Guidelines/Memory management

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Memory management

Apertis 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 users of Apertis are familiar with the idea of heap allocation of memory using malloc() and free(), and know of the GLib equivalents, g_malloc() and g_free().

Summary

There are three situations to avoid, in order of descending importance:

  1. Using memory after freeing it (use-after-free).
  2. Using memory before allocating it.
  3. Not freeing memory after allocating it (leaking).

Key principles, in no particular order:

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. 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 (see #Single-path cleanup).

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, given the functions:

gchar *generate_string (const gchar *template);
void print_string (const gchar *str);

the following code has been annotated to note where the ownership transfers happen:

gchar *my_str = NULL;  /* owned */
const gchar *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 the ownership status of each variable: because my_str is owned by the local scope, it has type gchar, whereas template is const to denote it is unowned. Similarly, the template parameter of generate_string() and the str parameter of print_string() are 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 GObjects and subclasses, which can never be const. It is only the case for strings and simple structs.)

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, e.g. g_strdup() for strings, or g_object_ref() for GObjects.

Assignments

Assigning from/to Owned destination Unowned destination
Owned source Copy or move the source to the destination.
owned_dest = copy (owned_src) or
owned_dest = owned_src; owned_src = NULL.
Pure assignment, assuming the unowned variable is not used after the owned one is freed.
unowned_dest = owned_src.
Unowned source Copy the source to the destination.
owned_dest = copy (unowned_src).
Pure assignment.
unowned_dest = unowned_src.

Function calls

Call from/to Transfer full parameter Transfer none parameter
Owned source Copy or move the source for the parameter.
function_call (copy (owned_src)) or
function_call (owned_src); owned_src = NULL.
Pure parameter passing.
function_call (owned_src).
Unowned source Copy the source for the parameter.
function_call (copy (unowned_src)).
Pure parameter passing.
function_call (unowned_src).

Function returns

Return from/to Transfer full return Transfer none return
Owned source Pure variable return.
return owned_src.
Invalid. The source needs to be freed, so the return value would use freed memory — a use-after-free error.
Unowned source Copy the source for the return.
return copy (unowned_src).
Pure variable passing.
return unowned_src.

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. If a function is not public API, write a documentation comment for it anyway and include the (transfer) annotations. By doing so, the gobject-introspection tools can also read the annotations and use them to correctly introspect the API.

Further documentation on introspection is available.

For example:

/**
 * g_value_take_string:
 * @value: (transfer none): an initialised #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.

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:

GPtrArray/*<owned gchar*>*/ *some_unowned_string_array;  /* unowned */
GPtrArray/*<owned gchar*>*/ *some_owned_string_array = NULL;  /* owned */
GPtrArray/*<unowned GObject*>*/ *some_owned_object_array = NULL;  /* owned */

Note also that owned variables should always be initialised so that freeing them is more convenient. See #Convenience functions.

Also note that some types, e.g. 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 (e.g. by calling g_object_ref()). An instance can be ‘copied’ by calling g_object_ref() again. Ownership can be freed with g_object_unref() — even though this may not actually finalise the instance, it frees the current scope’s ownership of that instance.

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()). Some types, like GHashTable, support both reference counting and explicit finalisation. 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.

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_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, as discussed in #Single-path cleanup:

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()

g_list_free_full() frees 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() to free the GList elements themselves.

g_hash_table_new_full()

g_hash_table_new_full() is a newer 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. See #Container types for a discussion on ownership of elements within container types.

A similar function exists for GPtrArray: g_ptr_array_new_with_free_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:

/**
 * 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:

/**
 * g_ptr_array_add:
 * @array: a #GPtrArray
 * @str: (transfer none): string to add
 */
void
g_ptr_array_add (GPtrArray *array,
                 const gchar *str);

and constant strings can be added without copying them: g_ptr_array_add (array, "constant string").

See the #Documentation section for examples of comments to add to variable definitions to annotate them with the element type and ownership.

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:

  1. The function returns from a single point, and uses goto to reach that point from other paths.
  2. All owned variables are set to NULL when initialised 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:

GObject *
some_function (GError **error)
{
  gchar *some_str = NULL;  /* owned */
  GObject *temp_object = NULL;  /* owned */
  const gchar *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;
}

Verification

Memory leaks can be checked for in two ways: static analysis, and runtime leak checking.

Static analysis with tools like Coverity or Tartan can catch some leaks, but require knowledge of the ownership transfer of every function called in the code. Domain-specific static analysers 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 analyser, 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.

See the tooling page for more information on using Valgrind.

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