Managing Memory =============== The GNOME stack is predominantly written in C, so dynamically allocated memory has to be managed manually. Through use of GLib convenience APIs, memory management can be trivial, but programmers always need to keep memory in mind when writing code. It is assumed that the reader is familiar with the idea of heap allocation of memory using ``malloc()`` and ``free()``, and knows of the paired GLib equivalents, ``g_malloc()`` and ``g_free()``. Principles of memory management ------------------------------- The normal approach to memory management is for the programmer to keep track of which variables point to allocated memory, and to manually free them when they are no longer needed. This is correct, but can be clarified by introducing the concept of ownership, which is the piece of code (such as a function, struct or object) which is responsible for freeing a piece of allocated memory (an allocation). Each allocation has exactly one owner; this owner may change as the program runs, by transferring ownership to another piece of code. Each variable is owned or unowned, according to whether the scope containing it is always its owner. Each function parameter and return type either transfers ownership of the values passed to it, or it doesn’t. If code which owns some memory doesn’t deallocate that memory, that’s a memory leak. If code which doesn’t own some memory frees it, that’s a double-free. Both are bad. By statically calculating which variables are owned, memory management becomes a simple task of unconditionally freeing the owned variables before they leave their scope, and not freeing the unowned variables. The key question to answer for all memory is: which code has ownership of this memory? There is an important restriction here: variables must never change from owned to unowned (or vice-versa) at runtime. This restriction is key to simplifying memory management. For example, consider the functions: .. code-block:: c char * generate_string (const char *template); void print_string (const char *str); The following code has been annotated to note where the ownership transfers happen: .. code-block:: c char *my_str = NULL; /* owned */ const char *template; /* unowned */ GValue value = G_VALUE_INIT; /* owned */ g_value_init (&value, G_TYPE_STRING); /* Transfers ownership of a string from the function to the variable. */ template = "XXXXXX"; my_str = generate_string (template); /* No ownership transfer. */ print_string (my_str); /* Transfer ownership. We no longer have to free @my_str. */ g_value_take_string (&value, my_str); /* We still have ownership of @value, so free it before it goes out of scope. */ g_value_unset (&value); There are a few points here: Firstly, the ‘owned’ comments by the variable declarations denote that those variables are owned by the local scope, and hence need to be freed before they go out of scope. The alternative is ‘unowned’, which means the local scope does not have ownership, and must not free the variables before going out of scope. Similarly, ownership must not be transferred to them on assignment. Secondly, the variable type modifiers reflect whether they transfer ownership: because ``my_str`` is owned by the local scope, it has type ``char*``, whereas ``template`` is ``const`` to denote it is unowned. Similarly, the ``template`` parameter of ``generate_string()`` and the ``str`` parameter of ``print_string()`` are marked as ``const`` because no ownership is transferred when those functions are called. As ownership is transferred for the string parameter of ``g_value_take_string()``, we can expect its type to be gchar. (Note that this is not the case for GObject instances and subclasses, which can never be ``const``. It is only the case for strings and simple structs.) Finally, a few libraries use a function naming convention to indicate ownership transfer, for example using ‘take’ in a function name to indicate full transfer of parameters, as with ``g_value_take_string()``. Note that different libraries use different conventions, as shown below: .. list-table:: :widths: 25 25 25 25 :header-rows: 1 * - Function name - Convention 1 (standard) - Convention 2 (alternate) - Convention 3 (``gdbus-codegen``) * - ``get`` - transfer: none - any transfer - transfer: full * - ``dup`` - transfer: full - unused - unused * - ``peek`` - unused - unused - transfer: none * - ``set`` - transfer: none - transfer: none - transfer: none * - ``take`` - transfer: full - unused - unused * - ``steal`` - transfer: full - transfer: full - transfer: full Ideally, all functions have a ``(transfer)`` introspection annotation for all relevant parameters and the return value. Failing that, here is a set of guidelines to use to determine whether ownership of a return value is transferred: #. If the type has an introspection ``(transfer)`` annotation, look