GObject_manual.md

📜 GObject Reference Manual

for GObject 2.78.0; The latest version of this documentation can be found on-line at https://developer.gnome.org/gobject/unstable/ https://docs.gtk.org/gobject/index.html

🍀 Introduction

Most modern programming languages come with their own native object systems and additional fundamental algorithmic language constructs. Just as GLib serves as an implementation of such fundamental types and algorithms (linked lists, hash tables and so forth), the GLib Object System provides the required implementations of a flexible, extensible, and intentionally easy to map (into other languages) object-oriented framework for C.

The substantial elements that are provided can be summarized as:

0. A generic type system to register arbitrary single-inherited flat and deep derived types as well as interfaces for structured types. It takes care of creation, initialization and memory management of the assorted object and class structures, maintains parent/child relationships and deals with dynamic implementations of such types. That is, their type specific implementations are relocatable/unloadable during runtime.
1. A collection of fundamental type implementations, such as integers, doubles, enums and structured types, to name a few.
2. A sample fundamental type implementation to base object hierarchies upon - the GObject fundamental type.
3. A signal system that allows very flexible user customization of virtual/overridable object methods and can serve as a powerful notification mechanism.
4. An extensible parameter/value system, supporting all the provided fundamental types that can be used to generically handle object properties or otherwise parameterized types.

🍀 Concepts

1. GObject Introduction docs/reference/tut_intro.xml
2. The GLib Dynamic Type System docs/reference/tut_gtype.xml
3. The GObject base class docs/reference/tut_gobject.xml
4. The GObject messaging system docs/reference/tut_gsignal.xml

🍀 Tools Reference

1. glib-mkenums User Commands docs/reference/glib-mkenums.xml
2. glib-genmarshal User Commands docs/reference/glib-genmarshal.xml
3. gobject-query User Commands docs/reference/gobject-query.xml
4. GObject Tutorial docs/reference/tut_howto.xml
5. Related Tools docs/reference/tut_tools.xml

🍀 Symbols References

1. Classes Hierarchy
2. GObject Sections

📜 GObject Introduction

🍀 Type System Concepts

GObject, and its lower-level type system, GType, are used by GTK and most GNOME libraries to provide:

0. object-oriented C-based APIs and
1. automatic transparent API bindings to other compiled or interpreted languages.

A lot of programmers are used to working with compiled-only or dynamically interpreted-only languages and do not understand the challenges associated with cross-language interoperability. This introduction tries to provide an insight into these challenges and briefly describes the solution chosen by GLib.

The following chapters go into greater detail into how GType and GObject work and how you can use them as a C programmer. It is useful to keep in mind that allowing access to C objects from other interpreted languages was one of the major design goals: this can often explain the sometimes rather convoluted APIs and features present in this library.

🍀 Data types and programming

One could say that a programming language is merely a way to create data types and manipulate them. Most languages provide a number of language-native types and a few primitives to create more complex types based on these primitive types.

In C, the language provides types such as char, long, pointer. During compilation of C code, the compiler maps these language types to the compiler's target architecture machine types. If you are using a C interpreter (assuming one exists), the interpreter (the program which interprets the source code and executes it) maps the language types to the machine types of the target machine at runtime, during the program execution (or just before execution if it uses a Just In Time compiler engine).

Perl and Python are interpreted languages which do not really provide type definitions similar to those used by C. Perl and Python programmers manipulate variables and the type of the variables is decided only upon the first assignment or upon the first use which forces a type on the variable. The interpreter also often provides a lot of automatic conversions from one type to the other. For example, in Perl, a variable which holds an integer can be automatically converted to a string given the required context:

my $tmp = 10;
print "this is an integer converted to a string:" . $tmp . "\n";

Of course, it is also often possible to explicitly specify conversions when the default conversions provided by the language are not intuitive.

🍀 Exporting a C API

C APIs are defined by a set of functions and global variables which are usually exported from a binary. C functions have an arbitrary number of arguments and one return value. Each function is thus uniquely identified by the function name and the set of C types which describe the function arguments and return value. The global variables exported by the API are similarly identified by their name and their type.

A C API is thus merely defined by a set of names to which a set of types are associated. If you know the function calling convention and the mapping of the C types to the machine types used by the platform you are on, you can resolve the name of each function to find where the code associated to this function is located in memory, and then construct a valid argument list for the function. Finally, all you have to do is trigger a call to the target C function with the argument list.

For the sake of discussion, here is a sample C function and the associated 32 bit x86 assembly code generated by GCC on a Linux computer:

static void
function_foo (int foo) { }

int
main (int   argc,
      char *argv[])
{
  function_foo (10);

  return 0;
}

push   $0xa
call   0x80482f4 <function_foo>

The assembly code shown above is pretty straightforward: the first instruction pushes the hexadecimal value 0xa (decimal value 10) as a 32-bit integer on the stack and calls function_foo. As you can see, C function calls are implemented by GCC as native function calls (this is probably the fastest implementation possible).

Now, let's say we want to call the C function function_foo from a Python program. To do this, the Python interpreter needs to:

0. Find where the function is located. This probably means finding the binary generated by the C compiler which exports this function.
1. Load the code of the function in executable memory.
2. Convert the Python parameters to C-compatible parameters before calling the function.
3. Call the function with the right calling convention.
4. Convert the return values of the C function to Python-compatible variables to return them to the Python code.

The process described above is pretty complex and there are a lot of ways to make it entirely automatic and transparent to C and Python programmers:

0. The first solution is to write by hand a lot of glue code, once for each function exported or imported, which does the Python-to-C parameter conversion and the C-to-Python return value conversion. This glue code is then linked with the interpreter which allows Python programs to call Python functions which delegate work to C functions.
1. Another, nicer solution is to automatically generate the glue code, once for each function exported or imported, with a special compiler which reads the original function signature.

The solution used by GLib is to use the GType library which holds at runtime a description of all the objects manipulated by the programmer. This so-called dynamic type library is then used by special generic glue code to automatically convert function parameters and function calling conventions between different runtime domains.

There are numerous different implementations of dynamic type systems: all C++ 
compilers have one, Java and .NET have one too. A dynamic type system allows you
to get information about every instantiated object at runtime. It can be implemented
by a process-specific database: every new object created registers the characteristics 
of its associated type in the type system. It can also be implemented by introspection
interfaces. The common point between all these different type systems and implementations
is that they all allow you to query for object metadata at runtime.

The greatest advantage of the solution implemented by GType is that the glue code sitting at the runtime domain boundaries is written once: the figure below states this more clearly.

https://docs.gtk.org/gobject/glue.png

Currently, there exist at least Python and Perl generic glue code which makes it possible to use C objects written with GType directly in Python or Perl, with a minimum amount of work: there is no need to generate huge amounts of glue code either automatically or by hand.

Although that goal was arguably laudable, its pursuit has had a major influence on the whole GType/GObject library. C programmers are likely to be puzzled at the complexity of the features exposed in the following chapters if they forget that the GType/GObject library was not only designed to offer OO-like features to C programmers but also transparent cross-language interoperability.

📜 The GLib Dynamic Type System

A type, as manipulated by the GLib type system, is much more generic than what is usually understood as an Object type. It is best explained by looking at the structure and the functions used to register new types in the type system.

typedef struct _GTypeInfo               GTypeInfo;
struct _GTypeInfo
{
  /* interface types, classed types, instantiated types */
  guint16                class_size;
  
  GBaseInitFunc          base_init;
  GBaseFinalizeFunc      base_finalize;
  
  /* classed types, instantiated types */
  GClassInitFunc         class_init;
  GClassFinalizeFunc     class_finalize;
  gconstpointer          class_data;
  
  /* instantiated types */
  guint16                instance_size;
  guint16                n_preallocs;
  GInstanceInitFunc      instance_init;
  
  /* value handling */
  const GTypeValueTable *value_table;
};
GType g_type_register_static (GType             parent_type,
                              const gchar      *type_name,
                              const GTypeInfo  *info,
                              GTypeFlags        flags);
GType g_type_register_fundamental (GType                  type_id,
                              const gchar                *type_name,
                              const GTypeInfo            *info,
                              const GTypeFundamentalInfo *finfo,
                              GTypeFlags                  flags);

g_type_register_static, g_type_register_dynamic and g_type_register_fundamental are the C functions, defined in gtype.h and implemented in gtype.c which you should use to register a new GType in the program's type system. It is not likely you will ever need to use g_type_register_fundamental but in case you want to, the last chapter explains how to create new fundamental types.

Fundamental types are top-level types which do not derive from any other type while other non-fundamental types derive from other types. Upon initialization, the type system not only initializes its internal data structures but it also registers a number of core types: some of these are fundamental types. Others are types derived from these fundamental types.

Fundamental and non-fundamental types are defined by: class size: the class_size field in GTypeInfo. class initialization functions (C++ constructor): the base_init and class_init fields in GTypeInfo.

class destruction functions (C++ destructor): the base_finalize and class_finalize fields in `GTypeInfo`.

instance size (C++ parameter to new): the instance_size field in `GTypeInfo`.

instantiation policy (C++ type of new operator): the n_preallocs field in `GTypeInfo`.

copy functions (C++ copy operators): the value_table field in `GTypeInfo`.

type characteristic flags: `GTypeFlags`. 

Fundamental types are also defined by a set of GTypeFundamentalFlags which are stored in a GTypeFundamentalInfo. Non-fundamental types are furthermore defined by the type of their parent which is passed as the parent_type parameter to g_type_register_static and g_type_register_dynamic.

🍀 Copy functions

                                                   *gtype-copy*

The major common point between all GLib types (fundamental and non-fundamental, classed and non-classed, instantiatable and non-instantiatable) is that they can all be manipulated through a single API to copy/assign them.

The GValue structure is used as an abstract container for all of these types. Its simplistic API (defined in gobject/gvalue.h) can be used to invoke the value_table functions registered during type registration: for example g_value_copy copies the content of a GValue to another GValue. This is similar to a C++ assignment which invokes the C++ copy operator to modify the default bit-by-bit copy semantics of C++/C structures/classes.

The following code shows how you can copy around a 64 bit integer, as well as a GObject instance pointer:

static void test_int (void)
{
  GValue a_value = G_VALUE_INIT;
  GValue b_value = G_VALUE_INIT;
  guint64 a, b;

  a = 0xdeadbeef;

  g_value_init (&a_value, G_TYPE_UINT64);
  g_value_set_uint64 (&a_value, a);

  g_value_init (&b_value, G_TYPE_UINT64);
  g_value_copy (&a_value, &b_value);

  b = g_value_get_uint64 (&b_value);

  if (a == b) {
    g_print ("Yay !! 10 lines of code to copy around a uint64.\n");
  } else {
    g_print ("Are you sure this is not a Z80 ?\n");
  }
}

static void test_object (void)
{
  GObject *obj;
  GValue obj_vala = G_VALUE_INIT;
  GValue obj_valb = G_VALUE_INIT;
  obj = g_object_new (VIEWER_TYPE_FILE, NULL);

  g_value_init (&obj_vala, VIEWER_TYPE_FILE);
  g_value_set_object (&obj_vala, obj);

  g_value_init (&obj_valb, G_TYPE_OBJECT);

  /* g_value_copy's semantics for G_TYPE_OBJECT types is to copy the reference.
   * This function thus calls g_object_ref.
   * It is interesting to note that the assignment works here because
   * VIEWER_TYPE_FILE is a G_TYPE_OBJECT.
   */
  g_value_copy (&obj_vala, &obj_valb);

  g_object_unref (G_OBJECT (obj));
  g_object_unref (G_OBJECT (obj));
}

The important point about the above code is that the exact semantics of the copy calls is undefined since they depend on the implementation of the copy function. Certain copy functions might decide to allocate a new chunk of memory and then to copy the data from the source to the destination. Others might want to simply increment the reference count of the instance and copy the reference to the new GValue.

The value table used to specify these assignment functions is documented in GTypeValueTable.

Interestingly, it is also very unlikely you will ever need to specify a value_table during type registration because these value_tables are inherited from the parent types for non-fundamental types.

🍀 Conventions

                                            *gtype-conventions*

There are a number of conventions users are expected to follow when creating new types which are to be exported in a header file:

0. Type names (including object names) must be at least three characters long and start with ‘a–z’, ‘A–Z’ or _.

1. Use the object_method pattern for function names: to invoke the method named save on an instance of object type file, call file_save.

2. If your library (or application) is named Viewer, prefix all your function names with viewer_. For example: viewer_object_method.

3. returns the GType for the associated object type. For an object of type File in the Viewer namespace, use: VIEWER_TYPE_FILE. This macro is implemented using a function named prefix_object_get_type; for example, viewer_file_get_type.

4. Use G_DECLARE_FINAL_TYPE or G_DECLARE_DERIVABLE_TYPE to define various other conventional macros for your object:

   0. PREFIX_OBJECT (obj) , which returns a pointer of type PrefixObject. This macro is used to enforce static type safety by doing explicit casts wherever needed. It also enforces dynamic type safety by doing runtime checks. It is possible to disable the dynamic type checks in production builds (see building GLib). For example, we would create VIEWER_FILE (obj) to keep the previous example.

   1. PREFIX_OBJECT_CLASS (klass), which is strictly equivalent to the previous casting macro: it does static casting with dynamic type checking of class structures. It is expected to return a pointer to a class structure of type PrefixObjectClass. An example is: VIEWER_FILE_CLASS.

   2. PREFIX_IS_OBJECT (obj), which returns a gboolean which indicates whether the input object instance pointer is non-NULL and of type OBJECT. For example, VIEWER_IS_FILE.

   3. PREFIX_IS_OBJECT_CLASS (klass), which returns a boolean if the input class pointer is a pointer to a class of type OBJECT. For example, VIEWER_IS_FILE_CLASS.

   4. PREFIX_OBJECT_GET_CLASS (obj), which returns the class pointer associated to an instance of a given type. This macro is used for static and dynamic type safety purposes (just like the previous casting macros). For example, VIEWER_FILE_GET_CLASS.

The implementation of these macros is pretty straightforward: a number of simple-to-use macros are provided in gtype.h. For the example we used above, we would write the following trivial code to declare the macros:

#define VIEWER_TYPE_FILE viewer_file_get_type ()
G_DECLARE_FINAL_TYPE (ViewerFile, viewer_file, VIEWER, FILE, GObject)

Unless your code has special requirements, you can use the G_DEFINE_TYPE macro to define a class:

G_DEFINE_TYPE (ViewerFile, viewer_file, G_TYPE_OBJECT)

Otherwise, the viewer_file_get_type function must be implemented manually:

GType viewer_file_get_type (void)
{
  static GType type = 0;
  if (type == 0) {
    const GTypeInfo info = {
      /* You fill this structure. */
    };
    type = g_type_register_static (G_TYPE_OBJECT,
                                   "ViewerFile",
                                   &info, 0);
  }
  return type;
}

🍀 Non-instantiatable non-classed fundamental types

                                     *gtype-non-instantiatable*

A lot of types are not instantiatable by the type system and do not have a class. Most of these types are fundamental trivial types such as gchar, and are already registered by GLib.

In the rare case of needing to register such a type in the type system, fill a GTypeInfo structure with zeros since these types are also most of the time fundamental:

  GTypeInfo info = {
    0,                                /* class_size */
    NULL,                        /* base_init */
    NULL,                        /* base_destroy */
    NULL,                        /* class_init */
    NULL,                        /* class_destroy */
    NULL,                        /* class_data */
    0,                                /* instance_size */
    0,                                /* n_preallocs */
    NULL,                        /* instance_init */
    NULL,                        /* value_table */
  };
  static const GTypeValueTable value_table = {
    value_init_long0,                /* value_init */
    NULL,                        /* value_free */
    value_copy_long0,                /* value_copy */
    NULL,                        /* value_peek_pointer */
    "i",                        /* collect_format */
    value_collect_int,        /* collect_value */
    "p",                        /* lcopy_format */
    value_lcopy_char,                /* lcopy_value */
  };
  info.value_table = &value_table;
  type = g_type_register_fundamental (G_TYPE_CHAR, "gchar", &info, &finfo, 0);

Having non-instantiatable types might seem a bit useless: what good is a type if you cannot instantiate an instance of that type ? Most of these types are used in conjunction with GValues: a GValue is initialized with an integer or a string and it is passed around by using the registered type's value_table. GValues (and by extension these trivial fundamental types) are most useful when used in conjunction with object properties and signals.

🍀 Instantiatable classed types: objects

                                 *gtype-instantiatable-classed*

This section covers the theory behind objects. See for the recommended way to define a GObject.

Types which are registered with a class and are declared instantiatable are what most closely resembles an object. Although GObjects (detailed in ) are the most well known type of instantiatable classed types, other kinds of similar objects used as the base of an inheritance hierarchy have been externally developed and they are all built on the fundamental features described below.

For example, the code below shows how you could register such a fundamental object type in the type system (using none of the GObject convenience API):

typedef struct {
  GObject parent;

  /* instance members */
  gchar *filename;
} ViewerFile;

typedef struct {
  GObjectClass parent;

  /* class members */
  /* the first is public, pure and virtual */
  void (*open)  (ViewerFile  *self,
                 GError     **error);

  /* the second is public and virtual */
  void (*close) (ViewerFile  *self,
                 GError     **error);
} ViewerFileClass;

#define VIEWER_TYPE_FILE (viewer_file_get_type ())

GType 
viewer_file_get_type (void)
{
  static GType type = 0;
  if (type == 0) {
    const GTypeInfo info = {
      sizeof (ViewerFileClass),
      NULL,           /* base_init */
      NULL,           /* base_finalize */
      (GClassInitFunc) viewer_file_class_init,
      NULL,           /* class_finalize */
      NULL,           /* class_data */
      sizeof (ViewerFile),
      0,              /* n_preallocs */
      (GInstanceInitFunc) NULL /* instance_init */
    };
    type = g_type_register_static (G_TYPE_OBJECT,
                                   "ViewerFile",
                                   &info, 0);
  }
  return type;
}

Upon the first call to viewer_file_get_type, the type named ViewerFile will be registered in the type system as inheriting from the type G_TYPE_OBJECT.

Every object must define two structures: its class structure and its instance structure. All class structures must contain as first member a GTypeClass structure. All instance structures must contain as first member a GTypeInstance structure. The declaration of these C types, coming from gtype.h is shown below:

struct _GTypeClass
{
  GType g_type;
};
struct _GTypeInstance
{
  GTypeClass *g_class;
};

These constraints allow the type system to make sure that every object instance (identified by a pointer to the object's instance structure) contains in its first bytes a pointer to the object's class structure.

This relationship is best explained by an example: let's take object B which inherits from object A:

/* A definitions */
typedef struct {
  GTypeInstance parent;
  int field_a;
  int field_b;
} A;
typedef struct {
  GTypeClass parent_class;
  void (*method_a) (void);
  void (*method_b) (void);
} AClass;

/* B definitions. */
typedef struct {
  A parent;
  int field_c;
  int field_d;
} B;
typedef struct {
  AClass parent_class;
  void (*method_c) (void);
  void (*method_d) (void);
} BClass;

The C standard mandates that the first field of a C structure is stored starting in the first byte of the buffer used to hold the structure's fields in memory. This means that the first field of an instance of an object B is A's first field which in turn is GTypeInstance's first field which in turn is g_class, a pointer to B's class structure.

Thanks to these simple conditions, it is possible to detect the type of every object instance by doing:

B *b;
b->parent.parent.g_class->g_type

or, more quickly:

B *b;
((GTypeInstance *) b)->g_class->g_type

🐣 Initialization and Destruction

instantiation of these types can be done with g_type_create_instance, which will look up the type information structure associated with the type requested. Then, the instance size and instantiation policy (if the n_preallocs field is set to a non-zero value, the type system allocates the object's instance structures in chunks rather than mallocing for every instance) declared by the user are used to get a buffer to hold the object's instance structure.

If this is the first instance of the object ever created, the type system must create a class structure. It allocates a buffer to hold the object's class structure and initializes it. The first part of the class structure (ie: the embedded parent class structure) is initialized by copying the contents from the class structure of the parent class. The rest of class structure is initialized to zero. If there is no parent, the entire class structure is initialized to zero. The type system then invokes the base_class_initialization functions (GBaseInitFunc) from topmost fundamental object to bottom-most most derived object. The object's class_init (GClassInitFunc) function is invoked afterwards to complete initialization of the class structure. Finally, the object's interfaces are initialized (we will discuss interface initialization in more detail later).

Once the type system has a pointer to an initialized class structure, it sets the object's instance class pointer to the object's class structure and invokes the object's instance_init (GInstanceInitFunc) functions, from top-most fundamental type to bottom-most most-derived type.

Object instance destruction through g_type_free_instance is very simple: the instance structure is returned to the instance pool if there is one and if this was the last living instance of the object, the class is destroyed.

Class destruction (the concept of destruction is sometimes partly referred to as finalization in GType) is the symmetric process of the initialization: interfaces are destroyed first. Then, the most derived class_finalize (GClassFinalizeFunc) function is invoked. Finally, the base_class_finalize (GBaseFinalizeFunc) functions are invoked from bottom-most most-derived type to top-most fundamental type and the class structure is freed.

