gint y; }`` structure).

.. code-block:: c

    void         gtk_widget_get_pointer     (GtkWidget      *widget,
                                             gint           *x,
                                             gint           *y);


In-out parameters
~~~~~~~~~~~~~~~~~

Don't use in-out arguments, especially not for non-scalar values. It's difficult
to enforce or validate the conventions for in-out arguments, which can easily
lead to crashes.

Instead, pass the input as an in argument, and receive the output as either a
return value or an out argument.

.. code-block:: c

    FooBoxed *foo_bar_scale_boxed(FooBar   *self,
                                  FooBoxed *boxed);

    void foo_bar_scale_boxed(FooBar    *self,
                             FooBoxed  *boxed_in,
                             FooBoxed **boxed_out);

In particular, do not require the caller to pass an initialized ``GValue`` to
avoid the in-out annotation; instead, pass a ``GValue`` as an out argument, and
have the function initialize it.


Arrays
~~~~~~

For reference types, zero-terminated arrays are the easiest to work with.
Arrays of primitive type such as "int" will require length metadata.

In a general-purpose library, it's best not to expose GLib array and hash types
such as ``GArray``, ``GPtrArray``, ``GByteArray``, ``GList``, ``GSList``,
``GQueue``, and ``GHashTable`` in the public API. They are fine for internal
libraries, but difficult in general for consumers of introspected libraries to
deal with.


Strings
~~~~~~~

C treats strings as zero-terminated arrays of bytes, but many other languages do not. So don't
write APIs that treat ``const char *`` parameters as arrays that need an
``array length`` annotation.

Treat all ``const char *`` parameters as zero-terminated strings. Don't use the
same entry point for zero-terminated strings as for byte arrays which may
contain embedded zeroes.

.. code-block:: c

    void foo_bar_snarf_string(FooBar     *self,
                              const char *str);

    void foo_bar_snarf_bytes(FooBar        *self,
                             const uint8_t *bytes,
                             size_t         length);

In particular, avoid functions taking a ``const char *`` with a signed length
that can be set to a negative value to let the function compute the string
length in bytes. These functions are hard to bind, and require manual overrides.

Callbacks
~~~~~~~~~

Callbacks are hard to support for introspection bindings because of their
complex life-cycle. Try to avoid having more than one callback in the same
function, and consider using GClosure when you need more.


Using a different name for error domain quarks from the enum name
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Error domain quarks should always be named in the form
<namespace>_<module>_error_quark() for an error enum called
<Namespace><Module>Error. Example to avoid:

.. code-block:: c

    typedef enum FooBarError {
      FOO_BAR_ERROR_MOO,
      FOO_BAR_ERROR_BLEAT
    };

    GQuark foo_bar_errors_quark();


Don't have properties and methods with the same name
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Some bindings for dynamic languages expose GObject properties and methods in the
same way, as properties on an object instance. So don't make a GObject property
with the same name as a method, e.g. a property named ``add-feature`` on a class
named ``SoupSession`` which also has a method ``soup_session_add_feature()``.


Custom code in constructors
~~~~~~~~~~~~~~~~~~~~~~~~~~~

Creating an object via ``foo_bar_new()`` shouldn't execute any code
differently than creating the same object via ``g_object_new()``, since many
bindings (and also GtkBuilder/Glade) create objects using ``g_object_new()``.
That is, don't do this:

.. code-block:: c

    FooBar *
    foo_bar_new (void)
    {
        FooBar *retval = FOO_BAR (g_object_new (FOO_TYPE_BAR, NULL));
        retval->priv->some_variable = 5;  /* Don't do this! */
        return retval;
    }

Instead, put initialization code in the ``foo_bar_init()`` function or the
``foo_bar_constructed()`` virtual function.


Transfer-none return values from the binding
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

If your library expects to call a function from C which may be implemented in
another language and exposed through the binding (for example, a signal handler,
or a GObject vfunc), it's best not to return transfer-none values, because what
you assume about storage lifetime in C may not apply in other languages.

For example,

.. code-block:: c

    typedef struct {
        GTypeInterface iface;

        const char * (*my_vfunc)        (FooBaz *self);  /* Don't do this! */
        char       * (*my_better_vfunc) (FooBaz *self);  /* Do this instead! */
    } FooBazIface;

A class that implements ``FooBazIface`` in another programming language may not
be able to return a static string here, because the language may not have a
concept of static storage lifetime, or it may not store strings as
zero-terminated UTF-8 bytes as C code would expect. This can cause memory leaks.
Instead, duplicate the string before returning it, and use transfer-full. This
recommendation applies to any data type with an ownership, including boxed and
object types.