at that. #. Otherwise, if the type is const, there is no transfer. #. Otherwise, if the function documentation explicitly specifies the return value must be freed, there is full or container transfer. #. Otherwise, if the function is named ‘dup’, ‘take’ or ‘steal’, there is full or container transfer. #. Otherwise, if the function is named ‘peek’, there is no transfer. #. Otherwise, you need to look at the function’s code to determine whether it intends ownership to be transferred. Then file a bug against the documentation for that function, and ask for an introspection annotation to be added. Given this ownership and transfer infrastructure, the correct approach to memory allocation can be mechanically determined for each situation. In each case, the ``copy()`` function must be appropriate to the data type, for example ``g_strdup()`` for strings, or ``g_object_ref()`` for GObjects. When thinking about ownership transfer, ``malloc()``/``free()`` and reference counting are equivalent: in the former case, a newly allocated piece of heap memory is transferred; in the latter, a newly incremented reference. Documentation ------------- Documenting the ownership transfer for each function parameter and return, and the ownership for each variable, is important. While they may be clear when writing the code, they are not clear a few months later; and may never be clear to users of an API. They should always be documented. The best way to document ownership transfer is to use the ``(transfer)`` annotation introduced by `gobject-introspection`_. Include this in the API documentation comment for each function parameter and return type. By doing so, the introspection tools can also read the annotations and use them to correctly introspect the API. .. _gobject-introspection: https://gi.readthedocs.io/en/latest/annotations/giannotations.html#memory-and-lifecycle-management .. tip:: If a function is not public API, write a documentation comment for it anyway and include the ``(transfer)`` annotations. This will help you and other people working on your code. For example: .. code-block:: c /** * g_value_take_string: * @value: (transfer none): an initialized #GValue * @str: (transfer full): string to set it to * * Function documentation goes here. */ /** * generate_string: * @template: (transfer none): a template to follow when generating the string * * Function documentation goes here. * * Returns: (transfer full): a newly generated string */ Ownership for variables can be documented using inline comments. These are non-standard, and not read by any tools, but can form a convention if used consistently: .. code-block:: c GObject *some_owned_object = NULL; /* owned */ GObject *some_unowned_object; /* unowned */ The documentation for container types is similarly only a convention; it includes the type of the contained elements too: .. code-block:: c /* PtrArray */ GPtrArray *some_unowned_string_array; /* unowned */ /* PtrArray */ GPtrArray *some_owned_string_array = NULL; /* owned */ /* PtrArray */ GPtrArray *some_owned_object_array = NULL; /* owned */ Note also that owned variables should always be initialized so that freeing them is more convenient. .. note:: Some types, for example basic C types like strings, can have the ``const`` modifier added if they are unowned, to take advantage of compiler warnings resulting from assigning those variables to owned variables (which must **not** use the ``const`` modifier). If so, the ``/* unowned */`` comment may be omitted. Reference counting ------------------ As well as conventional ``malloc()``/``free()``-style types, GLib has various reference counted types — GObject being a prime example. The concepts of ownership and transfer apply just as well to reference counted types as they do to allocated types. A scope owns a reference counted type if it holds a strong reference to the instance (for example by calling ``g_object_ref()``). An instance can be ‘copied’ by calling ``g_object_ref()`` again. Ownership can be released with ``g_object_unref()`` — even though this may not actually finalize the instance, it frees the current scope’s ownership of that instance. .. tip:: See ``g_clear_object()`` for a convenient way of handling GObject references. There are other reference counted types in GLib, such as ``GHashTable`` (using ``g_hash_table_ref()`` and ``g_hash_table_unref()``), or ``GVariant`` (``g_variant_ref()``, ``g_variant_unref()``). For historical reasons, some types, like ``GHashTable``, support both reference counting and explicit finalization. Reference counting should always be used in preference, because it allows instances to be easily shared between multiple scopes (each holding their own reference) without having to allocate multiple copies of the instance. This saves memory. .. tip:: GLib `provides API `__ for easily implementing reference counted