The base initialization/finalization process is very similar to the C++ constructor/destructor paradigm. The practical details are different though and it is important not to get confused by superficial similarities. GTypes have no instance destruction mechanism. It is the user's responsibility to implement correct destruction semantics on top of the existing GType code. (This is what GObject does: see gobject.) Furthermore, C++ code equivalent to the base_init and class_init callbacks of GType is usually not needed because C++ cannot really create object types at runtime.

The instantiation/finalization process can be summarized as follows:

<title>GType Instantiation/Finalization</title> Invocation time Function invoked Function's parameters First call to `g_type_create_instance` for target type type's `base_init` function On the inheritance tree of classes from fundamental type to target type. `base_init` is invoked once for each class structure. target type's `class_init` function On target type's class structure interface initialization, see Non-instantiatable classed types: interfaces. Each call to `g_type_create_instance` for target type target type's `instance_init` function On object's instance Last call to `g_type_free_instance` for target type interface destruction, see Non-instantiatable non-classed fundamental types. target type's `class_finalize` function On target type's class structure type's `base_finalize` function On the inheritance tree of classes from fundamental type to target type. `base_finalize` is invoked once for each class structure.

🍀 Non-instantiatable non-classed types: interfaces

                         *gtype-non-instantiatable-non-classed*

This section covers the theory behind interfaces. See for the recommended way to define an interface.

GType's interfaces are very similar to Java's interfaces. They allow to describe a common API that several classes will adhere to. Imagine the play, pause and stop buttons on hi-fi equipment — those can be seen as a playback interface. Once you know what they do, you can control your CD player, MP3 player or anything that uses these symbols. To declare an interface you have to register a non-instantiatable non-classed type which derives from GTypeInterface. The following piece of code declares such an interface.

#define VIEWER_TYPE_EDITABLE viewer_editable_get_type ()
G_DECLARE_INTERFACE (ViewerEditable, viewer_editable, VIEWER, EDITABLE, GObject)

struct _ViewerEditableInterface {
  GTypeInterface parent;

  void (*save) (ViewerEditable  *self,
                GError         **error);
};

void viewer_editable_save (ViewerEditable  *self,
                           GError         **error);

The interface function, viewer_editable_save is implemented in a pretty simple way:

void
viewer_editable_save (ViewerEditable  *self,
                      GError         **error)
{
  ViewerEditableinterface *iface;

  g_return_if_fail (VIEWER_IS_EDITABLE (self));
  g_return_if_fail (error == NULL || *error == NULL);

  iface = VIEWER_EDITABLE_GET_IFACE (self);
  g_return_if_fail (iface->save != NULL);
  iface->save (self);
}

viewer_editable_get_type registers a type named ViewerEditable which inherits from G_TYPE_INTERFACE. All interfaces must be children of G_TYPE_INTERFACE in the inheritance tree.

An interface is defined by only one structure which must contain as first member a GTypeInterface structure. The interface structure is expected to contain the function pointers of the interface methods. It is good style to define helper functions for each of the interface methods which simply call the interface's method directly: viewer_editable_save is one of these.

If you have no special requirements you can use the G_IMPLEMENT_INTERFACE macro to implement an interface:

static void
viewer_file_save (ViewerEditable *self)
{
  g_print ("File implementation of editable interface save method.\n");
}

static void
viewer_file_editable_interface_init (ViewerEditableInterface *iface)
{
  iface->save = viewer_file_save;
}

G_DEFINE_TYPE_WITH_CODE (ViewerFile, viewer_file, VIEWER_TYPE_FILE,
                         G_IMPLEMENT_INTERFACE (VIEWER_TYPE_EDITABLE,
                                                viewer_file_editable_interface_init))

If your code does have special requirements, you must write a custom get_type function to register your GType which inherits from some GObject and which implements the interface ViewerEditable. For example, this code registers a new ViewerFile class which implements ViewerEditable:

static void
viewer_file_save (ViewerEditable *editable)
{
  g_print ("File implementation of editable interface save method.\n");
}

static void
viewer_file_editable_interface_init (gpointer g_iface,
                                     gpointer iface_data)
{
  ViewerEditableInterface *iface = g_iface;

  iface->save = viewer_file_save;
}

GType 
viewer_file_get_type (void)
{
  static GType type = 0;
  if (type == 0) {
    const GTypeInfo info = {
      sizeof (ViewerFileClass),
      NULL,   /* base_init */
      NULL,   /* base_finalize */
      NULL,   /* class_init */
      NULL,   /* class_finalize */
      NULL,   /* class_data */
      sizeof (ViewerFile),
      0,      /* n_preallocs */
      NULL    /* instance_init */
    };
    const GInterfaceInfo editable_info = {
      (GInterfaceInitFunc) viewer_file_editable_interface_init,  /* interface_init */
      NULL,   /* interface_finalize */
      NULL    /* interface_data */
    };
    type = g_type_register_static (VIEWER_TYPE_FILE,
                                   "ViewerFile",
                                   &info, 0);
    g_type_add_interface_static (type,
                                 VIEWER_TYPE_EDITABLE,
                                 &editable_info);
  }
  return type;
}

g_type_add_interface_static records in the type system that a given type implements also FooInterface (foo_interface_get_type returns the type of FooInterface). The GInterfaceInfo structure holds information about the implementation of the interface:

struct _GInterfaceInfo
{
  GInterfaceInitFunc     interface_init;
  GInterfaceFinalizeFunc interface_finalize;
  gpointer               interface_data;
};

🐣 Interface Initialization

When an instantiatable classed type which implements an interface (either directly or by inheriting an implementation from a superclass) is created for the first time, its class structure is initialized following the process described in . After that, the interface implementations associated with the type are initialized.

First a memory buffer is allocated to hold the interface structure. The parent's interface structure is then copied over to the new interface structure (the parent interface is already initialized at that point). If there is no parent interface, the interface structure is initialized with zeros. The g_type and the g_instance_type fields are then initialized: g_type is set to the type of the most-derived interface and g_instance_type is set to the type of the most derived type which implements this interface.

The interface's base_init function is called, and then the interface's default_init is invoked. Finally if the type has registered an implementation of the interface, the implementation's interface_init function is invoked. If there are multiple implementations of an interface the base_init and interface_init functions will be invoked once for each implementation initialized.

It is thus recommended to use a default_init function to initialize an interface. This function is called only once for the interface no matter how many implementations there are. The default_init function is declared by G_DEFINE_INTERFACE which can be used to define the interface:

G_DEFINE_INTERFACE (ViewerEditable, viewer_editable, G_TYPE_OBJECT)

static void
viewer_editable_default_init (ViewerEditableInterface *iface)
{
  /* add properties and signals here, will only be called once */
}

Or you can do that yourself in a GType function for your interface:

GType
viewer_editable_get_type (void)
{
  static gsize type_id = 0;
  if (g_once_init_enter (&type_id)) {
    const GTypeInfo info = {
      sizeof (ViewerEditableInterface),
      NULL,   /* base_init */
      NULL,   /* base_finalize */
      viewer_editable_default_init, /* class_init */
      NULL,   /* class_finalize */
      NULL,   /* class_data */
      0,      /* instance_size */
      0,      /* n_preallocs */
      NULL    /* instance_init */
    };
    GType type = g_type_register_static (G_TYPE_INTERFACE,
                                         "ViewerEditable",
                                         &info, 0);
    g_once_init_leave (&type_id, type);
  }
  return type_id;
}

static void
viewer_editable_default_init (ViewerEditableInterface *iface)
{
  /* add properties and signals here, will only called once */
}

In summary, interface initialization uses the following functions:

<title>Interface Initialization</title> Invocation time Function Invoked Function's parameters Remark First call to `g_type_create_instance` for *any* type implementing interface interface's `base_init` function On interface's vtable Rarely necessary to use this. Called once per instantiated classed type implementing the interface. First call to `g_type_create_instance` for *each* type implementing interface interface's `default_init` function On interface's vtable Register interface's signals, properties, etc. here. Will be called once. First call to `g_type_create_instance` for *any* type implementing interface implementation's `interface_init` function On interface's vtable Initialize interface implementation. Called for each class that that implements the interface. Initialize the interface method pointers in the interface structure to the implementing class's implementation.

🐣 Interface Destruction

When the last instance of an instantiatable type which registered an interface implementation is destroyed, the interface's implementations associated to the type are destroyed.

To destroy an interface implementation, GType first calls the implementation's interface_finalize function and then the interface's most-derived base_finalize function.

Again, it is important to understand, as in , that both interface_finalize and base_finalize are invoked exactly once for the destruction of each implementation of an interface. Thus, if you were to use one of these functions, you would need to use a static integer variable which would hold the number of instances of implementations of an interface such that the interface's class is destroyed only once (when the integer variable reaches zero).

The above process can be summarized as follows:

Interface Finalization

| Invocation time | Function Invoked | Function's parameters | Last call to g_type_free_instance for type implementing interface | interface's interface_finalize function | On interface's vtable | Last call to g_type_free_instancefor type implementing interface | interface's base_finalize function | On interface's vtable

📜 The GObject base class

The previous chapter discussed the details of GLib's Dynamic Type System. The GObject library also contains an implementation for a base fundamental type named GObject.

GObject is a fundamental classed instantiatable type. It implements:

0. Memory management with reference counting
1. Construction/Destruction of instances
2. Generic per-object properties with set/get function pairs
3. Easy use of signals

All the GNOME libraries which use the GLib type system (like GTK and GStreamer) inherit from GObject which is why it is important to understand the details of how it works.

🍀 Object instantiation

The g_object_new family of functions can be used to instantiate any GType which inherits from the GObject base type. All these functions make sure the class and instance structures have been correctly initialized by GLib's type system and then invoke at one point or another the constructor class method which is used to:

0. Allocate and clear memory through g_type_create_instance,
1. Initialize the object's instance with the construction properties.

Although one can expect all class and instance members (except the fields pointing to the parents) to be set to zero, some consider it good practice to explicitly set them.

Once all construction operations have been completed and constructor properties set, the constructed class method is called.

Objects which inherit from GObject are allowed to override this constructed class method.

The example below shows how ViewerFile overrides the parent's construction process:

#define VIEWER_TYPE_FILE viewer_file_get_type ()
G_DECLARE_FINAL_TYPE (ViewerFile, viewer_file, VIEWER, FILE, GObject)

struct _ViewerFile
{
  GObject parent_instance;

  /* instance members */
  gchar *filename;
  guint zoom_level;
};

/* will create viewer_file_get_type and set viewer_file_parent_class */
G_DEFINE_TYPE (ViewerFile, viewer_file, G_TYPE_OBJECT)

static void
viewer_file_constructed (GObject *obj)
{
  /* update the object state depending on constructor properties */

  /* Always chain up to the parent constructed function to complete object
   * initialisation. */
  G_OBJECT_CLASS (viewer_file_parent_class)->constructed (obj);
}

static void
viewer_file_finalize (GObject *obj)
{
  ViewerFile *self = VIEWER_FILE (obj);

  g_free (self->filename);

  /* Always chain up to the parent finalize function to complete object
   * destruction. */
  G_OBJECT_CLASS (viewer_file_parent_class)->finalize (obj);
}

static void
viewer_file_class_init (ViewerFileClass *klass)
{
  GObjectClass *object_class = G_OBJECT_CLASS (klass);

  object_class->constructed = viewer_file_constructed;
  object_class->finalize = viewer_file_finalize;
}

static void
viewer_file_init (ViewerFile *self)
{
  /* initialize the object */
}

If the user instantiates an object ViewerFile with:

ViewerFile *file = g_object_new (VIEWER_TYPE_FILE, NULL);

If this is the first instantiation of such an object, the viewer_file_class_init function will be invoked after any viewer_file_base_class_init function. This will make sure the class structure of this new object is correctly initialized. Here, viewer_file_class_init is expected to override the object's class methods and setup the class' own methods. In the example above, the constructed method is the only overridden method: it is set to viewer_file_constructed.

Once g_object_new has obtained a reference to an initialized class structure, it invokes its constructor method to create an instance of the new object, if the constructor has been overridden in viewer_file_class_init. Overridden constructors must chain up to their parent’s constructor. In order to find the parent class and chain up to the parent class constructor, we can use the viewer_file_parent_class pointer that has been set up for us by the G_DEFINE_TYPE macro.

Finally, at one point or another, g_object_constructor is invoked by the last constructor in the chain. This function allocates the object's instance buffer through g_type_create_instance which means that the instance_init function is invoked at this point if one was registered. After instance_init returns, the object is fully initialized and should be ready to have its methods called by the user. When g_type_create_instance returns, g_object_constructor sets the construction properties (i.e. the properties which were given to g_object_new) and returns to the user's constructor.

The process described above might seem a bit complicated, but it can be summarized easily by the table below which lists the functions invoked by g_object_new and their order of invocation:

<title>`g_object_new`</title> Invocation time Function invoked Function's parameters Remark First call to `g_object_new` for target type target type's `base_init` function On the inheritance tree of classes from fundamental type to target type. `base_init` is invoked once for each class structure. Never used in practice. Unlikely you will need it. target type's `class_init` function On target type's class structure Here, you should make sure to initialize or override class methods (that is, assign to each class' method its function pointer) and create the signals and the properties associated to your object. interface's `base_init` function On interface's vtable interface's `interface_init` function On interface's vtable Each call to `g_object_new` for target type target type's class `constructor` method: `GObjectClass->constructor` On object's instance If you need to handle construct properties in a custom way, or implement a singleton class, override the constructor method and make sure to chain up to the object's parent class before doing your own initialization. In doubt, do not override the constructor method. type's `instance_init` function On the inheritance tree of classes from fundamental type to target type. the `instance_init` provided for each type is invoked once for each instance structure. Provide an `instance_init` function to initialize your object before its construction properties are set. This is the preferred way to initialize a GObject instance. This function is equivalent to C++ constructors. target type's class `constructed` method: `GObjectClass->constructed` On object's instance If you need to perform object initialization steps after all construct properties have been set. This is the final step in the object initialization process, and is only called if the `constructor` method returned a new object instance (rather than, for example, an existing singleton).

Readers should feel concerned about one little twist in the order in which functions are invoked: while, technically, the class' constructor method is called before the GType's instance_init function (since g_type_create_instance which calls instance_init is called by g_object_constructor which is the top-level class constructor method and to which users are expected to chain to), the user's code which runs in a user-provided constructor will always run after GType's instance_init function since the user-provided constructor must (you've been warned) chain up before doing anything useful.

🍀 Object memory management

The memory-management API for GObjects is a bit complicated but the idea behind it is pretty simple: the goal is to provide a flexible model based on reference counting which can be integrated in applications which use or require different memory management models (such as garbage collection). The methods which are used to manipulate this reference count are described below.

🐣 Reference count

The functions g_object_ref/g_object_unref respectively increase and decrease the reference count. These functions are thread-safe. g_clear_object is a convenience wrapper around g_object_unref which also clears the pointer passed to it. The reference count is initialized to one by g_object_new which means that the caller is currently the sole owner of the newly-created reference. (If the object is derived from GInitiallyUnowned, this reference count is floating.) When the reference count reaches zero, that is, when g_object_unref is called by the last client holding a reference to the object, the dispose and the finalize class methods are invoked. Finally, after finalize is invoked, g_type_free_instance is called to free the object instance. Depending on the memory allocation policy decided when the type was registered (through one of the g_type_register_* functions), the object's instance memory will be freed or returned to the object pool for this type. Once the object has been freed, if it was the last instance of the type, the type's class will be destroyed as described in gtype-instantiatable-classed and gtype-non-instantiatable-non-classed.

The table below summarizes the destruction process of a GObject:

<title>`g_object_unref`</title> Invocation time Function invoked Function's parameters Remark Last call to `g_object_unref` for an instance of target type target type's dispose class function GObject instance When dispose ends, the object should not hold any reference to any other member object. The object is also expected to be able to answer client method invocations (with possibly an error code but no memory violation) until finalize is executed. dispose can be executed more than once. dispose should chain up to its parent implementation just before returning to the caller. target type's finalize class function GObject instance Finalize is expected to complete the destruction process initiated by dispose. It should complete the object's destruction. finalize will be executed only once. finalize should chain up to its parent implementation just before returning to the caller. The reason why the destruction process is split is two different phases is explained in gobject-memory-cycles. Last call to `g_object_unref` for the last instance of target type interface's `interface_finalize` function On interface's vtable Never used in practice. Unlikely you will need it. interface's `base_finalize` function On interface's vtable Never used in practice. Unlikely you will need it. target type's `class_finalize` function On target type's class structure Never used in practice. Unlikely you will need it. type's `base_finalize` function On the inheritance tree of classes from fundamental type to target type. `base_init` is invoked once for each class structure. Never used in practice. Unlikely you will need it.

🐣 Weak References

Weak references are used to monitor object finalization: g_object_weak_ref adds a monitoring callback which does not hold a reference to the object but which is invoked when the object runs its dispose method. As such, each weak ref can be invoked more than once upon object finalization (since dispose can run more than once during object finalization).

g_object_weak_unref can be used to remove a monitoring callback from the object.

Weak references are also used to implement g_object_add_weak_pointer and g_object_remove_weak_pointer. These functions add a weak reference to the object they are applied to which makes sure to nullify the pointer given by the user when object is finalized.

Similarly, GWeakRef can be used to implement weak references if thread safety is required.

🐣 Reference counts and cycles

GObject's memory management model was designed to be easily integrated in existing code using garbage collection. This is why the destruction process is split in two phases: the first phase, executed in the dispose handler is supposed to release all references to other member objects. The second phase, executed by the finalize handler is supposed to complete the object's destruction process. Object methods should be able to run without program error in-between the two phases.

This two-step destruction process is very useful to break reference counting cycles. While the detection of the cycles is up to the external code, once the cycles have been detected, the external code can invoke g_object_run_dispose which will indeed break any existing cycles since it will run the dispose handler associated to the object and thus release all references to other objects.

This explains one of the rules about the dispose handler stated earlier: the dispose handler can be invoked multiple times. Let's say we have a reference count cycle: object A references B which itself references object A. Let's say we have detected the cycle and we want to destroy the two objects. One way to do this would be to invoke g_object_run_dispose on one of the objects.

If object A releases all its references to all objects, this means it releases its reference to object B. If object B was not owned by anyone else, this is its last reference count which means this last unref runs B's dispose handler which, in turn, releases B's reference on object A. If this is A's last reference count, this last unref runs A's dispose handler which is running for the second time before A's finalize handler is invoked !

The above example, which might seem a bit contrived, can really happen if GObjects are being handled by language bindings — hence the rules for object destruction should be closely followed.

🍀 Object properties

One of GObject's nice features is its generic get/set mechanism for object properties. When an object is instantiated, the object's class_init handler should be used to register the object's properties with g_object_class_install_properties.

The best way to understand how object properties work is by looking at a real example

of how it is used:

/************************************************/
/* Implementation                               */
/************************************************/

typedef enum
{
  PROP_FILENAME = 1,
  PROP_ZOOM_LEVEL,
  N_PROPERTIES
} ViewerFileProperty;

static GParamSpec *obj_properties[N_PROPERTIES] = { NULL, };

static void
viewer_file_set_property (GObject      *object,
                          guint         property_id,
                          const GValue *value,
                          GParamSpec   *pspec)
{
  ViewerFile *self = VIEWER_FILE (object);

  switch ((ViewerFileProperty) property_id)
    {
    case PROP_FILENAME:
      g_free (self->filename);
      self->filename = g_value_dup_string (value);
      g_print ("filename: %s\n", self->filename);
      break;

    case PROP_ZOOM_LEVEL:
      self->zoom_level = g_value_get_uint (value);
      g_print ("zoom level: %u\n", self->zoom_level);
      break;

    default:
      /* We don't have any other property... */
      G_OBJECT_WARN_INVALID_PROPERTY_ID (object, property_id, pspec);
      break;
    }
}

static void
viewer_file_get_property (GObject    *object,
                          guint       property_id,
                          GValue     *value,
                          GParamSpec *pspec)
{
  ViewerFile *self = VIEWER_FILE (object);

  switch ((ViewerFileProperty) property_id)
    {
    case PROP_FILENAME:
      g_value_set_string (value, self->filename);
      break;

    case PROP_ZOOM_LEVEL:
      g_value_set_uint (value, self->zoom_level);
      break;

    default:
      /* We don't have any other property... */
      G_OBJECT_WARN_INVALID_PROPERTY_ID (object, property_id, pspec);
      break;
    }
}

static void
viewer_file_class_init (ViewerFileClass *klass)
{
  GObjectClass *object_class = G_OBJECT_CLASS (klass);

  object_class->set_property = viewer_file_set_property;
  object_class->get_property = viewer_file_get_property;

  obj_properties[PROP_FILENAME] =
    g_param_spec_string ("filename",
                         "Filename",
                         "Name of the file to load and display from.",
                         NULL  /* default value */,
                         G_PARAM_CONSTRUCT_ONLY | G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS);

  obj_properties[PROP_ZOOM_LEVEL] =
    g_param_spec_uint ("zoom-level",
                       "Zoom level",
                       "Zoom level to view the file at.",
                       0  /* minimum value */,
                       10 /* maximum value */,
                       2  /* default value */,
                       G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS);

  g_object_class_install_properties (object_class,
                                     N_PROPERTIES,
                                     obj_properties);
}

/************************************************/
/* Use                                          */
/************************************************/

ViewerFile *file;
GValue val = G_VALUE_INIT;

file = g_object_new (VIEWER_TYPE_FILE, NULL);

g_value_init (&val, G_TYPE_UINT);
g_value_set_char (&val, 11);

g_object_set_property (G_OBJECT (file), "zoom-level", &val);

g_value_unset (&val);

The client code above looks simple but a lot of things happen under the hood:

g_object_set_property first ensures a property with this name was registered in file's class_init handler. If so it walks the class hierarchy, from bottom-most most-derived type, to top-most fundamental type to find the class which registered that property. It then tries to convert the user-provided GValue into a GValue whose type is that of the associated property.