types, in the form of ``g_rc_box_new()`` and ``g_atomic_rc_box_new()``. Floating references ~~~~~~~~~~~~~~~~~~~ Classes which are derived from ``GInitiallyUnowned``, as opposed to GObject, have an initial reference which is *floating*, meaning that no code owns the reference. As soon as ``g_object_ref_sink()`` is called on the object, the floating reference is converted to a strong reference, and the calling code assumes ownership of the object. Floating references are a convenience for use in C in APIs, such as GTK, where large numbers of objects must be created and organized into a hierarchy. In these cases, calling ``g_object_unref()`` to drop all the strong references would result in a lot of code: .. code:: // Without floating references GtkWidget *new_widget; new_widget = gtk_some_widget_new (); gtk_container_add (some_container, new_widget); g_object_unref (new_widget); // With floating references gtk_container_add (some_container, gtk_some_widget_new ()); .. note:: Note that g_object_ref_sink() is equivalent to g_object_ref() when called on a non-floating reference, making gtk_container_add() no different from any other function in such cases. Floating references are only used by a few APIs — in particular, ``GtkWidget`` and all its subclasses, or ``GVariant``. You must learn which APIs support it, and which APIs consume floating references, and only use them together. .. important:: When designing the API of newer libraries you should **not** use floating references, as they require special handling inside language bindings. Floating references can be effectively replaced by having your ``container_add()`` function transfer the ownership of the object you are adding, and annotating the argument using ``(transfer full)``: .. code-block:: c /** * your_container_add: * @container: a container object * @element: (transfer full): the element to add * * Adds a new element to a container. */ void your_container_add (YourContainer *container, YourElement *element) { g_ptr_array_add (container->elements, element); } Convenience functions --------------------- GLib provides various convenience functions for memory management, especially for GObjects. Three will be covered here, but others exist — check the GLib API documentation for more. They typically follow similar naming schemas to these three (using ‘_full’ suffixes, or the verb ‘clear’ in the function name). ``g_clear_pointer()`` ~~~~~~~~~~~~~~~~~~~~~ ``g_clear_pointer()`` is a function which frees the contents of a pointer, using the given ``GDestroyNotify`` function, and then clears the pointer by setting it to ``NULL``. .. tip:: The C standard guarantees that ``free()`` is always ``NULL``-safe, and so does GLib for ``g_free()``. Nevertheless, there is no guarantee made for any other free function to be ``NULL``-safe, so it's recommended to use ``g_clear_pointer()`` to avoid use-after-free issues. ``g_clear_object()`` ~~~~~~~~~~~~~~~~~~~~ ``g_clear_object()`` is a version of ``g_object_unref()`` which unrefs a GObject and then clears the pointer to it to ``NULL``. This makes it easier to implement code that guarantees a GObject pointer is always either ``NULL``, or has ownership of a GObject (but which never points to a GObject it no longer owns). By initialising all owned GObject pointers to ``NULL``, freeing them at the end of the scope is as simple as calling ``g_clear_object()`` without any checks: .. code-block:: c void my_function (void) { GObject *some_object = NULL; /* owned */ if (rand ()) { some_object = create_new_object (); /* do something with the object */ } g_clear_object (&some_object); } ``g_list_free_full()`` and ``g_slist_free_full()`` ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ``g_list_free_full()`` and ``g_slist_free_full()`` free all the elements in a linked list, and all their data. It is much more convenient than iterating through the list to free all the elements’ data, then calling ``g_list_free()``, or ``g_slist_free()`` to free the list elements themselves. ``g_hash_table_new_full()`` ~~~~~~~~~~~~~~~~~~~~~~~~~~~ ``g_hash_table_new_full()`` is a version of ``g_hash_table_new()`` which allows setting functions to destroy each key and value in the hash table when they are removed. These functions are then automatically called for all keys and values when the hash table is destroyed, or when an entry is removed using ``g_hash_table_remove()``. Essentially, it simplifies memory management of keys and values to the question of whether they are present in the hash table. Similar functions exist for ``GPtrArray`` and ``GArray``: * ``g_ptr_array_new_with_free_func()`` * ``g_array_set_clear_func()`` Container types --------------- When using container types, such as ``GPtrArray`` or ``GList``, an additional level of ownership is introduced: as well as the ownership of the container instance, each element in