If the user provides a signed char GValue, as is shown here, and if the object's property was registered as an unsigned int, g_value_transform will try to transform the input signed char into an unsigned int. Of course, the success of the transformation depends on the availability of the required transform function. In practice, there will almost always be a transformation

Its behaviour might not be what you expect but it is up to you to actually avoid relying on these transformations.

which matches and conversion will be carried out if needed.

After transformation, the GValue is validated by g_param_value_validate which makes sure the user's data stored in the GValue matches the characteristics specified by the property's GParamSpec. Here, the GParamSpec we provided in class_init has a validation function which makes sure that the GValue contains a value which respects the minimum and maximum bounds of the GParamSpec. In the example above, the client's GValue does not respect these constraints (it is set to 11, while the maximum is 10). As such, the g_object_set_property function will return with an error.

If the user's GValue had been set to a valid value, g_object_set_property would have proceeded with calling the object's set_property class method. Here, since our implementation of ViewerFile did override this method, execution would jump to viewer_file_set_property after having retrieved from the GParamSpec the param_id which had been stored by g_object_class_install_property.

It should be noted that the param_id used here need only to uniquely identify each GParamSpec within the ViewerFileClass such that the switch used in the set and get methods actually works. Of course, this locally-unique integer is purely an optimization: it would have been possible to use a set of if (strcmp (a, b) == 0) {} else if (strcmp (a, b) == 0) {} statements.

Once the property has been set by the object's set_property class method, execution returns to g_object_set_property which makes sure that the "notify" signal is emitted on the object's instance with the changed property as parameter unless notifications were frozen by g_object_freeze_notify.

g_object_thaw_notify can be used to re-enable notification of property modifications through the “notify” signal. It is important to remember that even if properties are changed while property change notification is frozen, the "notify" signal will be emitted once for each of these changed properties as soon as the property change notification is thawed: no property change is lost for the "notify" signal, although multiple notifications for a single property are compressed. Signals can only be delayed by the notification freezing mechanism.

It sounds like a tedious task to set up GValues every time when one wants to modify a property. In practice one will rarely do this. The functions g_object_set_property and g_object_get_property are meant to be used by language bindings. For application there is an easier way and that is described next.

🐣 Accessing multiple properties at once

It is interesting to note that the g_object_set and g_object_set_valist (variadic version) functions can be used to set multiple properties at once. The client code shown above can then be re-written as:

ViewerFile *file;
file = /* */;
g_object_set (G_OBJECT (file),
              "zoom-level", 6, 
              "filename", "~/some-file.txt", 
              NULL);

This saves us from managing the GValues that we were needing to handle when using g_object_set_property. The code above will trigger one notify signal emission for each property modified.

Equivalent _get versions are also available: g_object_get and g_object_get_valist (variadic version) can be used to get numerous properties at once.

These high level functions have one drawback — they don't provide a return value. One should pay attention to the argument types and ranges when using them. A known source of errors is to pass a different type from what the property expects; for instance, passing an integer when the property expects a floating point value and thus shifting all subsequent parameters by some number of bytes. Also forgetting the terminating NULL will lead to undefined behaviour.

This explains how g_object_new, g_object_newv and g_object_new_valist work: they parse the user-provided variable number of parameters and invoke g_object_set on the parameters only after the object has been successfully constructed. The "notify" signal will be emitted for each property set.

📜 The GObject messaging system

🍀 Closures

Closures are central to the concept of asynchronous signal delivery which is widely used throughout GTK and GNOME applications. A closure is an abstraction, a generic representation of a callback. It is a small structure which contains three objects:

A function pointer (the callback itself) whose prototype looks like:

return_type function_callback (… , gpointer user_data);

The user_data pointer which is passed to the callback upon invocation of the closure.

A function pointer which represents the destructor of the closure: whenever the closure's refcount reaches zero, this function will be called before the closure structure is freed.

The GClosure structure represents the common functionality of all closure implementations: there exists a different closure implementation for each separate runtime which wants to use the GObject type system.

In practice, closures sit at the boundary of language runtimes: if you are writing Python code and one of your Python callbacks receives a signal from a GTK widget, the C code in GTK needs to execute your Python code. The closure invoked by the GTK object invokes the Python callback: it behaves as a normal C object for GTK and as a normal Python object for Python code.

The GObject library provides a simple GCClosure type which is a specific implementation of closures to be used with C/C++ callbacks.

A GClosure provides simple services:

0. Invocation (g_closure_invoke): this is what closures were created for: they hide the details of callback invocation from the callback invoker.

1. Notification: the closure notifies listeners of certain events such as closure invocation, closure invalidation and closure finalization. Listeners can be registered with g_closure_add_finalize_notifier (finalization notification), g_closure_add_invalidate_notifier (invalidation notification) and g_closure_add_marshal_guards (invocation notification). There exist symmetric deregistration functions for finalization and invalidation events (g_closure_remove_finalize_notifier and g_closure_remove_invalidate_notifier) but not for the invocation process.

Closures are reference counted and notify listeners of their destruction in a two-stage process: the invalidation notifiers are invoked before the finalization notifiers.

🐣 C Closures

If you are using C or C++ to connect a callback to a given event, you will either use simple GCClosures which have a pretty minimal API or the even simpler g_signal_connect functions (which will be presented a bit later).

g_cclosure_new will create a new closure which can invoke the user-provided callback_func with the user-provided user_data as its last parameter. When the closure is finalized (second stage of the destruction process), it will invoke the destroy_data function if the user has supplied one.

g_cclosure_new_swap will create a new closure which can invoke the user-provided callback_func with the user-provided user_data as its first parameter (instead of being the last parameter as with g_cclosure_new). When the closure is finalized (second stage of the destruction process), it will invoke the destroy_data function if the user has supplied one.

🐣 Non-C closures (for the fearless)

As was explained above, closures hide the details of callback invocation. In C, callback invocation is just like function invocation: it is a matter of creating the correct stack frame for the called function and executing a call assembly instruction.

C closure marshallers transform the array of GValues which represent the parameters to the target function into a C-style function parameter list, invoke the user-supplied C function with this new parameter list, get the return value of the function, transform it into a GValue and return this GValue to the marshaller caller.

A generic C closure marshaller is available as g_cclosure_marshal_generic which implements marshalling for all function types using libffi. Custom marshallers for different types are not needed apart from performance critical code where the libffi-based marshaller may be too slow.

An example of a custom marshaller is given below, illustrating how GValues can be converted to a C function call. The marshaller is for a C function which takes an integer as its first parameter and returns void.

g_cclosure_marshal_VOID__INT (GClosure     *closure,
                              GValue       *return_value,
                              guint         n_param_values,
                              const GValue *param_values,
                              gpointer      invocation_hint,
                              gpointer      marshal_data)
{
  typedef void (*GMarshalFunc_VOID__INT) (gpointer     data1,
                                          gint         arg_1,
                                          gpointer     data2);
  register GMarshalFunc_VOID__INT callback;
  register GCClosure *cc = (GCClosure*) closure;
  register gpointer data1, data2;

  g_return_if_fail (n_param_values == 2);

  data1 = g_value_peek_pointer (param_values + 0);
  data2 = closure->data;

  callback = (GMarshalFunc_VOID__INT) (marshal_data ? marshal_data : cc->callback);

  callback (data1,
            g_marshal_value_peek_int (param_values + 1),
            data2);
}

There exist other kinds of marshallers, for example there is a generic Python marshaller which is used by all Python closures (a Python closure is used to invoke a callback written in Python). This Python marshaller transforms the input GValue list representing the function parameters into a Python tuple which is the equivalent structure in Python.

🍀 Signals

GObject's signals have nothing to do with standard UNIX signals: they connect arbitrary application-specific events with any number of listeners. For example, in GTK, every user event (keystroke or mouse move) is received from the windowing system and generates a GTK event in the form of a signal emission on the widget object instance.

Each signal is registered in the type system together with the type on which it can be emitted: users of the type are said to connect to the signal on a given type instance when they register a closure to be
invoked upon the signal emission. The closure will be called synchronously on emission. Users can also emit the signal by themselves or stop the emission of the signal from within one of the closures connected to the signal.

When a signal is emitted on a given type instance, all the closures connected to this signal on this type instance will be invoked. All the closures connected to such a signal represent callbacks whose signature looks like:

return_type function_callback (gpointer instance, …, gpointer user_data);

🐣 Signal registration

To register a new signal on an existing type, we can use any of g_signal_newv, g_signal_new_valist or g_signal_new functions:

guint g_signal_newv (const gchar        *signal_name,
                     GType               itype,
                     GSignalFlags        signal_flags,
                     GClosure           *class_closure,
                     GSignalAccumulator  accumulator,
                     gpointer            accu_data,
                     GSignalCMarshaller  c_marshaller,
                     GType               return_type,
                     guint               n_params,
                     GType              *param_types);

The number of parameters to these functions is a bit intimidating but they are relatively simple:

0. signal_name: is a string which can be used to uniquely identify a given signal.
1. itype: is the instance type on which this signal can be emitted.
2. signal_flags: partly defines the order in which closures which were connected to the signal are invoked.
3. class_closure: this is the default closure for the signal: if it is not NULL upon the signal emission, it will be invoked upon this emission of the signal. The moment where this closure is invoked compared to other closures connected to that signal depends partly on the signal_flags.
4. accumulator: this is a function pointer which is invoked after each closure has been invoked. If it returns FALSE, signal emission is stopped. If it returns TRUE, signal emission proceeds normally. It is also used to compute the return value of the signal based on the return value of all the invoked closures. For example, an accumulator could ignore NULL returns from closures; or it could build a list of the values returned by the closures.
5. accu_data: this pointer will be passed down to each invocation of the accumulator during emission.
6. c_marshaller: this is the default C marshaller for any closure which is connected to this signal.
7. return_type: this is the type of the return value of the signal.
8. n_params: this is the number of parameters this signal takes.
9. param_types: this is an array of GTypes which indicate the type of each parameter of the signal. The length of this array is indicated by n_params.

As you can see from the above definition, a signal is basically a description of the closures which can be connected to this signal and a description of the order in which the closures connected to this signal will be invoked.

🐣 Signal connection

If you want to connect to a signal with a closure, you have three possibilities:

0. You can register a class closure at signal registration: this is a system-wide operation. i.e.: the class closure will be invoked during each emission of a given signal on any of the instances of the type which supports that signal.
1. You can use g_signal_override_class_closure which overrides the class closure of a given type. It is possible to call this function only on a derived type of the type on which the signal was registered. This function is of use only to language bindings.
2. You can register a closure with the g_signal_connect family of functions. This is an instance-specific operation: the closure will be invoked only during emission of a given signal on a given instance.

It is also possible to connect a different kind of callback on a given signal: emission hooks are invoked whenever a given signal is emitted whatever the instance on which it is emitted. Emission hooks are used for example to get all mouse_clicked emissions in an application to be able to emit the small mouse click sound. Emission hooks are connected with g_signal_add_emission_hook and removed with g_signal_remove_emission_hook.

🐣 Signal emission

Signal emission is done through the use of the g_signal_emit family of functions.

void g_signal_emitv (const GValue *instance_and_params,
                     guint         signal_id,
                     GQuark        detail,
                     GValue       *return_value);

0. The instance_and_params array of GValues contains the list of input parameters to the signal. The first element of the array is the instance pointer on which to invoke the signal. The following elements of the array contain the list of parameters to the signal.
1. signal_id identifies the signal to invoke.
2. detail identifies the specific detail of the signal to invoke. A detail is a kind of magic token/argument which is passed around during signal emission and which is used by closures connected to the signal to filter out unwanted signal emissions. In most cases, you can safely set this value to zero. See signal-detail for more details about this parameter.
3. return_value holds the return value of the last closure invoked during emission if no accumulator was specified. If an accumulator was specified during signal creation, this accumulator is used to calculate the return value as a function of the return values of all the closures invoked during emission. If no closure is invoked during emission, the return_value is nonetheless initialized to zero/null.

Signal emission is done synchronously and can be decomposed in 5 steps:

0. RUN_FIRST: if the G_SIGNAL_RUN_FIRST flag was used during signal registration and if there exists a class closure for this signal, the class closure is invoked.
1. EMISSION_HOOK: if any emission hook was added to the signal, they are invoked from first to last added. Accumulate return values.
2. HANDLER_RUN_FIRST: if any closure were connected with the g_signal_connect family of functions, and if they are not blocked (with the g_signal_handler_block family of functions) they are run here, from first to last connected.
3. RUN_LAST: if the G_SIGNAL_RUN_LAST flag was set during registration and if a class closure was set, it is invoked here.
4. HANDLER_RUN_LAST: if any closure were connected with the g_signal_connect_after family of functions, if they were not invoked during HANDLER_RUN_FIRST and if they are not blocked, they are run here, from first to last connected.
5. RUN_CLEANUP: if the G_SIGNAL_RUN_CLEANUP flag was set during registration and if a class closure was set, it is invoked here. Signal emission is completed here.

If, at any point during emission (except in RUN_CLEANUP or EMISSION_HOOK state), one of the closures stops the signal emission with g_signal_stop_emission, emission jumps to RUN_CLEANUP state.

If, at any point during emission, one of the closures or emission hook emits the same signal on the same instance, emission is restarted from the RUN_FIRST state.

The accumulator function is invoked in all states, after invocation of each closure (except in RUN_EMISSION_HOOK and RUN_CLEANUP). It accumulates the closure return value into the signal return value and returns TRUE or FALSE. If, at any point, it does not return TRUE, emission jumps to RUN_CLEANUP state.

If no accumulator function was provided, the value returned by the last handler run will be returned by g_signal_emit.

🍀 The detail argument

All the functions related to signal emission or signal connection have a parameter named the detail. Sometimes, this parameter is hidden by the API but it is always there, in one form or another.

Of the three main connection functions, only one has an explicit detail parameter as a GQuark: g_signal_connect_closure_by_id.

A GQuark is an integer which uniquely represents a string. It is possible to transform back and forth between the integer and string representations with the functions g_quark_from_string and g_quark_to_string.

The two other functions, g_signal_connect_closure and g_signal_connect_data hide the detail parameter in the signal name identification. Their detailed_signal parameter is a string which identifies the name of the signal to connect to. The format of this string should match signal_name::detail_name. For example, connecting to the signal named notify::cursor_position will actually connect to the signal named notify with the cursor_position detail. Internally, the detail string is transformed to a GQuark if it is present.

Of the four main signal emission functions, one hides it in its signal name parameter: g_signal_connect. The other three have an explicit detail parameter as a GQuark again: g_signal_emit, g_signal_emitv and g_signal_emit_valist.

If a detail is provided by the user to the emission function, it is used during emission to match against the closures which also provide a detail. If a closure's detail does not match the detail provided by the user, it will not be invoked (even though it is connected to a signal which is being emitted).

This completely optional filtering mechanism is mainly used as an optimization for signals which are often emitted for many different reasons: the clients can filter out which events they are interested in before the closure's marshalling code runs. For example, this is used extensively by the notify signal of GObject: whenever a property is modified on a GObject, instead of just emitting the notify signal, GObject associates as a detail to this signal emission the name of the property modified. This allows clients who wish to be notified of changes to only one property to filter most events before receiving them.

As a simple rule, users can and should set the detail parameter to zero: this will disable completely this optional filtering for that signal.

📜 GObject Howto Tutorial

This chapter tries to answer the real-life questions of users and presents the most common use cases in order from most likely to least likely.

🍀 How to define and implement a new GObject

This chapter focuses on the implementation of a subtype of GObject, for example to create a custom class hierarchy, or to subclass a GTK widget.

Throughout the chapter, a running example of a file viewer program is used, which has a ViewerFile class to represent a single file being viewed, and various derived classes for different types of files with special functionality, such as audio files. The example application also supports editing files (for example, to tweak a photo being viewed), using a ViewerEditable interface.

🐣 Boilerplate header code

The first step before writing the code for your GObject is to write the type's header which contains the needed type, function and macro definitions. Each of these elements is nothing but a convention which is followed by almost all users of GObject, and has been refined over multiple years of experience developing GObject-based code. If you are writing a library, it is particularly important for you to adhere closely to these conventions; users of your library will assume that you have. Even if not writing a library, it will help other people who want to work on your project. Pick a name convention for your headers and source code and stick to it:

0. use a dash to separate the prefix from the typename: viewer-file.h and viewer-file.c (this is the convention used by Nautilus and most GNOME libraries).
1. use an underscore to separate the prefix from the typename: viewer_file.h and viewer_file.c.
2. Do not separate the prefix from the typename: viewerfile.h and viewerfile.c. (this is the convention used by GTK)

Some people like the first two solutions better: it makes reading file names easier for those with poor eyesight. The basic conventions for any header which exposes a GType are described in gtype-conventions. If you want to declare a type named ‘file’ in namespace ‘viewer’, name the type instance ViewerFile and its class ViewerFileClass (names are case sensitive). The recommended method of declaring a type differs based on whether the type is final or derivable. Final types cannot be subclassed further, and should be the default choice for new types — changing a final type to be derivable is always a change that will be compatible with existing uses of the code, but the converse will often cause problems. Final types are declared using G_DECLARE_FINAL_TYPE, and require a structure to hold the instance data to be declared in the source code (not the header file).

/*
 * Copyright/Licensing information.
 */

/* inclusion guard */
#ifndef __VIEWER_FILE_H__
#define __VIEWER_FILE_H__

#include <glib-object.h>
/*
 * Potentially, include other headers on which this header depends.
 */

G_BEGIN_DECLS

/*
 * Type declaration.
 */
#define VIEWER_TYPE_FILE viewer_file_get_type ()
G_DECLARE_FINAL_TYPE (ViewerFile, viewer_file, VIEWER, FILE, GObject)

/*
 * Method definitions.
 */
ViewerFile *viewer_file_new (void);

G_END_DECLS

#endif /* __VIEWER_FILE_H__ */

Derivable types can be subclassed further, and their class and instance structures form part of the public API which must not be changed if API stability is cared about. They are declared using G_DECLARE_DERIVABLE_TYPE:

/*
 * Copyright/Licensing information.
 */

/* inclusion guard */
#ifndef __VIEWER_FILE_H__
#define __VIEWER_FILE_H__

#include <glib-object.h>
/*
 * Potentially, include other headers on which this header depends.
 */

G_BEGIN_DECLS

/*
 * Type declaration.
 */
#define VIEWER_TYPE_FILE viewer_file_get_type ()
G_DECLARE_DERIVABLE_TYPE (ViewerFile, viewer_file, VIEWER, FILE, GObject)

struct _ViewerFileClass
{
  GObjectClass parent_class;

  /* Class virtual function fields. */
  void (* open) (ViewerFile  *file,
                 GError     **error);

  /* Padding to allow adding up to 12 new virtual functions without
   * breaking ABI. */
  gpointer padding[12];
};

/*
 * Method definitions.
 */
ViewerFile *viewer_file_new (void);

G_END_DECLS

#endif /* __VIEWER_FILE_H__ */

The convention for header includes is to add the minimum number of #include directives to the top of your headers needed to compile that header. This allows client code to simply #include "viewer-file.h", without needing to know the prerequisites for viewer-file.h.

🐣 Boilerplate code

In your code, the first step is to #include the needed headers:

/*
 * Copyright information
 */

#include "viewer-file.h"

/* Private structure definition. */
typedef struct {
  gchar *filename;
  /* stuff */
} ViewerFilePrivate;

/* 
 * forward definitions
 */

If the class is being declared as final using G_DECLARE_FINAL_TYPE, its instance structure should be defined in the C file:

struct _ViewerFile
{
  GObject parent_instance;

  /* Other members, including private data. */
};

Call the G_DEFINE_TYPE macro (or G_DEFINE_TYPE_WITH_PRIVATE if your class needs private data — final types do not need private data) using the name of the type, the prefix of the functions and the parent GType to reduce the amount of boilerplate needed. This macro will:

0. implement the viewer_file_get_type function
1. define a parent class pointer accessible from the whole .c file
2. add private instance data to the type (if using G_DEFINE_TYPE_WITH_PRIVATE)

If the class has been declared as final using G_DECLARE_FINAL_TYPE (see howto-gobject-header), private data should be placed in the instance structure, ViewerFile, and G_DEFINE_TYPE should be used instead of G_DEFINE_TYPE_WITH_PRIVATE. The instance structure for a final class is not exposed publicly, and is not embedded in the instance structures of any derived classes (because the class is final); so its size can vary without causing incompatibilities for code which uses the class. Conversely, private data for derivable classes must be included in a private structure, and G_DEFINE_TYPE_WITH_PRIVATE must be used.

G_DEFINE_TYPE (ViewerFile, viewer_file, G_TYPE_OBJECT)

or

G_DEFINE_TYPE_WITH_PRIVATE (ViewerFile, viewer_file, G_TYPE_OBJECT)

It is also possible to use the G_DEFINE_TYPE_WITH_CODE macro to control the get_type function implementation — for instance, to add a call to the G_IMPLEMENT_INTERFACE macro to implement an interface.

🐣 Object construction

People often get confused when trying to construct their GObjects because of the sheer number of different ways to hook into the objects's construction process: it is difficult to figure which is the correct, recommended way. gobject-construction-table shows what user-provided functions are invoked during object instantiation and in which order they are invoked. A user looking for the equivalent of the simple C++ constructor function should use the instance_init method. It will be invoked after all the parents’ instance_init functions have been invoked. It cannot take arbitrary construction parameters (as in C++) but if your object needs arbitrary parameters to complete initialization, you can use construction properties. Construction properties will be set only after all instance_init functions have run. No object reference will be returned to the client of g_object_new until all the construction properties have been set. It is important to note that object construction cannot ever fail. If you require a fallible GObject construction, you can use the GInitable and GAsyncInitable interfaces provided by the GIO library. You should write the following code first:

G_DEFINE_TYPE_WITH_PRIVATE (ViewerFile, viewer_file, G_TYPE_OBJECT)

static void
viewer_file_class_init (ViewerFileClass *klass) { }

static void
viewer_file_init (ViewerFile *self)
{
  ViewerFilePrivate *priv = viewer_file_get_instance_private (self);

  /* initialize all public and private members to reasonable default values.
   * They are all automatically initialized to 0 to begin with. */
}

If you need special construction properties (with G_PARAM_CONSTRUCT_ONLY set), install the properties in the class_init() function, override the set_property() and get_property() methods of the GObject class, and implement them as described by gobject-properties. Property IDs must start from 1, as 0 is reserved for internal use by GObject.

enum
{
  PROP_FILENAME = 1,
  PROP_ZOOM_LEVEL,
  N_PROPERTIES
};

static GParamSpec *obj_properties[N_PROPERTIES] = { NULL, };

static void
viewer_file_class_init (ViewerFileClass *klass)
{
  GObjectClass *object_class = G_OBJECT_CLASS (klass);

  object_class->set_property = viewer_file_set_property;
  object_class->get_property = viewer_file_get_property;

  obj_properties[PROP_FILENAME] =
    g_param_spec_string ("filename",
                         "Filename",
                         "Name of the file to load and display from.",
                         NULL  /* default value */,
                         G_PARAM_CONSTRUCT_ONLY | G_PARAM_READWRITE);

  obj_properties[PROP_ZOOM_LEVEL] =
    g_param_spec_uint ("zoom-level",
                       "Zoom level",
                       "Zoom level to view the file at.",
                       0  /* minimum value */,
                       10 /* maximum value */,
                       2  /* default value */,
                       G_PARAM_READWRITE);

  g_object_class_install_properties (object_class,
                                     N_PROPERTIES,
                                     obj_properties);
}

If you need this, make sure you can build and run code similar to the code shown above. Also, make sure your construct properties can be set without side effects during construction. Some people sometimes need to complete the initialization of an instance of a type only after the properties passed to the constructors have been set. This is possible through the use of the constructor() class method as described in gobject-instantiation or, more simply, using the constructed() class method. Note that the constructed() virtual function will only be invoked after the properties marked as G_PARAM_CONSTRUCT_ONLY or G_PARAM_CONSTRUCT have been consumed, but before the regular properties passed to g_object_new() have been set.

🐣 Object destruction

Again, it is often difficult to figure out which mechanism to use to hook into the object's destruction process: when the last g_object_unref function call is made, a lot of things happen as described in gobject-destruction-table. The destruction process of your object is in two phases: dispose and finalize. This split is necessary to handle potential cycles due to the nature of the reference counting mechanism used by GObject, as well as dealing with temporary revival of instances in case of signal emission during the destruction sequence. See gobject-memory-cycles for more information.

struct _ViewerFilePrivate
{
  gchar *filename;
  guint zoom_level;

  GInputStream *input_stream;
};

G_DEFINE_TYPE_WITH_PRIVATE (ViewerFile, viewer_file, G_TYPE_OBJECT)

static void
viewer_file_dispose (GObject *gobject)
{
  ViewerFilePrivate *priv = viewer_file_get_instance_private (VIEWER_FILE (gobject));

  /* In dispose(), you are supposed to free all types referenced from this
   * object which might themselves hold a reference to self. Generally,
   * the most simple solution is to unref all members on which you own a 
   * reference.
   */

  /* dispose() might be called multiple times, so we must guard against
   * calling g_object_unref() on an invalid GObject by setting the member
   * NULL; g_clear_object() does this for us.
   */
  g_clear_object (&priv->input_stream);

  /* Always chain up to the parent class; there is no need to check if
   * the parent class implements the dispose() virtual function: it is
   * always guaranteed to do so
   */
  G_OBJECT_CLASS (viewer_file_parent_class)->dispose (gobject);
}

static void
viewer_file_finalize (GObject *gobject)
{
  ViewerFilePrivate *priv = viewer_file_get_instance_private (VIEWER_FILE (gobject));

  g_free (priv->filename);

  /* Always chain up to the parent class; as with dispose(), finalize()
   * is guaranteed to exist on the parent's class virtual function table
   */
  G_OBJECT_CLASS (viewer_file_parent_class)->finalize (gobject);
}

static void
viewer_file_class_init (ViewerFileClass *klass)
{
  GObjectClass *object_class = G_OBJECT_CLASS (klass);

  object_class->dispose = viewer_file_dispose;
  object_class->finalize = viewer_file_finalize;
}

static void
viewer_file_init (ViewerFile *self);
{
  ViewerFilePrivate *priv = viewer_file_get_instance_private (self);

  priv->input_stream = g_object_new (VIEWER_TYPE_INPUT_STREAM, NULL);
  priv->filename = /* would be set as a property */;
}

It is possible that object methods might be invoked after dispose is run and before finalize runs. GObject does not consider this to be a program error: you must gracefully detect this and neither crash nor warn the user, by having a disposed instance revert to an inert state.

🐣 Object methods

Just as with C++, there are many different ways to define object methods and extend them: the following list and sections draw on C++ vocabulary. (Readers are expected to know basic C++ concepts. Those who have not had to write C++ code recently can refer to e.g. to refresh their memories.)

0. non-virtual public methods,
1. virtual public methods and
2. virtual private methods

🐣 Non-virtual public methods

These are the simplest, providing a simple method which acts on the object. Provide a function prototype in the header and an implementation of that prototype in the source file.

/* declaration in the header. */
void viewer_file_open (ViewerFile  *self,
                       GError     **error);

/* implementation in the source file */
void
viewer_file_open (ViewerFile  *self,
                  GError     **error)
{
  g_return_if_fail (VIEWER_IS_FILE (self));
  g_return_if_fail (error == NULL || *error == NULL);

  /* do stuff here. */
}

🐣 Virtual public methods

This is the preferred way to create GObjects with overridable methods:

0. Define the common method and its virtual function in the class structure in the public header

1. Define the common method in the header file and implement it in the source file

2. Implement a base version of the virtual function in the source file and initialize the virtual function pointer to this implementation in the object’s class_init function; or leave it as NULL for a ‘pure virtual’ method which must be overridden by derived classes

3. Re-implement the virtual function in each derived class which needs to override it

Note that virtual functions can only be defined if the class is derivable, declared using G_DECLARE_DERIVABLE_TYPE so the class structure can be defined.

/* declaration in viewer-file.h. */
#define VIEWER_TYPE_FILE viewer_file_get_type ()
G_DECLARE_DERIVABLE_TYPE (ViewerFile, viewer_file, VIEWER, FILE, GObject)

struct _ViewerFileClass
{
  GObjectClass parent_class;

  /* stuff */
  void (*open) (ViewerFile  *self,
                GError     **error);

  /* Padding to allow adding up to 12 new virtual functions without
   * breaking ABI. */
  gpointer padding[12];
};

void viewer_file_open (ViewerFile  *self,
                       GError     **error);

/* implementation in viewer-file.c */
void
viewer_file_open (ViewerFile  *self,
                  GError     **error)
{
  ViewerFileClass *klass;

  g_return_if_fail (VIEWER_IS_FILE (self));
  g_return_if_fail (error == NULL || *error == NULL);

  klass = VIEWER_FILE_GET_CLASS (self);
  g_return_if_fail (klass->open != NULL);

  klass->open (self, error);
}

The code above simply redirects the open call to the relevant virtual function.

It is possible to provide a default implementation for this class method in the object's class_init function: initialize the klass->open field to a pointer to the actual implementation. By default, class methods that are not inherited are initialized to NULL, and thus are to be considered "pure virtual".

static void
viewer_file_real_close (ViewerFile  *self,
                        GError     **error)
{
  /* Default implementation for the virtual method. */
}

static void
viewer_file_class_init (ViewerFileClass *klass)
{
  /* this is not necessary, except for demonstration purposes.
   *
   * pure virtual method: mandates implementation in children.
   */
  klass->open = NULL;

  /* merely virtual method. */
  klass->close = viewer_file_real_close;
}

void
viewer_file_open (ViewerFile  *self,
                  GError     **error)
{
  ViewerFileClass *klass;

  g_return_if_fail (VIEWER_IS_FILE (self));
  g_return_if_fail (error == NULL || *error == NULL);

  klass = VIEWER_FILE_GET_CLASS (self);

  /* if the method is purely virtual, then it is a good idea to
   * check that it has been overridden before calling it, and,
   * depending on the intent of the class, either ignore it silently
   * or warn the user.
   */
  g_return_if_fail (klass->open != NULL);
  klass->open (self, error);
}

void
viewer_file_close (ViewerFile  *self,
                   GError     **error)
{
  ViewerFileClass *klass;

  g_return_if_fail (VIEWER_IS_FILE (self));
  g_return_if_fail (error == NULL || *error == NULL);

  klass = VIEWER_FILE_GET_CLASS (self);
  if (klass->close != NULL)
    klass->close (self, error);
}

🐣 Virtual private Methods

These are very similar to virtual public methods. They just don't have a public function to call directly. The header file contains only a declaration of the virtual function:

/* declaration in viewer-file.h. */
struct _ViewerFileClass
{
  GObjectClass parent;

  /* Public virtual method as before. */
  void     (*open)           (ViewerFile  *self,
                              GError     **error);

  /* Private helper function to work out whether the file can be loaded via
   * memory mapped I/O, or whether it has to be read as a stream. */
  gboolean (*can_memory_map) (ViewerFile *self);

  /* Padding to allow adding up to 12 new virtual functions without
   * breaking ABI. */
  gpointer padding[12];
};

void viewer_file_open (ViewerFile *self, GError **error);

These virtual functions are often used to delegate part of the job to child classes:

/* this accessor function is static: it is not exported outside of this file. */
static gboolean 
viewer_file_can_memory_map (ViewerFile *self)
{
  return VIEWER_FILE_GET_CLASS (self)->can_memory_map (self);
}

void
viewer_file_open (ViewerFile  *self,
                  GError     **error)
{
  g_return_if_fail (VIEWER_IS_FILE (self));
  g_return_if_fail (error == NULL || *error == NULL);

  /*
   * Try to load the file using memory mapped I/O, if the implementation of the
   * class determines that is possible using its private virtual method.
   */
  if (viewer_file_can_memory_map (self))
    {
      /* Load the file using memory mapped I/O. */
    }
  else
    {
      /* Fall back to trying to load the file using streaming I/O… */
    }
}

Again, it is possible to provide a default implementation for this private virtual function:

static gboolean
viewer_file_real_can_memory_map (ViewerFile *self)
{
  /* As an example, always return false. Or, potentially return true if the
   * file is local. */
  return FALSE;
}

static void
viewer_file_class_init (ViewerFileClass *klass)
{
  /* non-pure virtual method; does not have to be implemented in children. */
  klass->can_memory_map = viewer_file_real_can_memory_map;
}

Derived classes can then override the method with code such as:

static void
viewer_audio_file_class_init (ViewerAudioFileClass *klass)
{
  ViewerFileClass *file_class = VIEWER_FILE_CLASS (klass);

  /* implement pure virtual function. */
  file_class->can_memory_map = viewer_audio_file_can_memory_map;
}

🐣 Chaining up

Chaining up is often loosely defined by the following set of conditions:

0. Parent class A defines a public virtual method named foo and provides a default implementation.
1. Child class B re-implements method foo.
2. B’s implementation of foo calls (‘chains up to’) its parent class A’s implementation of foo. There are various uses of this idiom:

0. You need to extend the behaviour of a class without modifying its code. You create a subclass to inherit its implementation, re-implement a public virtual method to modify the behaviour and chain up to ensure that the previous behaviour is not really modified, just extended.

0. You need to implement the Chain Of Responsibility pattern: each object of the inheritance tree chains up to its parent (typically, at the beginning or the end of the method) to ensure that each handler is run in turn. To explicitly chain up to the implementation of the virtual method in the parent class, you first need a handle to the original parent class structure. This pointer can then be used to access the original virtual function pointer and invoke it directly.

The *original* adjective used in this sentence is not innocuous. To fully understand its meaning, recall how class structures are initialized: for each object type, the class structure associated with this object is created by first copying the class structure of its parent type (a simple `memcpy`) and then by invoking the `class_init` callback on the resulting class structure. Since the `class_init` callback is responsible for overwriting the class structure with the user re-implementations of the class methods, the modified copy of the parent class structure stored in the derived instance cannot be used. A copy of the class structure of an instance of the parent class is needed. Use the `parent_class` pointer created and initialized by the `G_DEFINE_TYPE` family of macros, for instance:

static void
b_method_to_call (B *obj, gint some_param)
{
  /* do stuff before chain up */

  /* call the method_to_call() virtual function on the
   * parent of BClass, AClass.
   *
   * remember the explicit cast to AClass*
   */
  A_CLASS (b_parent_class)->method_to_call (obj, some_param);

  /* do stuff after chain up */
}

🍀 How to define and implement interfaces

🐣 Defining interfaces

The theory behind how GObject interfaces work is given in gtype-non-instantiatable-non-classed; this section covers how to define and implement an interface.

The first step is to get the header right. This interface defines three methods:

/*
 * Copyright/Licensing information.
 */

#ifndef __VIEWER_EDITABLE_H__
#define __VIEWER_EDITABLE_H__

#include <glib-object.h>

G_BEGIN_DECLS

#define VIEWER_TYPE_EDITABLE viewer_editable_get_type ()
G_DECLARE_INTERFACE (ViewerEditable, viewer_editable, VIEWER, EDITABLE, GObject)

struct _ViewerEditableInterface
{
  GTypeInterface parent_iface;

  void (*save) (ViewerEditable  *self,
                GError         **error);
  void (*undo) (ViewerEditable  *self,
                guint            n_steps);
  void (*redo) (ViewerEditable  *self,
                guint            n_steps);
};

void viewer_editable_save (ViewerEditable  *self,
                           GError         **error);
void viewer_editable_undo (ViewerEditable  *self,
                           guint            n_steps);
void viewer_editable_redo (ViewerEditable  *self,
                           guint            n_steps);

G_END_DECLS

#endif /* __VIEWER_EDITABLE_H__ */

This code is the same as the code for a normal GType which derives from a GObject except for a few details:

0. The _GET_CLASS function is called _GET_IFACE (and is defined by G_DECLARE_INTERFACE).

1. The instance type, ViewerEditable, is not fully defined: it is used merely as an abstract type which represents an instance of whatever object which implements the interface.

2. The parent of the ViewerEditableInterface is GTypeInterface, not GObjectClass.

The implementation of the ViewerEditable type itself is trivial:

0. G_DEFINE_INTERFACE creates a viewer_editable_get_type function which registers the type in the type system. The third argument is used to define a prerequisite interface (which we'll talk about more later). Just pass 0 for this argument when an interface has no prerequisite.

1. viewer_editable_default_init is expected to register the interface's signals if there are any (we will see a bit later how to use them).

2. The interface methods viewer_editable_save, viewer_editable_undo and viewer_editable_redo dereference the interface structure to access its associated interface function and call it.

G_DEFINE_INTERFACE (ViewerEditable, viewer_editable, G_TYPE_OBJECT)

static void
viewer_editable_default_init (ViewerEditableInterface *iface)
{
    /* add properties and signals to the interface here */
}

void
viewer_editable_save (ViewerEditable  *self,
                      GError         **error)
{
  ViewerEditableInterface *iface;

  g_return_if_fail (VIEWER_IS_EDITABLE (self));
  g_return_if_fail (error == NULL || *error == NULL);

  iface = VIEWER_EDITABLE_GET_IFACE (self);
  g_return_if_fail (iface->save != NULL);
  iface->save (self, error);
}

void
viewer_editable_undo (ViewerEditable *self,
                      guint           n_steps)
{
  ViewerEditableInterface *iface;

  g_return_if_fail (VIEWER_IS_EDITABLE (self));

  iface = VIEWER_EDITABLE_GET_IFACE (self);
  g_return_if_fail (iface->undo != NULL);
  iface->undo (self, n_steps);
}

void
viewer_editable_redo (ViewerEditable *self,
                      guint           n_steps)
{
  ViewerEditableInterface *iface;

  g_return_if_fail (VIEWER_IS_EDITABLE (self));

  iface = VIEWER_EDITABLE_GET_IFACE (self);
  g_return_if_fail (iface->redo != NULL);
  iface->redo (self, n_steps);
}

🐣 Implementing interfaces

Once the interface is defined, implementing it is rather trivial. The first step is to define a normal final GObject class exactly as in howto-gobject-header. The second step is to implement ViewerFile by defining it using G_DEFINE_TYPE_WITH_CODE and G_IMPLEMENT_INTERFACE instead of G_DEFINE_TYPE:

static void viewer_file_editable_interface_init (ViewerEditableInterface *iface);

G_DEFINE_TYPE_WITH_CODE (ViewerFile, viewer_file, G_TYPE_OBJECT,
                         G_IMPLEMENT_INTERFACE (VIEWER_TYPE_EDITABLE,
                                                viewer_file_editable_interface_init))

This definition is very much like all the similar functions seen previously. The only interface-specific code present here is the use of G_IMPLEMENT_INTERFACE.

Classes can implement multiple interfaces by using multiple calls to G_IMPLEMENT_INTERFACE inside the call to G_DEFINE_TYPE_WITH_CODE.

viewer_file_editable_interface_init, the interface initialization function: inside it every virtual method of the interface must be assigned to its implementation:

static void
viewer_file_editable_save (ViewerFile  *self,
                           GError     **error)
{
  g_print ("File implementation of editable interface save method: %s.\n",
           self->filename);
}

static void
viewer_file_editable_undo (ViewerFile *self,
                           guint       n_steps)
{
  g_print ("File implementation of editable interface undo method: %s.\n",
           self->filename);
}

static void
viewer_file_editable_redo (ViewerFile *self,
                           guint       n_steps)
{
  g_print ("File implementation of editable interface redo method: %s.\n",
           self->filename);
}

static void
viewer_file_editable_interface_init (ViewerEditableInterface *iface)
{
  iface->save = viewer_file_editable_save;
  iface->undo = viewer_file_editable_undo;
  iface->redo = viewer_file_editable_redo;
}

static void
viewer_file_init (ViewerFile *self)
{
  /* Instance variable initialisation code. */
}

If the object is not of final type, e.g. was declared using G_DECLARE_DERIVABLE_TYPE then G_ADD_PRIVATE macro should be added. The private structure should be declared exactly as for a normal derivable object, see howto-gobject-code.