the container is either owned or unowned too. By nesting containers, multiple levels of ownership must be tracked. Ownership of owned elements belongs to the container; ownership of the container belongs to the scope it’s in (which may be another container). A key principle for simplifying this is to ensure that all elements in a container have the same ownership: they are either all owned, or all unowned. This happens automatically if the normal convenience functions are used for types like ``GPtrArray`` and ``GHashTable``. If elements in a container are owned, adding them to the container is essentially an ownership transfer. For example, for an array of strings, if the elements are owned, the definition of ``g_ptr_array_add()`` is effectively: .. code-block:: c /** * g_ptr_array_add: * @array: a #GPtrArray * @str: (transfer full): string to add */ void g_ptr_array_add (GPtrArray *array, gchar *str); So, for example, constant (unowned) strings must be added to the array using ``g_ptr_array_add (array, g_strdup ("constant string"))``. Whereas if the elements are unowned, the definition is effectively: .. code-block:: c /** * g_ptr_array_add: * @array: a #GPtrArray * @str: (transfer none): string to add */ void g_ptr_array_add (GPtrArray *array, const gchar *str); Here, constant strings can be added without copying them: ``g_ptr_array_add (array, "constant string")``. .. _single-path-cleanup: Single-path cleanup ------------------- A useful design pattern for more complex functions is to have a single control path which cleans up (frees) allocations and returns to the caller. This vastly simplifies tracking of allocations, as it’s no longer necessary to mentally work out which allocations have been freed on each code path — all code paths end at the same point, so perform all the frees then. The benefits of this approach rapidly become greater for larger functions with more owned local variables; it may not make sense to apply the pattern to smaller functions. This approach has two requirements: #. The function returns from a single point, and uses ``goto`` to reach that point from other paths. #. All owned variables are set to ``NULL`` when initialized or when ownership is transferred away from them. The example below is for a small function (for brevity), but should illustrate the principles for application of the pattern to larger functions: .. code-block:: c GObject * some_function (GError **error) { char *some_str = NULL; /* owned */ GObject *temp_object = NULL; /* owned */ const char *temp_str; GObject *my_object = NULL; /* owned */ GError *child_error = NULL; /* owned */ temp_object = generate_object (); temp_str = "example string"; if (rand ()) { some_str = g_strconcat (temp_str, temp_str, NULL); } else { some_operation_which_might_fail (&child_error); if (child_error != NULL) goto done; my_object = generate_wrapped_object (temp_object); } done: /* Here, @some_str is either NULL or a string to be freed, so can be passed to * g_free() unconditionally. * * Similarly, @temp_object is either NULL or an object to be unreffed, so can * be passed to g_clear_object() unconditionally. */ g_free (some_str); g_clear_object (&temp_object); /* The pattern can also be used to ensure that the function always returns * either an error or a return value (but never both). */ if (child_error != NULL) { g_propagate_error (error, child_error); g_clear_object (&my_object); } return my_object; } .. tip:: If you are writing code that targets GCC or other GCC-compatible compilers and will not be ported to other platforms, you can avoid the use of ``goto`` by using the ``g_autoptr`` macro that GLib `provides `__. Verification ------------ Memory leaks can be checked for in two ways: static analysis, and runtime leak checking. Static analysis with tools like Coverity, the Clang static analyzer or `Tartan `__ can catch some leaks, but require knowledge of the ownership transfer of every function called in the code. Domain-specific static analyzers like Tartan (which knows about GLib memory allocation and transfer) can perform better here, but Tartan is quite a young project and still misses things (a low true positive rate). It is recommended that code be put through a static analyzer, but the primary tool for detecting leaks should be runtime leak checking. Runtime leak checking is done using Valgrind, using its memcheck tool. Any leak it detects as ‘definitely losing memory’ should be fixed. Many of the leaks which ‘potentially’ lose memory are not real leaks, and should be added to the suppression file. If compiling with a recent version of Clang or GCC, the address sanitizer can be enabled instead, and it will detect memory leaks and overflow problems at runtime, but without the difficulty of running Valgrind in the right environment. Note, however, that it is still a young tool, so may fail in some cases.