G_DEFINE_TYPE_WITH_CODE (ViewerFile, viewer_file, G_TYPE_OBJECT,
                         G_ADD_PRIVATE (ViewerFile)
                         G_IMPLEMENT_INTERFACE (VIEWER_TYPE_EDITABLE,
                                                viewer_file_editable_interface_init))

🐣 Interface definition prerequisites

To specify that an interface requires the presence of other interfaces when implemented, GObject introduces the concept of prerequisites: it is possible to associate a list of prerequisite types to an interface. For example, if object A wishes to implement interface I1, and if interface I1 has a prerequisite on interface I2, A has to implement both I1 and I2. The mechanism described above is, in practice, very similar to Java's interface I1 extends interface I2. The example below shows the GObject equivalent:

/* Make the ViewerEditableLossy interface require ViewerEditable interface. */
G_DEFINE_INTERFACE (ViewerEditableLossy, viewer_editable_lossy, VIEWER_TYPE_EDITABLE)

In the G_DEFINE_INTERFACE call above, the third parameter defines the prerequisite type. This is the GType of either an interface or a class. In this case the ViewerEditable interface is a prerequisite of ViewerEditableLossy. The code below shows how an implementation can implement both interfaces and register their implementations:

static void
viewer_file_editable_lossy_compress (ViewerEditableLossy *editable)
{
  ViewerFile *self = VIEWER_FILE (editable);

  g_print ("File implementation of lossy editable interface compress method: %s.\n",
           self->filename);
}

static void
viewer_file_editable_lossy_interface_init (ViewerEditableLossyInterface *iface)
{
  iface->compress = viewer_file_editable_lossy_compress;
}

static void
viewer_file_editable_save (ViewerEditable  *editable,
                           GError         **error)
{
  ViewerFile *self = VIEWER_FILE (editable);

  g_print ("File implementation of editable interface save method: %s.\n",
           self->filename);
}

static void
viewer_file_editable_undo (ViewerEditable *editable,
                           guint           n_steps)
{
  ViewerFile *self = VIEWER_FILE (editable);

  g_print ("File implementation of editable interface undo method: %s.\n",
           self->filename);
}

static void
viewer_file_editable_redo (ViewerEditable *editable,
                           guint           n_steps)
{
  ViewerFile *self = VIEWER_FILE (editable);

  g_print ("File implementation of editable interface redo method: %s.\n",
           self->filename);
}

static void
viewer_file_editable_interface_init (ViewerEditableInterface *iface)
{
  iface->save = viewer_file_editable_save;
  iface->undo = viewer_file_editable_undo;
  iface->redo = viewer_file_editable_redo;
}

static void
viewer_file_class_init (ViewerFileClass *klass)
{
  /* Nothing here. */
}

static void
viewer_file_init (ViewerFile *self)
{
  /* Instance variable initialisation code. */
}

G_DEFINE_TYPE_WITH_CODE (ViewerFile, viewer_file, G_TYPE_OBJECT,
                         G_IMPLEMENT_INTERFACE (VIEWER_TYPE_EDITABLE,
                                                viewer_file_editable_interface_init)
                         G_IMPLEMENT_INTERFACE (VIEWER_TYPE_EDITABLE_LOSSY,
                                                viewer_file_editable_lossy_interface_init))

It is very important to notice that the order in which interface implementations are added to the main object is not random: g_type_add_interface_static, which is called by G_IMPLEMENT_INTERFACE, must be invoked first on the interfaces which have no prerequisites and then on the others.

🐣 Interface properties

GObject interfaces can also have properties. Declaration of the interface properties is similar to declaring the properties of ordinary GObject types as explained in gobject-properties, except that g_object_interface_install_property is used to declare the properties instead of g_object_class_install_property. To include a property named 'autosave-frequency' of type gdouble in the ViewerEditable interface example code above, we only need to add one call in viewer_editable_default_init as shown below:

static void
viewer_editable_default_init (ViewerEditableInterface *iface)
{
  g_object_interface_install_property (iface,
         g_param_spec_double ("autosave-frequency",
          "Autosave frequency",
          "Frequency (in per-seconds) to autosave backups of the editable content at. "
          "Or zero to disable autosaves.",
          0.0,  /* minimum */
          G_MAXDOUBLE,  /* maximum */
          0.0,  /* default */
          G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS));
  }

One point worth noting is that the declared property wasn't assigned an integer ID. The reason being that integer IDs of properties are used only inside the get_property and set_property virtual methods. Since interfaces declare but do not implement properties, there is no need to assign integer IDs to them. An implementation declares and defines its properties in the usual way as explained in gobject-properties, except for one small change: it can declare the properties of the interface it implements using g_object_class_override_property instead of g_object_class_install_property. The following code snippet shows the modifications needed in the ViewerFile declaration and implementation above:

struct _ViewerFile
{
  GObject parent_instance;

  gdouble autosave_frequency;
};

enum
{
  PROP_AUTOSAVE_FREQUENCY = 1,
  N_PROPERTIES
};

static void
viewer_file_set_property (GObject      *object,
                          guint         prop_id,
                          const GValue *value,
                          GParamSpec   *pspec)
{
  ViewerFile *file = VIEWER_FILE (object);

  switch (prop_id)
    {
    case PROP_AUTOSAVE_FREQUENCY:
      file->autosave_frequency = g_value_get_double (value);
      break;

    default:
      G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
      break;
    }
}

static void
viewer_file_get_property (GObject    *object,
                          guint       prop_id,
                          GValue     *value,
                          GParamSpec *pspec)
{
  ViewerFile *file = VIEWER_FILE (object);

  switch (prop_id)
    {
    case PROP_AUTOSAVE_FREQUENCY:
      g_value_set_double (value, file->autosave_frequency);
      break;

    default:
      G_OBJECT_WARN_INVALID_PROPERTY_ID (object, prop_id, pspec);
      break;
    }
}

static void
viewer_file_class_init (ViewerFileClass *klass)
{
  GObjectClass *object_class = G_OBJECT_CLASS (klass);

  object_class->set_property = viewer_file_set_property;
  object_class->get_property = viewer_file_get_property;

  g_object_class_override_property (object_class, PROP_AUTOSAVE_FREQUENCY, "autosave-frequency");
}

🐣 Overriding interface methods

If a base class already implements an interface and a derived class needs to implement the same interface but needs to override certain methods, you must reimplement the interface and set only the interface methods which need overriding. In this example, ViewerAudioFile is derived from ViewerFile. Both implement the ViewerEditable interface. ViewerAudioFile only implements one method of the ViewerEditable interface and uses the base class implementation of the other.

static void
viewer_audio_file_editable_save (ViewerEditable  *editable,
                                 GError         **error)
{
  ViewerAudioFile *self = VIEWER_AUDIO_FILE (editable);

  g_print ("Audio file implementation of editable interface save method.\n");
}

static void
viewer_audio_file_editable_interface_init (ViewerEditableInterface *iface)
{
  /* Override the implementation of save(). */
  iface->save = viewer_audio_file_editable_save;

  /*
   * Leave iface->undo and ->redo alone, they are already set to the
   * base class implementation.
   */
}

G_DEFINE_TYPE_WITH_CODE (ViewerAudioFile, viewer_audio_file, VIEWER_TYPE_FILE,
                         G_IMPLEMENT_INTERFACE (VIEWER_TYPE_EDITABLE,
                                                viewer_audio_file_editable_interface_init))

static void
viewer_audio_file_class_init (ViewerAudioFileClass *klass)
{
  /* Nothing here. */
}

static void
viewer_audio_file_init (ViewerAudioFile *self)
{
  /* Nothing here. */
}

To access the base class interface implementation use g_type_interface_peek_parent from within an interface's default_init function. To call the base class implementation of an interface method from a derived class where than interface method has been overridden, stash away the pointer returned from g_type_interface_peek_parent in a global variable. In this example ViewerAudioFile overrides the save interface method. In its overridden method it calls the base class implementation of the same interface method.

static ViewerEditableInterface *viewer_editable_parent_interface = NULL;

static void
viewer_audio_file_editable_save (ViewerEditable  *editable,
                                 GError         **error)
{
  ViewerAudioFile *self = VIEWER_AUDIO_FILE (editable);

  g_print ("Audio file implementation of editable interface save method.\n");

  /* Now call the base implementation */
  viewer_editable_parent_interface->save (editable, error);
}

static void
viewer_audio_file_editable_interface_init (ViewerEditableInterface *iface)
{
  viewer_editable_parent_interface = g_type_interface_peek_parent (iface);

  iface->save = viewer_audio_file_editable_save;
}

G_DEFINE_TYPE_WITH_CODE (ViewerAudioFile, viewer_audio_file, VIEWER_TYPE_FILE,
                         G_IMPLEMENT_INTERFACE (VIEWER_TYPE_EDITABLE,
                                                viewer_audio_file_editable_interface_init))

static void
viewer_audio_file_class_init (ViewerAudioFileClass *klass)
{
  /* Nothing here. */
}

static void
viewer_audio_file_init (ViewerAudioFile *self)
{
  /* Nothing here. */
}

🍀 How to create and use signals

The signal system in GType is pretty complex and flexible: it is possible for its users to connect at runtime any number of callbacks (implemented in any language for which a binding exists) to any signal and to stop the emission of any signal at any state of the signal emission process. This flexibility makes it possible to use GSignal for much more than just emitting signals to multiple clients.

A Python callback can be connected to any signal on any C-based GObject, and vice versa, assuming that the Python object inherits from GObject.

🐣 Simple use of signals

The most basic use of signals is to implement event notification. For example, given a ViewerFile object with a write method, a signal could be emitted whenever the file is changed using that method. The code below shows how the user can connect a callback to the "changed" signal.

file = g_object_new (VIEWER_FILE_TYPE, NULL);

g_signal_connect (file, "changed", (GCallback) changed_event, NULL);

viewer_file_write (file, buffer, strlen (buffer));

The ViewerFile signal is registered in the class_init function:

file_signals[CHANGED] = 
  g_signal_newv ("changed",
                 G_TYPE_FROM_CLASS (object_class),
                 G_SIGNAL_RUN_LAST | G_SIGNAL_NO_RECURSE | G_SIGNAL_NO_HOOKS,
                 NULL /* closure */,
                 NULL /* accumulator */,
                 NULL /* accumulator data */,
                 NULL /* C marshaller */,
                 G_TYPE_NONE /* return_type */,
                 0     /* n_params */,
                 NULL  /* param_types */);

and the signal is emitted in viewer_file_write:

void
viewer_file_write (ViewerFile   *self,
                   const guint8 *buffer,
                   gsize         size)
{
  g_return_if_fail (VIEWER_IS_FILE (self));
  g_return_if_fail (buffer != NULL || size == 0);

  /* First write data. */

  /* Then, notify user of data written. */
  g_signal_emit (self, file_signals[CHANGED], 0 /* details */);
}

As shown above, the details parameter can safely be set to zero if no detail needs to be conveyed. For a discussion of what it can be used for, see signal-detail The C signal marshaller should always be NULL, in which case the best marshaller for the given closure type will be chosen by GLib. This may be an internal marshaller specific to the closure type, or g_cclosure_marshal_generic, which implements generic conversion of arrays of parameters to C callback invocations. GLib used to require the user to write or generate a type-specific marshaller and pass that, but that has been deprecated in favour of automatic selection of marshallers. Note that g_cclosure_marshal_generic is slower than non-generic marshallers, so should be avoided for performance critical code. However, performance critical code should rarely be using signals anyway, as emitting a signal blocks on emitting it to all listeners, which has potentially unbounded cost.

📜 Floating references

Note: Floating references are a C convenience API and should not be used in modern GObject code. Language bindings in particular find the concept highly problematic, as floating references are not identifiable through annotations, and neither are deviations from the floating reference behavior, like types that inherit from GInitiallyUnowned and still return a full reference from g_object_new().

GInitiallyUnowned is derived from GObject. The only difference between the two is that the initial reference of a GInitiallyUnowned is flagged as a “floating” reference. This means that it is not specifically claimed to be “owned” by any code portion. The main motivation for providing floating references is C convenience. In particular, it allows code to be written as:

container = create_container ();
container_add_child (container, create_child());

If container_add_child() calls g_object_ref_sink() on the passed-in child, no reference of the newly created child is leaked. Without floating references, container_add_child() can only acquire a reference the new child, so to implement this code without reference leaks, it would have to be written as:

Child *child;
container = create_container ();
child = create_child ();
container_add_child (container, child);
g_object_unref (child);

The floating reference can be converted into an ordinary reference by calling g_object_ref_sink(). For already sunken objects (objects that don’t have a floating reference anymore), g_object_ref_sink() is equivalent to g_object_ref() and returns a new reference.

Since floating references are useful almost exclusively for C convenience, language bindings that provide automated reference and memory ownership maintenance (such as smart pointers or garbage collection) should not expose floating references in their API. The best practice for handling types that have initially floating references is to immediately sink those references after g_object_new() returns, by checking if the GType inherits from GInitiallyUnowned. For instance:

GObject *res = g_object_new_with_properties (gtype,
                                             n_props,
                                             prop_names,
                                             prop_values);

// or: if (g_type_is_a (gtype, G_TYPE_INITIALLY_UNOWNED))
if (G_IS_INITIALLY_UNOWNED (res))
  g_object_ref_sink (res);

return res;

Some object implementations may need to save an object’s floating state across certain code portions (an example is GtkMenu in GTK3), to achieve this, the following sequence can be used:

// save floating state
gboolean was_floating = g_object_is_floating (object);
g_object_ref_sink (object);
// protected code portion

...

// restore floating state
if (was_floating)
  g_object_force_floating (object);
else
  g_object_unref (object); // release previously acquired reference

🍀 Replacing floating references with annotations

You should avoid basing your object hierarchy on floating references, as they are hard to understand even in C, and introduce additional limitations when consuming a GObject-based API in other languages.

One way to express the ownership transfer between “container” and “child” instances is to use ownership transfer annotations in your documentation and introspection data. For instance, you can implement this pattern:

container_add_child (container, create_child ());

without leaking by having container_add_child() defined as:

/**
 * container_add_child:
 * @self: a container
 * @child: (transfer full): the child to add to the container
 *
 * ...
 */
void
container_add_child (Container *container,
                     Child *child)
{
  container->children = g_list_append (container->children, child);
}

The container does not explicitly acquire a reference on the child; instead, the ownership of the child is transferred to the container. The transfer annotation will be used by language bindings to ensure that there are no leaks; and documentation tools will explicitly note that the callee now owns the value passed by the caller.

🍀 Replacing floating references with weak references

Another option for replacing floating references is to use weak references in place of strong ones. A “container” can acquire a weak reference on the “child” instance by using g_object_weak_ref(); once the child instance loses its last strong reference, the container holding a weak reference is notified, and it can either remove the child from its internal list, or turn a weak reference into a strong one.

📜 Boxed types

A “boxed type” is a generic wrapper mechanism for arbitrary C structures. The only thing the type system needs to know about the structures is how to copy them (a GBoxedCopyFunc) and how to free them (a GBoxedFreeFunc)—beyond that they are treated as opaque chunks of memory.

Boxed types are useful for simple value-holder structures like rectangles or points. They can also be used for wrapping structures defined in non-GObject based libraries. They allow arbitrary structures to be handled in a uniform way, allowing uniform copying (or referencing) and freeing (or unreferencing) of them, and uniform representation of the type of the contained structure. In turn, this allows any type which can be boxed to be set as the data in a GValue, which allows for polymorphic handling of a much wider range of data types, and hence usage of such types as GObject property values.

All boxed types inherit from the G_TYPE_BOXED fundamental type.

It is very important to note that boxed types are not deeply inheritable: you cannot register a boxed type that inherits from another boxed type. This means you cannot create your own custom, parallel type hierarchy and map it to GType using boxed types. If you want to have deeply inheritable types without using GObject, you will need to use GTypeInstance.

🍀 Registering a new boxed type

The recommended way to register a new boxed type is to use the G_DEFINE_BOXED_TYPE macro:

// In the header

#define EXAMPLE_TYPE_RECTANGLE (example_rectangle_get_type())

typedef struct {
  double x, y;
  double width, height;
} ExampleRectangle;

GType
example_rectangle_get_type (void);

ExampleRectangle *
example_rectangle_copy (ExampleRectangle *r);

void
example_rectangle_free (ExampleRectangle *r);

// In the source
G_DEFINE_BOXED_TYPE (ExampleRectangle, example_rectangle,
                     example_rectangle_copy,
                     example_rectangle_free)
obj_props[PROP_BOUNDS] =
  g_param_spec_boxed ("bounds", NULL, NULL,
                      EXAMPLE_TYPE_RECTANGLE,
                      G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS);
switch (prop_id)
  {
  // ...
  case PROP_BOUNDS:
    example_object_set_bounds (self, g_value_get_boxed (value));
    break;
  // ...
  }
switch (prop_id)
  {
  // ...
  case PROP_BOUNDS:
    g_value_set_boxed (self, &self->bounds);
    break;
  // ...
  }

Just like G_DEFINE_TYPE and G_DEFINE_INTERFACE_TYPE, the G_DEFINE_BOXED_TYPE macro will provide the definition of the get_type() function, which will call g_boxed_type_register_static() with the given type name as well as the GBoxedCopyFunc and GBoxedFreeFunc functions.

🍀 Using boxed types

Object properties In order to use a boxed type with GObject properties you will need to register the property using g_param_spec_boxed(), e.g.

// In the header

#define EXAMPLE_TYPE_RECTANGLE (example_rectangle_get_type())

typedef struct {
  double x, y;
  double width, height;
} ExampleRectangle;

GType
example_rectangle_get_type (void);

ExampleRectangle *
example_rectangle_copy (ExampleRectangle *r);

void
example_rectangle_free (ExampleRectangle *r);

// In the source
G_DEFINE_BOXED_TYPE (ExampleRectangle, example_rectangle,
                     example_rectangle_copy,
                     example_rectangle_free)
obj_props[PROP_BOUNDS] =
  g_param_spec_boxed ("bounds", NULL, NULL,
                      EXAMPLE_TYPE_RECTANGLE,
                      G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS);
switch (prop_id)
  {
  // ...
  case PROP_BOUNDS:
    example_object_set_bounds (self, g_value_get_boxed (value));
    break;
  // ...
  }
switch (prop_id)
  {
  // ...
  case PROP_BOUNDS:
    g_value_set_boxed (self, &self->bounds);
    break;
  // ...
  }

In the set_property implementation you can use g_value_get_boxed() to retrieve a pointer to the boxed type:

// In the header

#define EXAMPLE_TYPE_RECTANGLE (example_rectangle_get_type())

typedef struct {
  double x, y;
  double width, height;
} ExampleRectangle;

GType
example_rectangle_get_type (void);

ExampleRectangle *
example_rectangle_copy (ExampleRectangle *r);

void
example_rectangle_free (ExampleRectangle *r);

// In the source
G_DEFINE_BOXED_TYPE (ExampleRectangle, example_rectangle,
                     example_rectangle_copy,
                     example_rectangle_free)
obj_props[PROP_BOUNDS] =
  g_param_spec_boxed ("bounds", NULL, NULL,
                      EXAMPLE_TYPE_RECTANGLE,
                      G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS);
switch (prop_id)
  {
  // ...
  case PROP_BOUNDS:
    example_object_set_bounds (self, g_value_get_boxed (value));
    break;
  // ...
  }
switch (prop_id)
  {
  // ...
  case PROP_BOUNDS:
    g_value_set_boxed (self, &self->bounds);
    break;
  // ...
  }

Similarly, you can use g_value_set_boxed() in the implementation of the get_property virtual function:

// In the header

#define EXAMPLE_TYPE_RECTANGLE (example_rectangle_get_type())

typedef struct {
  double x, y;
  double width, height;
} ExampleRectangle;

GType
example_rectangle_get_type (void);

ExampleRectangle *
example_rectangle_copy (ExampleRectangle *r);

void
example_rectangle_free (ExampleRectangle *r);

// In the source
G_DEFINE_BOXED_TYPE (ExampleRectangle, example_rectangle,
                     example_rectangle_copy,
                     example_rectangle_free)
obj_props[PROP_BOUNDS] =
  g_param_spec_boxed ("bounds", NULL, NULL,
                      EXAMPLE_TYPE_RECTANGLE,
                      G_PARAM_READWRITE | G_PARAM_STATIC_STRINGS);
switch (prop_id)
  {
  // ...
  case PROP_BOUNDS:
    example_object_set_bounds (self, g_value_get_boxed (value));
    break;
  // ...
  }
switch (prop_id)
  {
  // ...
  case PROP_BOUNDS:
    g_value_set_boxed (self, &self->bounds);
    break;
  // ...
  }

🍀 Reference counting

Boxed types are designed so that reference counted types can be boxed. Use the type’s ‘ref’ function as the GBoxedCopyFunc, and its ‘unref’ function as the GBoxedFreeFunc. For example, for GBytes, the GBoxedCopyFunc is g_bytes_ref(), and the GBoxedFreeFunc is g_bytes_unref().

📜 Enumeration types

The GLib type system provides fundamental types for enumeration and flags types. Enumerations types are collection of identifiers associated with a numeric value; flags types are like enumerations, but allow their values to be combined by bitwise or.

A registered enumeration or flags type associates a name and a nickname with each allowed value, and the methods g_enum_get_value_by_name(), g_enum_get_value_by_nick(), g_flags_get_value_by_name() and g_flags_get_value_by_nick() can look up values by their name or nickname.

When an enumeration or flags type is registered with the GLib type system, it can be used as value type for object properties, using g_param_spec_enum() or g_param_spec_flags().

GObject ships with a utility called glib-mkenums, that can construct suitable type registration functions from C enumeration definitions.

Example of how to get a string representation of an enum value:

GEnumClass *enum_class;
GEnumValue *enum_value;

enum_class = g_type_class_ref (EXAMPLE_TYPE_ENUM);
enum_value = g_enum_get_value (enum_class, EXAMPLE_ENUM_FOO);

g_print ("Name: %s\n", enum_value->value_name);

g_type_class_unref (enum_class);
Copy
Alternatively, you can use g_enum_to_string().

📜 Generic value container

The GValue structure is basically a variable container that consists of a type identifier and a specific value of that type. The type identifier within a GValue structure always determines the type of the associated value.

To create an undefined GValue structure, simply create a zero-filled GValue structure. To initialize the GValue, use the g_value_init() function. A GValue cannot be used until it is initialized.

Once you have finished using a GValue, you must call g_value_unset() to ensure that all the resources associated with the GValue are freed.

The basic type operations (such as freeing and copying) are determined by the GTypeValueTable associated with the type ID stored in the GValue. Other GValue operations (such as converting values between types) are provided by this interface.

The code in the example program below demonstrates GValue‘s features:

#include <glib-object.h>

static void
int2string (const GValue *src_value,
            GValue       *dest_value)
{
  if (g_value_get_int (src_value) == 42)
    g_value_set_static_string (dest_value, "An important number");
  else
    g_value_set_static_string (dest_value, "What's that?");
}

int
main (int   argc,
      char *argv[])
{
  // GValues must be initialized
  GValue a = G_VALUE_INIT;
  GValue b = G_VALUE_INIT;
  const char *message;

  // The GValue starts empty
  g_assert (!G_VALUE_HOLDS_STRING (&a));

  // Put a string in it
  g_value_init (&a, G_TYPE_STRING);
  g_assert (G_VALUE_HOLDS_STRING (&a));
  g_value_set_static_string (&a, "Hello, world!");
  g_printf ("%s\n", g_value_get_string (&a));

  // Reset it to its pristine state
  g_value_unset (&a);

  // It can then be reused for another type
  g_value_init (&a, G_TYPE_INT);
  g_value_set_int (&a, 42);

  // Attempt to transform it into a GValue of type STRING
  g_value_init (&b, G_TYPE_STRING);

  // An INT is transformable to a STRING
  g_assert (g_value_type_transformable (G_TYPE_INT, G_TYPE_STRING));

  g_value_transform (&a, &b);
  g_printf ("%s\n", g_value_get_string (&b));

  // Attempt to transform it again using a custom transform function
  g_value_register_transform_func (G_TYPE_INT, G_TYPE_STRING, int2string);
  g_value_transform (&a, &b);
  g_printf ("%s\n", g_value_get_string (&b));

  g_value_unset (&b);
  g_value_unset (&a);

  return 0;
}

📜 glib-mkenums User Commands

glib-mkenums - C language enum description generation utility

glib-mkenums OPTION FILE

Description

glib-mkenums is a small utility that parses C code to extract enum definitions and produces enum descriptions based on text templates specified by the user. Typically, you can use this tool to generate enumeration types for the GType type system, for GObject properties and signal marshalling; additionally, you can use it to generate enumeration values of GSettings schemas.

glib-mkenums takes a list of valid C code files as input. The options specified control the text that generated, substituting various keywords enclosed in @ characters in the templates.

Since version 2.74, GLib provides the G_DEFINE_ENUM_TYPE and G_DEFINE_FLAGS_TYPE C pre-processor macros. These macros can be used to define a GType for projects that have few, small enumeration types without going through the complexities of generating code at build time.

🍀 Production text substitutions

Certain keywords enclosed in @ characters will be substituted in the emitted text. For the substitution examples of the keywords below, the following example enum definition is assumed:

typedef enum
{
  PREFIX_THE_XVALUE    = 1 << 3,
  PREFIX_ANOTHER_VALUE = 1 << 4
} PrefixTheXEnum;

0. @EnumName@ The name of the enum currently being processed, enum names are assumed to be properly namespaced and to use mixed capitalization to separate words (e.g. PrefixTheXEnum).
1. @enum_name@ The enum name with words lowercase and word-separated by underscores (e.g. prefix_the_xenum).
2. @ENUMNAME@ The enum name with words uppercase and word-separated by underscores (e.g. PREFIX_THE_XENUM).
3. @ENUMSHORT@ The enum name with words uppercase and word-separated by underscores, prefix stripped (e.g. THE_XENUM).
4. @ENUMPREFIX@ The prefix of the enum name (e.g. PREFIX).
5. @VALUENAME@ The enum value name currently being processed with words uppercase and word-separated by underscores, this is the assumed literal notation of enum values in the C sources (e.g. PREFIX_THE_XVALUE).
6. @valuenick@ A nick name for the enum value currently being processed, this is usually generated by stripping common prefix words of all the enum values of the current enum, the words are lowercase and underscores are substituted by a minus (e.g. the-xvalue).
7. @valuenum@ The integer value for the enum value currently being processed. If the evaluation fails then glib-mkenums will exit with an error status, but this only happens if @valuenum@ appears in your value production template. (Since: 2.26)
8. @type@ This is substituted either by "enum" or "flags", depending on whether the enum value definitions contained bit-shift operators or not (e.g. flags).
9. @Type@ The same as @type@ with the first letter capitalized (e.g. Flags).
10. @TYPE@ The same as @type@ with all letters uppercased (e.g. FLAGS).
11. @filename@ The full path of the input file currently being processed (e.g. /build/environment/project/src/foo.h).
12. @basename@ The base name of the input file currently being processed (e.g. foo.h). Typically you want to use @basename@ in place of @filename@ in your templates, to improve the reproducibility of the build. (Since: 2.22)

🍀 Trigraph extensions

Some C comments are treated specially in the parsed enum definitions, such comments start out with the trigraph sequence /*< and end with the trigraph sequence >*/.

The following options can be specified per enum definition:

0. skip Indicates this enum definition should be skipped.
1. flags Indicates this enum should be treated as a flags definition.
2. underscore_name Specifies the word separation used in the *_get_type() function. For instance, /*< underscore_name=gnome_vfs_uri_hide_options >*/.
3. since Specifies the version tag that will be used to substitute the @enumsince@ keyword in the template, useful when documenting methods generated from the enums (e.g. Since: @enumsince@). (Since: 2.66)

The following options can be specified per value definition:

0. skip Indicates the value should be skipped.
1. nick Specifies the otherwise auto-generated nickname.

Examples:

typedef enum /*< skip >*/
{
  PREFIX_FOO
} PrefixThisEnumWillBeSkipped;
typedef enum /*< flags,prefix=PREFIX,since=1.0 >*/
{
  PREFIX_THE_ZEROTH_VALUE,  /*< skip >*/
  PREFIX_THE_FIRST_VALUE,
  PREFIX_THE_SECOND_VALUE,
  PREFIX_THE_THIRD_VALUE, /*< nick=the-last-value >*/
} PrefixTheFlagsEnum;

🍀 Options

0. --fhead TEXT Emits TEXT prior to processing input files.

   You can specify this option multiple times, and the TEXT will be concatenated.

   When used along with a template file, TEXT will be prepended to the template's file-header section.

1. --fprod TEXT Emits TEXT every time a new input file is being processed.

   You can specify this option multiple times, and the TEXT will be concatenated.

   When used along with a template file, TEXT will be appended to the template's file-production section.

2. --ftail TEXT Emits TEXT after all input files have been processed.

   You can specify this option multiple times, and the TEXT will be concatenated.

   When used along with a template file, TEXT will be appended to the template's file-tail section.

3. --eprod TEXT Emits TEXT every time an enum is encountered in the input files.

4. --vhead TEXT Emits TEXT before iterating over the set of values of an enum.

   You can specify this option multiple times, and the TEXT will be concatenated.

   When used along with a template file, TEXT will be prepended to the template's value-header section.

5. --vprod TEXT Emits TEXT for every value of an enum.

   You can specify this option multiple times, and the TEXT will be concatenated.

   When used along with a template file, TEXT will be appended to the template's value-production section.

6. --vtail TEXT Emits TEXT after iterating over all values of an enum.

   You can specify this option multiple times, and the TEXT will be concatenated.

   When used along with a template file, TEXT will be appended to the template's value-tail section.

7. --comments TEXT Template for auto-generated comments, the default (for C code generations) is "/* @comment@ */".

8. --template FILE Read templates from the given file. The templates are enclosed in specially-formatted C comments:

   /*** BEGIN section ***/
   /*** END section ***/

   section may be file-header, file-production, file-tail, enumeration-production, value-header, value-production, value-tail or comment.

9. --identifier-prefix PREFIX Indicates what portion of the enum name should be interpreted as the prefix (eg, the "Gtk" in "GtkDirectionType"). Normally this will be figured out automatically, but you may need to override the default if your namespace is capitalized oddly.

10. --symbol-prefix PREFIX Indicates what prefix should be used to correspond to the identifier prefix in related C function names (eg, the "gtk" in "gtk_direction_type_get_type". Equivalently, this is the lowercase version of the prefix component of the enum value names (eg, the "GTK" in "GTK_DIR_UP". The default value is the identifier prefix, converted to lowercase.

11. --help Print brief help and exit.

12. --version Print version and exit.

13. --output=FILE Write output to FILE instead of stdout.

14. @RSPFILE When passed as the sole argument, read and parse the actual arguments from RSPFILE. Useful on systems with a low command-line length limit. For example, Windows has a limit of 8191 characters.

🍀 Using templates

Instead of passing the various sections of the generated file to the command line of glib-mkenums, it's strongly recommended to use a template file, especially for generating C sources.

A C header template file will typically look like this:

/*** BEGIN file-header ***/
#pragma once

/* Include the main project header */
#include "project.h"

G_BEGIN_DECLS
/*** END file-header ***/

/*** BEGIN file-production ***/

/* enumerations from "@basename@" */
/*** END file-production ***/

/*** BEGIN value-header ***/
GType @enum_name@_get_type (void) G_GNUC_CONST;
#define @ENUMPREFIX@_TYPE_@ENUMSHORT@ (@enum_name@_get_type ())
/*** END value-header ***/

/*** BEGIN file-tail ***/
G_END_DECLS
/*** END file-tail ***/

A C source template file will typically look like this:

/*** BEGIN file-header ***/
#include "config.h"
#include "enum-types.h"

/*** END file-header ***/

/*** BEGIN file-production ***/
/* enumerations from "@basename@" */
/*** END file-production ***/

/*** BEGIN value-header ***/
GType
@enum_name@_get_type (void)
{
  static gsize static_g_@type@_type_id;

  if (g_once_init_enter (&static_g_@type@_type_id))
    {
      static const G@Type@Value values[] = {
/*** END value-header ***/

/*** BEGIN value-production ***/
            { @VALUENAME@, "@VALUENAME@", "@valuenick@" },
/*** END value-production ***/

/*** BEGIN value-tail ***/
            { 0, NULL, NULL }
      };

      GType g_@type@_type_id =
        g_@type@_register_static (g_intern_static_string ("@EnumName@"), values);

      g_once_init_leave (&static_g_@type@_type_id, g_@type@_type_id);
    }
  return static_g_@type@_type_id;
}

/*** END value-tail ***/

Template files are easier to modify and update, and can be used to generate various types of outputs using the same command line or tools during the build.

🍀 Using glib-mkenums with Meson

Meson supports generating enumeration types using glib-mkenums out of the box in its "gnome" module.

In your meson.build file you will typically call the gnome.mkenums_simple() method to generate idiomatic enumeration types from a list of headers to inspect:

project_headers = [
  'project-foo.h',
  'project-bar.h',
  'project-baz.h',
]

gnome = import('gnome')
enum_files = gnome.mkenums_simple('enum-types',
  sources: project_headers,
)

The enum_files variable will contain an array of two elements in the following order:

0. a build target for the source file
1. a build target for the header file

You should use the returned objects to provide a dependency on every other build target that references the source or header file; for instance, if you are using the source to build a library:

mainlib = library('project',
  sources: project_sources + enum_files,
  ...
)

Additionally, if you are including the generated header file inside a build target that depends on the library you just built, you must ensure that the internal dependency includes the generated header as a required source file:

mainlib_dep = declare_dependency(sources: enum_files[1], link_with: mainlib)

You should not include the generated source file as well, otherwise it will be built separately for every target that depends on it, causing build failures. To know more about why all this is required, please refer to the corresponding Meson FAQ entry.

If you are generating C header and source files that require special templates, you can use gnome.mkenums() to provide those headers, for instance:

enum_files = gnome.mkenums('enum-types',
  sources: project_headers,
  h_template: 'enum-types.h.in',
  c_template: 'enum-types.c.in',
  install_header: true,
)

For more information, see the Meson documentation for gnome.mkenums().

🍀 Using glib-mkenums with Autotools

In order to use glib-mkenums in your project when using Autotools as the build system, you will first need to modify your configure.ac file to ensure you find the appropriate command using pkg-config, similarly as to how you discover the compiler and linker flags for GLib.

PKG_PROG_PKG_CONFIG([0.28])

PKG_CHECK_VAR([GLIB_MKENUMS], [glib-2.0], [glib_mkenums])

In your Makefile.am file you will typically use rules like these:

# A list of headers to inspect
project_headers = \
        project-foo.h \
        project-bar.h \
        project-baz.h

enum-types.h: $(project_headers) enum-types.h.in
        $(AM_V_GEN)$(GLIB_MKENUMS) \
                --template=enum-types.h.in \
                --output=$@ \
                $(project_headers)

enum-types.c: $(project_headers) enum-types.c.in enum-types.h
        $(AM_V_GEN)$(GLIB_MKENUMS) \
                --template=enum-types.c.in \
                --output=$@ \
                $(project_headers)

# Build the enum types files before every other target
BUILT_SOURCES += enum-types.h enum-types.c
CLEANFILES += enum-types.h enum-types.c
EXTRA_DIST += enum-types.h.in enum-types.c.in

In the example above, we have a variable called project_headers where we reference all header files we want to inspect for generating enumeration GTypes. In the enum-types.h rule we use glib-mkenums with a template called enum-types.h.in in order to generate the header file; similarly, in the enum-types.c rule we use a template called enum-types.c.in.

🍀 glib-mkenums CLI

usage: glib-mkenums [-h] [--identifier-prefix IDPREFIX]
                    [--symbol-prefix SYMPREFIX] [--fhead FHEAD]
                    [--ftail FTAIL] [--fprod FPROD] [--eprod EPROD]
                    [--vhead VHEAD] [--vprod VPROD] [--vtail VTAIL]
                    [--comments COMMENT_TMPL] [--template TEMPLATE]
                    [--output OUTPUT] [--version]
                    [args ...]

positional arguments:
  args                  One or more input files, or a single argument
                        @rspfile_path pointing to a file that contains the
                        actual arguments

options:
  -h, --help            show this help message and exit
  --identifier-prefix IDPREFIX
                        Identifier prefix
  --symbol-prefix SYMPREFIX
                        Symbol prefix
  --fhead FHEAD         Output file header
  --ftail FTAIL         Output file footer
  --fprod FPROD         Put out TEXT every time a new input file is being
                        processed.
  --eprod EPROD         Per enum text, produced prior to value iterations
  --vhead VHEAD         Value header, produced before iterating over enum
                        values
  --vprod VPROD         Value text, produced for each enum value.
  --vtail VTAIL         Value tail, produced after iterating over enum values
  --comments COMMENT_TMPL
                        Comment structure
  --template TEMPLATE   Template file
  --output OUTPUT
  --version, -v         Print version information

Production text substitutions:
  @EnumName@            PrefixTheXEnum
  @enum_name@           prefix_the_xenum
  @ENUMNAME@            PREFIX_THE_XENUM
  @ENUMSHORT@           THE_XENUM
  @ENUMPREFIX@          PREFIX
  @enumsince@           the user-provided since value given
  @VALUENAME@           PREFIX_THE_XVALUE
  @valuenick@           the-xvalue
  @valuenum@            the integer value (limited support, Since: 2.26)
  @type@                either enum or flags
  @Type@                either Enum or Flags
  @TYPE@                either ENUM or FLAGS
  @filename@            name of current input file
  @basename@            base name of the current input file (Since: 2.22)

📜 glib-genmarshal User Commands

glib-genmarshal - C code marshaller generation utility for GLib closures

glib-genmarshal OPTION FILE

🍀 Description

glib-genmarshal is a small utility that generates C code marshallers for callback functions of the GClosure mechanism in the GObject sublibrary of GLib. The marshaller functions have a standard signature, they get passed in the invoking closure, an array of value structures holding the callback function parameters and a value structure for the return value of the callback. The marshaller is then responsible to call the respective C code function of the closure with all the parameters on the stack and to collect its return value.

glib-genmarshal takes a list of marshallers to generate as input. The marshaller list is either read from files passed as additional arguments on the command line; or from standard input, by using - as the input file.

🍀 Marshaller list format

The marshaller lists are processed line by line, a line can contain a comment in the form of

# this is a comment

or a marshaller specification of the form

RTYPE:PTYPE
RTYPE:PTYPE,PTYPE
RTYPE:PTYPE,PTYPE,PTYPE

The RTYPE part specifies the callback's return type and the PTYPEs right to the colon specify the callback's parameter list, except for the first and the last arguments which are always pointers.

🍀 Parameter types

Currently, the following types are supported:

0. VOID indicates no return type, or no extra parameters. If VOID is used as the parameter list, no additional parameters may be present.

1. BOOLEAN for boolean types (gboolean)

2. CHAR for signed char types (gchar)

3. UCHAR for unsigned char types (guchar)

4. INT for signed integer types (gint)

5. UINT for unsigned integer types (guint)

6. LONG for signed long integer types (glong)

7. ULONG for unsigned long integer types (gulong)

8. INT64 for signed 64bit integer types (gint64)

9. UINT64 for unsigned 64bit integer types (guint64)

10. ENUM for enumeration types (gint)

11. FLAGS for flag enumeration types (guint)

12. FLOAT for single-precision float types (gfloat)

13. DOUBLE for double-precision float types (gdouble)

14. STRING for string types (gchar*)

15. BOXED for boxed (anonymous but reference counted) types (GBoxed*)

16. PARAM for GParamSpec or derived types (GParamSpec*)

17. POINTER for anonymous pointer types (gpointer)

18. OBJECT for GObject or derived types (GObject*)

19. VARIANT for GVariant types (GVariant*)

20. NONE deprecated alias for VOID

21. BOOL deprecated alias for BOOLEAN

🍀 Options

0. --header Generate header file contents of the marshallers. This option is mutually exclusive with the --body option.

1. --body Generate C code file contents of the marshallers. This option is mutually exclusive with the --header option.

2. --prefix=PREFIX Specify marshaller prefix. The default prefix is g_cclosure_user_marshal.

3. --skip-source Skip source location remarks in generated comments.

4. --stdinc Use the standard marshallers of the GObject library, and include glib-object.h in generated header files. This option is mutually exclusive with the --nostdinc option.

5. --nostdinc Do not use the standard marshallers of the GObject library, and skip glib-object.h include directive in generated header files. This option is mutually exclusive with the --stdinc option.

6. --internal Mark generated functions as internal, using G_GNUC_INTERNAL.

7. --valist-marshallers Generate valist marshallers, for use with g_signal_set_va_marshaller().

8. -v, --version Print version information.

9. --g-fatal-warnings Make warnings fatal, that is, exit immediately once a warning occurs.

10. -h, --help Print brief help and exit.

11. -v, --version Print version and exit.

12. --output=FILE Write output to FILE instead of the standard output.

13. --prototypes Generate function prototypes before the function definition in the C source file, in order to avoid a missing-prototypes compiler warning. This option is only useful when using the --body option.

14. --pragma-once Use the once pragma instead of an old style header guard when generating the C header file. This option is only useful when using the --header option.

15. --include-header=HEADER Adds a #include directive for the given file in the C source file. This option is only useful when using the --body option.

16. -D SYMBOL[=VALUE] Adds a #define C pre-processor directive for SYMBOL and its given VALUE, or "1" if the value is unset. You can use this option multiple times; if you do, all the symbols will be defined in the same order given on the command line, before the symbols undefined using the -U option. This option is only useful when using the --body option.

17. -U SYMBOL Adds a #undef C pre-processor directive to undefine the given SYMBOL. You can use this option multiple times; if you do, all the symbols will be undefined in the same order given on the command line, after the symbols defined using the -D option. This option is only useful when using the --body option.

18. --quiet Minimizes the output of glib-genmarshal, by printing only warnings and errors. This option is mutually exclusive with the --verbose option.

19. --verbose Increases the verbosity of glib-genmarshal, by printing debugging information. This option is mutually exclusive with the --quiet option.

🍀 Using glib-genmarshal with Meson

Meson supports generating closure marshallers using glib-genmarshal out of the box in its "gnome" module.

In your meson.build file you will typically call the gnome.genmarshal() method with the source list of marshallers to generate:

gnome = import('gnome')
marshal_files = gnome.genmarshal('marshal',
  sources: 'marshal.list',
  internal: true,
)

The marshal_files variable will contain an array of two elements in the following order:

0. a build target for the source file
1. a build target for the header file

You should use the returned objects to provide a dependency on every other build target that references the source or header file; for instance, if you are using the source to build a library:

mainlib = library('project',
  sources: project_sources + marshal_files,
  ...
)

Additionally, if you are including the generated header file inside a build target that depends on the library you just built, you must ensure that the internal dependency includes the generated header as a required source file:

mainlib_dep = declare_dependency(sources: marshal_files[1], link_with: mainlib)

You should not include the generated source file as well, otherwise it will be built separately for every target that depends on it, causing build failures. To know more about why all this is required, please refer to the corresponding Meson FAQ entry. For more information on how to use the method, see the Meson documentation for gnome.genmarshal().

🍀 Using glib-genmarshal with Autotools

In order to use glib-genmarshal in your project when using Autotools as the build system, you will first need to modify your configure.ac file to ensure you find the appropriate command using pkg-config, similarly as to how you discover the compiler and linker flags for GLib.

PKG_PROG_PKG_CONFIG([0.28])

PKG_CHECK_VAR([GLIB_GENMARSHAL], [glib-2.0], [glib_genmarshal])

In your Makefile.am file you will typically need very simple rules to generate the C files needed for the build.

marshal.h: marshal.list
        $(AM_V_GEN)$(GLIB_GENMARSHAL) \
                --header \
                --output=$@ \
                $<

marshal.c: marshal.list marshal.h
        $(AM_V_GEN)$(GLIB_GENMARSHAL) \
                --include-header=marshal.h \
                --body \
                --output=$@ \
                $<

BUILT_SOURCES += marshal.h marshal.c
CLEANFILES += marshal.h marshal.c
EXTRA_DIST += marshal.list

In the example above, the first rule generates the header file and depends on a marshal.list file in order to regenerate the result in case the marshallers list is updated. The second rule generates the source file for the same marshal.list, and includes the file generated by the header rule.

🍀 Example

To generate marshallers for the following callback functions:

void   foo (gpointer data1,
            gpointer data2);
void   bar (gpointer data1,
            gint     param1,
            gpointer data2);
gfloat baz (gpointer data1,
            gboolean param1,
            guchar   param2,
            gpointer data2);

The marshaller.list file has to look like this:

VOID:VOID
VOID:INT
FLOAT:BOOLEAN,UCHAR

and you call glib-genmarshal like this:

glib-genmarshal --header marshaller.list > marshaller.h
glib-genmarshal --body marshaller.list > marshaller.c

The generated marshallers have the arguments encoded in their function name. For this particular list, they are

g_cclosure_user_marshal_VOID__VOID(...),
g_cclosure_user_marshal_VOID__INT(...),
g_cclosure_user_marshal_FLOAT__BOOLEAN_UCHAR(...).

They can be used directly for GClosures or be passed in as the GSignalCMarshaller c_marshaller; argument upon creation of signals:

GClosure *cc_foo, *cc_bar, *cc_baz;

cc_foo = g_cclosure_new (NULL, foo, NULL);
g_closure_set_marshal (cc_foo, g_cclosure_user_marshal_VOID__VOID);
cc_bar = g_cclosure_new (NULL, bar, NULL);
g_closure_set_marshal (cc_bar, g_cclosure_user_marshal_VOID__INT);
cc_baz = g_cclosure_new (NULL, baz, NULL);
g_closure_set_marshal (cc_baz, g_cclosure_user_marshal_FLOAT__BOOLEAN_UCHAR);

📜 gobject-query User Commands

gobject-query display a tree of types

gobject-query froots OPTION
gobject-query tree OPTION

gobject-query is a small utility that draws a tree of types.

gobject-query takes a mandatory argument that specifies whether it should iterate over the fundamental types or print a type tree.

Commands

froots ➡ iterate over fundamental roots tree ➡ print type tree

Options

-r TYPE ➡ specify the root type -n ➡ don't descend type tree -b STRING ➡ specify indent string -i STRING ➡ specify incremental indent string

-s NUMBER ➡ specify line spacing -h , --help ➡ Print brief help and exit. -v , --version ➡ Print version and exit.

usage: gobject-query <qualifier> [-r <type>] [-{i|b} ""] [-s #] [-{h|x|y}]
       -r       specify root type
       -n       don't descend type tree
       -h       show help
       -b       specify indent string
       -i       specify incremental indent string
       -s       specify line spacing
qualifiers:
       froots   iterate over fundamental roots
       tree     print type tree

📜 Related Tools

Several useful developer tools have been build around GObject technology. The next sections briefly introduce them and link to the respective project pages.

For example, writing GObjects is often seen as a tedious task. It requires a lot of typing and just doing a copy/paste requires a great deal of care. A lot of projects and scripts have been written to generate GObject skeleton form boilerplate code, or even translating higher-level language into plain C.

🍀 Vala

From the Vala homepage itself: Vala is a new programming language that aims to bring modern programming language features to GNOME developers without imposing any additional runtime requirements and without using a different ABI compared to applications and libraries written in C.

The syntax of Vala is similar to C#. The available compiler translates Vala into GObject C code. It can also compile non-GObject C, using plain C API.

🍀 GObject builder

In order to help a GObject class developer, one obvious idea is to use some sort of templates for the skeletons and then run them through a special tool to generate the real C files. GOB (or GOB2) is such a tool. It is a preprocessor which can be used to build GObjects with inline C code so that there is no need to edit the generated C code. The syntax is inspired by Java and Yacc or Lex. The implementation is intentionally kept simple: the inline C code provided by the user is not parsed.

🍀 Graphical inspection of GObjects

Yet another tool that you may find helpful when working with GObjects is G-Inspector. It is able to display GLib/GTK objects and their properties.

🍀 Debugging reference count problems

The reference counting scheme used by GObject does solve quite a few memory management problems but also introduces new sources of bugs. In large applications, finding the exact spot where the reference count of an Object is not properly handled can be very difficult. A useful tool in debugging reference counting problems is to set breakpoints in gdb on g_object_ref() and g_object_unref(). Once you know the address of the object you are interested in, you can make the breakpoints conditional:

break g_object_ref if _object == 0xcafebabe
break g_object_unref if _object == 0xcafebabe

🍀 Writing API docs

The API documentation for most of the GLib, GObject, GTK and GNOME libraries is built with a combination of complex tools. Typically, the part of the documentation which describes the behavior of each function is extracted from the specially-formatted source code comments by a tool named gtk-doc which generates DocBook XML and merges this DocBook XML with a set of template XML DocBook files. These XML DocBook files are finally processed with xsltproc (a small program part of the libxslt library) to generate the final HTML output. Other tools can be used to generate PDF output from the source XML. The following code excerpt shows what these comments look like.

/**
 * gtk_widget_freeze_child_notify:
 * @widget: a #GtkWidget
 * 
 * Stops emission of "child-notify" signals on @widget. The signals are
 * queued until gtk_widget_thaw_child_notify() is called on @widget. 
 *
 * This is the analogue of g_object_freeze_notify() for child properties.
 **/
void
gtk_widget_freeze_child_notify (GtkWidget *widget)
{
...

Thorough documentation on how to set up and use gtk-doc in your project is provided on the GNOME developer website.

📜 Classes Hierarchy

https://docs.gtk.org/gobject/classes_hierarchy.html

`GObject.TypeInstance`
+-- `Object`
|   +--- `Binding`  
|   +--- `BindingGroup` 
|   +--- `InitiallyUnowned` 
|   +--- `SignalGroup`  
|   +--- `TypeModule`   
+-- `ParamSpec`
    +--- `ParamSpecBoolean` 
    +--- `ParamSpecBoxed`   
    +--- `ParamSpecChar`
    +--- `ParamSpecDouble`  
    +--- `ParamSpecEnum`
    +--- `ParamSpecFlags`   
    +--- `ParamSpecFloat`   
    +--- `ParamSpecGType`   
    +--- `ParamSpecInt` 
    +--- `ParamSpecInt64`   
    +--- `ParamSpecLong`
    +--- `ParamSpecObject`  
    +--- `ParamSpecOverride`
    +--- `ParamSpecParam`   
    +--- `ParamSpecPointer` 
    +--- `ParamSpecString`  
    +--- `ParamSpecUChar`   
    +--- `ParamSpecUInt`
    +--- `ParamSpecUInt64`  
    +--- `ParamSpecULong`   
    +--- `ParamSpecUnichar` 
    +--- `ParamSpecValueArray`  
    +--- `ParamSpecVariant` 

📜 GObject Sections

glib-2.78.0\docs\reference\gobject\gobject-sections.txt

💻FILE: gtype

🍀 Type Information

    GType;
    G_TYPE_FUNDAMENTAL;
    G_TYPE_FUNDAMENTAL_MAX;
    G_TYPE_MAKE_FUNDAMENTAL;
    G_TYPE_IS_ABSTRACT;
    G_TYPE_IS_DERIVED;
    G_TYPE_IS_FUNDAMENTAL;
    G_TYPE_IS_VALUE_TYPE;
    G_TYPE_HAS_VALUE_TABLE;
    G_TYPE_IS_CLASSED;
    G_TYPE_IS_INSTANTIATABLE;
    G_TYPE_IS_DERIVABLE;
    G_TYPE_IS_DEEP_DERIVABLE;
    G_TYPE_IS_INTERFACE;
    G_TYPE_IS_FINAL;
    G_TYPE_IS_DEPRECATED;
    GTypeInterface;
    GTypeInstance;
    GTypeClass;
    GTypeInfo;
    GTypeFundamentalInfo;
    GInterfaceInfo;
    GTypeValueInitFunc;
    GTypeValueFreeFunc;
    GTypeValueCopyFunc;
    GTypeValuePeekPointerFunc;
    GTypeValueCollectFunc;
    GTypeValueLCopyFunc;
    GTypeValueTable;
    G_TYPE_FROM_INSTANCE;
    G_TYPE_FROM_CLASS;
    G_TYPE_FROM_INTERFACE;
    G_TYPE_INSTANCE_GET_CLASS;
    G_TYPE_INSTANCE_GET_INTERFACE;
    G_TYPE_INSTANCE_GET_PRIVATE;
    G_TYPE_CLASS_GET_PRIVATE;
    G_TYPE_CHECK_INSTANCE;
    G_TYPE_CHECK_INSTANCE_CAST;
    G_TYPE_CHECK_INSTANCE_TYPE;
    G_TYPE_CHECK_INSTANCE_FUNDAMENTAL_TYPE;
    G_TYPE_CHECK_CLASS_CAST;
    G_TYPE_CHECK_CLASS_TYPE;
    G_TYPE_CHECK_VALUE;
    G_TYPE_CHECK_VALUE_TYPE;
    G_TYPE_FLAG_RESERVED_ID_BIT;
    g_type_init;
    GTypeDebugFlags;
    g_type_init_with_debug_flags;
    g_type_name;
    g_type_qname;
    g_type_from_name;
    g_type_parent;
    g_type_depth;
    g_type_next_base;
    g_type_is_a;
    g_type_class_ref;
    g_type_class_peek;
    g_type_class_peek_static;
    g_type_class_unref;
    g_type_class_peek_parent;
    g_type_class_add_private;
    g_type_add_class_private;
    g_type_interface_peek;
    g_type_interface_peek_parent;
    g_type_default_interface_ref;
    g_type_default_interface_peek;
    g_type_default_interface_unref;
    g_type_children;
    g_type_interfaces;
    g_type_interface_prerequisites;
    g_type_interface_instantiatable_prerequisite;
    g_type_set_qdata;
    g_type_get_qdata;
    g_type_query;
    GTypeQuery;
    GBaseInitFunc;
    GBaseFinalizeFunc;
    GClassInitFunc;
    GClassFinalizeFunc;
    GInstanceInitFunc;
    GInterfaceInitFunc;
    GInterfaceFinalizeFunc;
    GTypeClassCacheFunc;
    GTypeFlags;
    GTypeFundamentalFlags;
    g_type_register_static;
    g_type_register_static_simple;
    g_type_register_dynamic;
    g_type_register_fundamental;
    g_type_add_interface_static;
    g_type_add_interface_dynamic;
    g_type_interface_add_prerequisite;
    g_type_get_plugin;
    g_type_interface_get_plugin;
    g_type_fundamental_next;
    g_type_fundamental;
    g_type_create_instance;
    g_type_free_instance;
    g_type_add_class_cache_func;
    g_type_remove_class_cache_func;
    g_type_class_unref_uncached;
    g_type_add_interface_check;
    g_type_remove_interface_check;
    GTypeInterfaceCheckFunc;
    g_type_value_table_peek;
    g_type_ensure;
    g_type_get_type_registration_serial;
    g_type_get_instance_count;

    G_DECLARE_FINAL_TYPE;
    G_DECLARE_DERIVABLE_TYPE;
    G_DECLARE_INTERFACE;
    G_DEFINE_TYPE;
    G_DEFINE_TYPE_WITH_PRIVATE;
    G_DEFINE_TYPE_WITH_CODE;
    G_DEFINE_ABSTRACT_TYPE;
    G_DEFINE_ABSTRACT_TYPE_WITH_PRIVATE;
    G_DEFINE_ABSTRACT_TYPE_WITH_CODE;
    G_DEFINE_FINAL_TYPE;
    G_DEFINE_FINAL_TYPE_WITH_PRIVATE;
    G_DEFINE_FINAL_TYPE_WITH_CODE;
    G_ADD_PRIVATE;
    G_PRIVATE_OFFSET;
    G_PRIVATE_FIELD;
    G_PRIVATE_FIELD_P;
    G_DEFINE_INTERFACE;
    G_DEFINE_INTERFACE_WITH_CODE;
    G_IMPLEMENT_INTERFACE;
    G_DEFINE_TYPE_EXTENDED;
    G_DEFINE_BOXED_TYPE;
    G_DEFINE_BOXED_TYPE_WITH_CODE;
    G_DEFINE_POINTER_TYPE;
    G_DEFINE_POINTER_TYPE_WITH_CODE;
    G_DEFINE_ENUM_VALUE;
    G_DEFINE_ENUM_TYPE;
    G_DEFINE_FLAGS_TYPE;

Subsection Private

    G_TYPE_FUNDAMENTAL_SHIFT;
    g_type_check_instance;
    g_type_check_instance_cast;
    g_type_check_instance_is_a;
    g_type_check_instance_is_fundamentally_a;
    g_type_check_class_cast;
    g_type_check_class_is_a;
    g_type_check_is_value_type;
    g_type_check_value;
    g_type_check_value_holds;
    g_type_class_adjust_private_offset;
    g_type_add_instance_private;
    g_type_instance_get_private;
    g_type_class_get_instance_private_offset;
    g_type_class_get_private;
    g_type_test_flags;
    g_type_name_from_instance;
    g_type_name_from_class;

Subsection

    G_TYPE_INVALID;
    G_TYPE_NONE;
    G_TYPE_INTERFACE;
    G_TYPE_CHAR;
    G_TYPE_UCHAR;
    G_TYPE_BOOLEAN;
    G_TYPE_INT;
    G_TYPE_UINT;
    G_TYPE_LONG;
    G_TYPE_ULONG;
    G_TYPE_INT64;
    G_TYPE_UINT64;
    G_TYPE_ENUM;
    G_TYPE_FLAGS;
    G_TYPE_FLOAT;
    G_TYPE_DOUBLE;
    G_TYPE_STRING;
    G_TYPE_POINTER;
    G_TYPE_BOXED;
    G_TYPE_PARAM;
    G_TYPE_OBJECT;
    G_TYPE_GTYPE;
    G_TYPE_VARIANT;
    G_TYPE_CHECKSUM;

Subsection

    G_TYPE_RESERVED_GLIB_FIRST;
    G_TYPE_RESERVED_GLIB_LAST;
    G_TYPE_RESERVED_BSE_FIRST;
    G_TYPE_RESERVED_BSE_LAST;
    G_TYPE_RESERVED_USER_FIRST;

💻FILE: gtypeplugin

🍀 GTypePlugin

    GTypePlugin;
    GTypePluginClass;
    GTypePluginUse;
    GTypePluginUnuse;
    GTypePluginCompleteTypeInfo;
    GTypePluginCompleteInterfaceInfo;
    g_type_plugin_use;
    g_type_plugin_unuse;
    g_type_plugin_complete_type_info;
    g_type_plugin_complete_interface_info;

Subsection Standard

    G_TYPE_PLUGIN;
    G_IS_TYPE_PLUGIN;
    G_TYPE_TYPE_PLUGIN;
    g_type_plugin_get_type;
    G_TYPE_PLUGIN_CLASS;
    G_IS_TYPE_PLUGIN_CLASS;
    G_TYPE_PLUGIN_GET_CLASS;

💻FILE: gtypemodule

🍀 GTypeModule

    GTypeModule;
    GTypeModuleClass;
    g_type_module_use;
    g_type_module_unuse;
    g_type_module_set_name;
    g_type_module_register_type;
    g_type_module_add_interface;
    g_type_module_register_enum;
    g_type_module_register_flags;
    G_DEFINE_DYNAMIC_TYPE;
    G_DEFINE_DYNAMIC_TYPE_EXTENDED;
    G_IMPLEMENT_INTERFACE_DYNAMIC;
    G_ADD_PRIVATE_DYNAMIC;

Subsection Standard

    G_TYPE_MODULE;
    G_IS_TYPE_MODULE;
    G_TYPE_TYPE_MODULE;
    g_type_module_get_type;
    G_TYPE_MODULE_CLASS;
    G_IS_TYPE_MODULE_CLASS;
    G_TYPE_MODULE_GET_CLASS;

🍀 The Base Object Type

💻FILE: objects

    GObject;
    GObjectClass;
    GObjectConstructParam;
    GObjectGetPropertyFunc;
    GObjectSetPropertyFunc;
    GObjectFinalizeFunc;
    G_TYPE_IS_OBJECT;
    G_OBJECT;
    G_IS_OBJECT;
    G_OBJECT_CLASS;
    G_IS_OBJECT_CLASS;
    G_OBJECT_GET_CLASS;
    G_OBJECT_TYPE;
    G_OBJECT_TYPE_NAME;
    G_OBJECT_CLASS_TYPE;
    G_OBJECT_CLASS_NAME;
    g_object_class_install_property;
    g_object_class_install_properties;
    g_object_class_find_property;
    g_object_class_list_properties;
    g_object_class_override_property;
    g_object_interface_install_property;
    g_object_interface_find_property;
    g_object_interface_list_properties;
    g_object_new;
    g_object_new_with_properties;
    g_object_newv;
    GParameter;
    g_object_ref;
    g_object_unref;
    g_object_ref_sink;
    g_object_take_ref;
    g_set_object;
    g_clear_object;
    GInitiallyUnowned;
    GInitiallyUnownedClass;
    G_TYPE_INITIALLY_UNOWNED;
    g_object_is_floating;
    g_object_force_floating;
    GWeakNotify;
    g_object_weak_ref;
    g_object_weak_unref;
    g_object_add_weak_pointer;
    g_object_remove_weak_pointer;
    g_set_weak_pointer;
    g_clear_weak_pointer;
    GToggleNotify;
    g_object_add_toggle_ref;
    g_object_remove_toggle_ref;
    g_object_connect;
    g_object_disconnect;
    g_object_set;
    g_object_setv;
    g_object_get;
    g_object_getv;
    g_object_notify;
    g_object_notify_by_pspec;
    g_object_freeze_notify;
    g_object_thaw_notify;
    g_object_get_data;
    g_object_set_data;
    g_object_set_data_full;
    g_object_steal_data;
    g_object_dup_data;
    g_object_replace_data;
    g_object_get_qdata;
    g_object_set_qdata;
    g_object_set_qdata_full;
    g_object_steal_qdata;
    g_object_dup_qdata;
    g_object_replace_qdata;
    g_object_set_property;
    g_object_get_property;
    g_object_new_valist;
    g_object_set_valist;
    g_object_get_valist;
    g_object_watch_closure;
    g_object_run_dispose;
    G_OBJECT_WARN_INVALID_PROPERTY_ID;

Subsection Weak references

    GWeakRef;
    g_weak_ref_init;
    g_weak_ref_clear;
    g_weak_ref_get;
    g_weak_ref_set;

Subsection Testing

    g_assert_finalize_object;

Subsection Standard

    G_INITIALLY_UNOWNED;
    G_INITIALLY_UNOWNED_CLASS;
    G_INITIALLY_UNOWNED_GET_CLASS;
    G_IS_INITIALLY_UNOWNED;
    G_IS_INITIALLY_UNOWNED_CLASS;

Subsection Private

    G_OBJECT_WARN_INVALID_PSPEC;
    g_initially_unowned_get_type;
    g_object_compat_control;
    g_object_get_type;

🍀 Enumeration and Flag Types

💻FILE: enumerations_flags

    GEnumClass;
    GFlagsClass;
    G_ENUM_CLASS_TYPE;
    G_ENUM_CLASS_TYPE_NAME;
    G_TYPE_IS_ENUM;
    G_ENUM_CLASS;
    G_IS_ENUM_CLASS;
    G_TYPE_IS_FLAGS;
    G_FLAGS_CLASS;
    G_IS_FLAGS_CLASS;
    G_FLAGS_CLASS_TYPE;
    G_FLAGS_CLASS_TYPE_NAME;
    GEnumValue;
    GFlagsValue;
    g_enum_get_value;
    g_enum_get_value_by_name;
    g_enum_get_value_by_nick;
    g_enum_to_string;
    g_flags_get_first_value;
    g_flags_get_value_by_name;
    g_flags_get_value_by_nick;
    g_flags_to_string;
    g_enum_register_static;
    g_flags_register_static;
    g_enum_complete_type_info;
    g_flags_complete_type_info;

💻FILE: gboxed

🍀 Boxed Types

    GBoxedCopyFunc;
    GBoxedFreeFunc;
    g_boxed_copy;
    g_boxed_free;
    g_boxed_type_register_static;
    g_pointer_type_register_static;

Subsection

    G_TYPE_HASH_TABLE;
    G_TYPE_DATE;
    G_TYPE_GSTRING;
    G_TYPE_STRV;
    G_TYPE_REGEX;
    G_TYPE_MATCH_INFO;
    G_TYPE_ARRAY;
    G_TYPE_BYTE_ARRAY;
    G_TYPE_PTR_ARRAY;
    G_TYPE_BYTES;
    G_TYPE_VARIANT_TYPE;
    G_TYPE_ERROR;
    G_TYPE_DATE_TIME;
    G_TYPE_TIME_ZONE;
    G_TYPE_IO_CHANNEL;
    G_TYPE_IO_CONDITION;
    G_TYPE_VARIANT_BUILDER;
    G_TYPE_VARIANT_DICT;
    G_TYPE_KEY_FILE;
    G_TYPE_MAIN_CONTEXT;
    G_TYPE_MAIN_LOOP;
    G_TYPE_MAPPED_FILE;
    G_TYPE_MARKUP_PARSE_CONTEXT;
    G_TYPE_SOURCE;
    G_TYPE_POLLFD;
    G_TYPE_THREAD;
    G_TYPE_OPTION_GROUP;
    G_TYPE_URI;
    G_TYPE_TREE;
    G_TYPE_PATTERN_SPEC;
    G_TYPE_BOOKMARK_FILE;

Subsection Standard

    G_TYPE_IS_BOXED;

Subsection Private

    g_gstring_get_type;
    g_strv_get_type;
    g_date_get_type;
    g_hash_table_get_type;
    g_regex_get_type;
    g_match_info_get_type;
    g_array_get_type;
    g_byte_array_get_type;
    g_ptr_array_get_type;
    g_error_get_type;
    g_date_time_get_type;
    g_time_zone_get_type;
    g_variant_get_gtype;
    g_variant_type_get_gtype;
    g_variant_builder_get_type;
    g_variant_dict_get_type;
    g_gtype_get_type;
    g_main_context_get_type;
    g_main_loop_get_type;
    g_source_get_type;
    g_pollfd_get_type;
    g_bytes_get_type;
    g_key_file_get_type;
    g_checksum_get_type;
    g_mapped_file_get_type;
    g_markup_parse_context_get_type;
    g_thread_get_type;
    g_option_group_get_type;
    g_uri_get_type;
    g_tree_get_type;
    g_pattern_spec_get_type;
    g_bookmark_file_get_type;

🍀 Generic values

💻FILE: generic_values

    G_VALUE_INIT;
    G_VALUE_HOLDS;
    G_VALUE_TYPE;
    G_VALUE_TYPE_NAME;
    G_TYPE_IS_VALUE;
    G_TYPE_IS_VALUE_ABSTRACT;
    G_IS_VALUE;
    GValue;
    G_TYPE_VALUE;
    G_TYPE_VALUE_ARRAY;
    g_value_init;
    g_value_copy;
    g_value_reset;
    g_value_unset;
    g_value_init_from_instance;
    g_value_set_instance;
    g_value_fits_pointer;
    g_value_peek_pointer;
    g_value_type_compatible;
    g_value_type_transformable;
    g_value_transform;
    GValueTransform;
    g_value_register_transform_func;
    g_strdup_value_contents;

Subsection Private

    G_VALUE_NOCOPY_CONTENTS;
    g_value_get_type;
    g_value_array_get_type;

🍀 Value arrays

💻FILE: value_arrays

    GValueArray;
    g_value_array_get_nth;
    g_value_array_new;
    g_value_array_copy;
    g_value_array_free;
    g_value_array_append;
    g_value_array_prepend;
    g_value_array_insert;
    g_value_array_remove;
    g_value_array_sort;
    g_value_array_sort_with_data;

🍀 GParamSpec

💻FILE: gparamspec

    G_TYPE_IS_PARAM;
    G_PARAM_SPEC;
    G_IS_PARAM_SPEC;
    G_PARAM_SPEC_CLASS;
    G_IS_PARAM_SPEC_CLASS;
    G_PARAM_SPEC_GET_CLASS;
    G_PARAM_SPEC_TYPE;
    G_PARAM_SPEC_TYPE_NAME;
    G_PARAM_SPEC_VALUE_TYPE;
    GParamSpec;
    GParamSpecClass;
    GParamFlags;
    G_PARAM_STATIC_STRINGS;
    G_PARAM_MASK;
    G_PARAM_USER_SHIFT;
    g_param_spec_ref;
    g_param_spec_unref;
    g_param_spec_sink;
    g_param_spec_ref_sink;
    g_param_spec_get_default_value;
    g_param_value_set_default;
    g_param_value_defaults;
    g_param_value_validate;
    g_param_value_is_valid;
    g_param_value_convert;
    g_param_values_cmp;
    g_param_spec_is_valid_name;
    g_param_spec_get_name;
    g_param_spec_get_name_quark;
    g_param_spec_get_nick;
    g_param_spec_get_blurb;
    g_param_spec_get_qdata;
    g_param_spec_set_qdata;
    g_param_spec_set_qdata_full;
    g_param_spec_steal_qdata;
    g_param_spec_get_redirect_target;
    g_param_spec_internal;
    GParamSpecTypeInfo;
    g_param_type_register_static;
    GParamSpecPool;
    g_param_spec_pool_new;
    g_param_spec_pool_insert;
    g_param_spec_pool_remove;
    g_param_spec_pool_lookup;
    g_param_spec_pool_list;
    g_param_spec_pool_list_owned;

🍀 Standard Parameter and Value Types

💻FILE: param_value_types

Subsection gboolean

    G_IS_PARAM_SPEC_BOOLEAN;
    G_PARAM_SPEC_BOOLEAN;
    G_VALUE_HOLDS_BOOLEAN;
    G_TYPE_PARAM_BOOLEAN;
    GParamSpecBoolean;
    g_param_spec_boolean;
    g_value_set_boolean;
    g_value_get_boolean;

Subsection gchar

    G_IS_PARAM_SPEC_CHAR;
    G_PARAM_SPEC_CHAR;
    G_VALUE_HOLDS_CHAR;
    G_TYPE_PARAM_CHAR;
    GParamSpecChar;
    g_param_spec_char;
    g_value_set_char;
    g_value_get_char;
    g_value_get_schar;
    g_value_set_schar;

Subsection guchar

    G_IS_PARAM_SPEC_UCHAR;
    G_PARAM_SPEC_UCHAR;
    G_VALUE_HOLDS_UCHAR;
    G_TYPE_PARAM_UCHAR;
    GParamSpecUChar;
    g_param_spec_uchar;
    g_value_set_uchar;
    g_value_get_uchar;

Subsection gint

    G_IS_PARAM_SPEC_INT;
    G_PARAM_SPEC_INT;
    G_VALUE_HOLDS_INT  ;
    G_TYPE_PARAM_INT;
    GParamSpecInt;
    g_param_spec_int;
    g_value_set_int;
    g_value_get_int;

Subsection guint

    G_IS_PARAM_SPEC_UINT;
    G_PARAM_SPEC_UINT;
    G_VALUE_HOLDS_UINT  ;
    G_TYPE_PARAM_UINT;
    GParamSpecUInt;
    g_param_spec_uint;
    g_value_set_uint;
    g_value_get_uint;

Subsection glong

    G_IS_PARAM_SPEC_LONG;
    G_PARAM_SPEC_LONG;
    G_VALUE_HOLDS_LONG ;
    G_TYPE_PARAM_LONG;
    GParamSpecLong;
    g_param_spec_long;
    g_value_set_long;
    g_value_get_long;

Subsection gulong

    G_IS_PARAM_SPEC_ULONG;
    G_PARAM_SPEC_ULONG;
    G_VALUE_HOLDS_ULONG ;
    G_TYPE_PARAM_ULONG;
    GParamSpecULong;
    g_param_spec_ulong;
    g_value_set_ulong;
    g_value_get_ulong;

Subsection gint64

    G_IS_PARAM_SPEC_INT64;
    G_PARAM_SPEC_INT64;
    G_VALUE_HOLDS_INT64;
    G_TYPE_PARAM_INT64;
    GParamSpecInt64;
    g_param_spec_int64;
    g_value_set_int64;
    g_value_get_int64;

Subsection guint64

    G_IS_PARAM_SPEC_UINT64;
    G_PARAM_SPEC_UINT64;
    G_VALUE_HOLDS_UINT64;
    G_TYPE_PARAM_UINT64;
    GParamSpecUInt64;
    g_param_spec_uint64;
    g_value_set_uint64;
    g_value_get_uint64;

Subsection gfloat

    G_IS_PARAM_SPEC_FLOAT;
    G_PARAM_SPEC_FLOAT;
    G_VALUE_HOLDS_FLOAT;
    G_TYPE_PARAM_FLOAT;
    GParamSpecFloat;
    g_param_spec_float;
    g_value_set_float;
    g_value_get_float;

Subsection gdouble

    G_IS_PARAM_SPEC_DOUBLE;
    G_PARAM_SPEC_DOUBLE;
    G_VALUE_HOLDS_DOUBLE;
    G_TYPE_PARAM_DOUBLE;
    GParamSpecDouble;
    g_param_spec_double;
    g_value_set_double;
    g_value_get_double;

Subsection GEnum

    G_IS_PARAM_SPEC_ENUM;
    G_PARAM_SPEC_ENUM;
    G_VALUE_HOLDS_ENUM;
    G_TYPE_PARAM_ENUM;
    GParamSpecEnum;
    g_param_spec_enum;
    g_value_set_enum;
    g_value_get_enum;

Subsection GFLags

    G_IS_PARAM_SPEC_FLAGS;
    G_PARAM_SPEC_FLAGS;
    G_VALUE_HOLDS_FLAGS;
    G_TYPE_PARAM_FLAGS;
    GParamSpecFlags;
    g_param_spec_flags;
    g_value_set_flags;
    g_value_get_flags;

Subsection gchararray

    G_IS_PARAM_SPEC_STRING;
    G_PARAM_SPEC_STRING;
    G_VALUE_HOLDS_STRING;
    G_TYPE_PARAM_STRING;
    G_VALUE_IS_INTERNED_STRING;
    G_VALUE_INTERNED_STRING;
    GParamSpecString;
    gchararray;
    g_param_spec_string;
    g_value_set_string;
    g_value_set_static_string;
    g_value_take_string;
    g_value_set_string_take_ownership;
    g_value_get_string;
    g_value_dup_string;
    g_value_set_interned_string;

Subsection GParamSpec

    G_IS_PARAM_SPEC_PARAM;
    G_PARAM_SPEC_PARAM;
    G_VALUE_HOLDS_PARAM;
    G_TYPE_PARAM_PARAM;
    GParamSpecParam;
    g_param_spec_param;
    g_value_set_param;
    g_value_take_param;
    g_value_set_param_take_ownership;
    g_value_get_param;
    g_value_dup_param;

Subsection GBoxed

    G_IS_PARAM_SPEC_BOXED;
    G_PARAM_SPEC_BOXED;
    G_VALUE_HOLDS_BOXED;
    G_TYPE_PARAM_BOXED;
    GParamSpecBoxed;
    g_param_spec_boxed;
    g_value_set_boxed;
    g_value_set_static_boxed;
    g_value_take_boxed;
    g_value_set_boxed_take_ownership;
    g_value_get_boxed;
    g_value_dup_boxed;

Subsection gpointer

    G_IS_PARAM_SPEC_POINTER;
    G_PARAM_SPEC_POINTER;
    G_VALUE_HOLDS_POINTER;
    G_TYPE_PARAM_POINTER;
    GParamSpecPointer;
    g_param_spec_pointer;
    g_value_set_pointer;
    g_value_get_pointer;

Subsection GObject

    G_IS_PARAM_SPEC_OBJECT;
    G_PARAM_SPEC_OBJECT;
    G_VALUE_HOLDS_OBJECT;
    G_TYPE_PARAM_OBJECT;
    GParamSpecObject;
    g_param_spec_object;
    g_value_set_object;
    g_value_take_object;
    g_value_set_object_take_ownership;
    g_value_get_object;
    g_value_dup_object;

Subsection gunichar

    G_IS_PARAM_SPEC_UNICHAR;
    G_PARAM_SPEC_UNICHAR;
    G_TYPE_PARAM_UNICHAR;
    GParamSpecUnichar;
    g_param_spec_unichar;

Subsection GValueArray

    G_IS_PARAM_SPEC_VALUE_ARRAY;
    G_PARAM_SPEC_VALUE_ARRAY;
    G_TYPE_PARAM_VALUE_ARRAY;
    GParamSpecValueArray;
    g_param_spec_value_array;

Subsection Override

    G_IS_PARAM_SPEC_OVERRIDE;
    G_PARAM_SPEC_OVERRIDE;
    G_TYPE_PARAM_OVERRIDE;
    GParamSpecOverride;
    g_param_spec_override;

Subsection GType

    G_IS_PARAM_SPEC_GTYPE;
    G_PARAM_SPEC_GTYPE;
    G_VALUE_HOLDS_GTYPE;
    G_TYPE_PARAM_GTYPE;
    GParamSpecGType;
    g_param_spec_gtype;
    g_value_get_gtype;
    g_value_set_gtype;

Subsection GVariant

    G_IS_PARAM_SPEC_VARIANT;
    G_PARAM_SPEC_VARIANT;
    G_VALUE_HOLDS_VARIANT;
    G_TYPE_PARAM_VARIANT;
    GParamSpecVariant;
    g_param_spec_variant;
    g_value_get_variant;
    g_value_dup_variant;
    g_value_set_variant;
    g_value_take_variant;

Subsection Private

    g_value_set_instance;
    g_param_spec_types;

🍀 Varargs Value Collection

💻FILE: value_collection

<INCLUDE>glib-object.h,gobject/gvaluecollector.h</INCLUDE>;

    GTypeCValue;
    G_VALUE_COLLECT_INIT;
    G_VALUE_COLLECT_INIT2;
    G_VALUE_COLLECT;
    G_VALUE_COLLECT_SKIP;
    G_VALUE_LCOPY;
    G_VALUE_COLLECT_FORMAT_MAX_LENGTH;

🍀 Signals

💻FILE: signals

    GSignalInvocationHint;
    GSignalAccumulator;
    GSignalCMarshaller;
    GSignalCVaMarshaller;
    GSignalEmissionHook;
    GSignalFlags;
    GSignalMatchType;
    GSignalQuery;
    G_SIGNAL_TYPE_STATIC_SCOPE;
    G_SIGNAL_MATCH_MASK;
    G_SIGNAL_FLAGS_MASK;
    g_signal_new;
    g_signal_newv;
    g_signal_new_valist;
    g_signal_set_va_marshaller;
    g_signal_query;
    g_signal_lookup;
    g_signal_name;
    g_signal_list_ids;
    g_signal_emit;
    g_signal_emit_by_name;
    g_signal_emitv;
    g_signal_emit_valist;
    g_signal_connect;
    g_signal_connect_after;
    g_signal_connect_swapped;
    g_signal_connect_object;
    GConnectFlags;
    g_signal_connect_data;
    g_signal_connect_closure;
    g_signal_connect_closure_by_id;
    g_signal_handler_block;
    g_signal_handler_unblock;
    g_signal_handler_disconnect;
    g_signal_handler_find;
    g_signal_handlers_block_matched;
    g_signal_handlers_unblock_matched;
    g_signal_handlers_disconnect_matched;
    g_signal_handler_is_connected;
    g_signal_handlers_block_by_func;
    g_signal_handlers_unblock_by_func;
    g_signal_handlers_disconnect_by_func;
    g_signal_handlers_disconnect_by_data;
    g_signal_has_handler_pending;
    g_signal_stop_emission;
    g_signal_stop_emission_by_name;
    g_signal_override_class_closure;
    g_signal_chain_from_overridden;
    g_signal_new_class_handler;
    g_signal_override_class_handler;
    g_signal_chain_from_overridden_handler;
    g_signal_add_emission_hook;
    g_signal_remove_emission_hook;
    g_signal_is_valid_name;
    g_signal_parse_name;
    g_signal_get_invocation_hint;
    g_signal_type_cclosure_new;
    g_signal_accumulator_first_wins;
    g_signal_accumulator_true_handled;
    g_clear_signal_handler;

Subsection Private

    g_signal_handlers_destroy;

💻FILE: gclosure

🍀 Closures

    G_CLOSURE_NEEDS_MARSHAL;
    G_CLOSURE_N_NOTIFIERS;
    G_CCLOSURE_SWAP_DATA;
    G_CALLBACK;
    GCallback;
    GClosure;
    G_TYPE_CLOSURE;
    GCClosure;
    GClosureMarshal;
    GVaClosureMarshal;
    GClosureNotify;
    g_cclosure_new;
    g_cclosure_new_swap;
    g_cclosure_new_object;
    g_cclosure_new_object_swap;
    g_cclosure_marshal_generic;
    g_closure_new_object;
    g_closure_ref;
    g_closure_sink;
    g_closure_unref;
    g_closure_invoke;
    g_closure_invalidate;
    g_closure_add_finalize_notifier;
    g_closure_add_invalidate_notifier;
    g_closure_remove_finalize_notifier;
    g_closure_remove_invalidate_notifier;
    g_closure_new_simple;
    g_closure_set_marshal;
    g_closure_add_marshal_guards;
    g_closure_set_meta_marshal;
    g_source_set_closure;
    g_source_set_dummy_callback;

Subsection

    g_cclosure_marshal_VOID__VOID;
    g_cclosure_marshal_VOID__BOOLEAN;
    g_cclosure_marshal_VOID__CHAR;
    g_cclosure_marshal_VOID__UCHAR;
    g_cclosure_marshal_VOID__INT;
    g_cclosure_marshal_VOID__UINT;
    g_cclosure_marshal_VOID__LONG;
    g_cclosure_marshal_VOID__ULONG;
    g_cclosure_marshal_VOID__ENUM;
    g_cclosure_marshal_VOID__FLAGS;
    g_cclosure_marshal_VOID__FLOAT;
    g_cclosure_marshal_VOID__DOUBLE;
    g_cclosure_marshal_VOID__STRING;
    g_cclosure_marshal_VOID__PARAM;
    g_cclosure_marshal_VOID__BOXED;
    g_cclosure_marshal_VOID__POINTER;
    g_cclosure_marshal_VOID__OBJECT;
    g_cclosure_marshal_VOID__VARIANT;
    g_cclosure_marshal_STRING__OBJECT_POINTER;
    g_cclosure_marshal_VOID__UINT_POINTER;
    g_cclosure_marshal_BOOLEAN__FLAGS;
    g_cclosure_marshal_BOOL__FLAGS;
    g_cclosure_marshal_BOOLEAN__BOXED_BOXED;
    g_cclosure_marshal_BOOL__BOXED_BOXED;

Subsection

    g_cclosure_marshal_generic_va;
    g_cclosure_marshal_VOID__VOIDv;
    g_cclosure_marshal_VOID__BOOLEANv;
    g_cclosure_marshal_VOID__CHARv;
    g_cclosure_marshal_VOID__UCHARv;
    g_cclosure_marshal_VOID__INTv;
    g_cclosure_marshal_VOID__UINTv;
    g_cclosure_marshal_VOID__LONGv;
    g_cclosure_marshal_VOID__ULONGv;
    g_cclosure_marshal_VOID__ENUMv;
    g_cclosure_marshal_VOID__FLAGSv;
    g_cclosure_marshal_VOID__FLOATv;
    g_cclosure_marshal_VOID__DOUBLEv;
    g_cclosure_marshal_VOID__STRINGv;
    g_cclosure_marshal_VOID__PARAMv;
    g_cclosure_marshal_VOID__BOXEDv;
    g_cclosure_marshal_VOID__POINTERv;
    g_cclosure_marshal_VOID__OBJECTv;
    g_cclosure_marshal_VOID__VARIANTv;
    g_cclosure_marshal_STRING__OBJECT_POINTERv;
    g_cclosure_marshal_VOID__UINT_POINTERv;
    g_cclosure_marshal_BOOLEAN__FLAGSv;
    g_cclosure_marshal_BOOLEAN__BOXED_BOXEDv;

Subsection Private

    GClosureNotifyData;
    g_closure_get_type;
    g_io_channel_get_type;
    g_io_condition_get_type;

💻FILE: gbinding

    GBinding;
    GBindingFlags;
    g_binding_get_source;
    g_binding_dup_source;
    g_binding_get_source_property;
    g_binding_get_target;
    g_binding_dup_target;
    g_binding_get_target_property;
    g_binding_get_flags;
    g_binding_unbind;

Subsection

    g_object_bind_property;
    GBindingTransformFunc;
    g_object_bind_property_full;
    g_object_bind_property_with_closures;

Subsection Standard

    G_TYPE_BINDING;
    G_TYPE_BINDING_FLAGS;
    G_BINDING;
    G_IS_BINDING;

Subsection Private

    g_binding_flags_get_type;
    g_binding_get_type;

💻FILE: gbindinggroup

    GBindingGroup;
    g_binding_group_new;
    g_binding_group_dup_source;
    g_binding_group_set_source;
    g_binding_group_bind;
    g_binding_group_bind_full;
    g_binding_group_bind_with_closures;

Subsection Standard

    G_TYPE_BINDING_GROUP;
    G_TYPE_BINDING_GROUP_CLASS;
    G_BINDING_GROUP;
    G_IS_BINDING_GROUP;

Subsection Private

    g_binding_group_get_type;

💻FILE: gsignalgroup

    GSignalGroup;
    g_signal_group_block;
    g_signal_group_connect;
    g_signal_group_connect_after;
    g_signal_group_connect_data;
    g_signal_group_connect_object;
    g_signal_group_connect_swapped;
    g_signal_group_connect_closure;
    g_signal_group_dup_target;
    g_signal_group_get_type;
    g_signal_group_new;
    g_signal_group_set_target;
    g_signal_group_unblock;

Subsection Standard

    G_IS_SIGNAL_GROUP;
    G_SIGNAL_GROUP;
    G_TYPE_SIGNAL_GROUP;

Subsection Private

    g_signal_group_get_type;