glib/docs/reference/glib/tmpl/memory_chunks.sgml

<!-- ##### SECTION Title ##### -->
Memory Chunks

<!-- ##### SECTION Short_Description ##### -->
efficient way to allocate groups of equal-sized chunks of memory.

<!-- ##### SECTION Long_Description ##### -->
<para>
Memory chunks provide an efficient way to allocate equal-sized pieces of
memory, called atoms. They are used extensively within GLib itself.
For example, the
<link linkend="glib-Doubly-Linked-lists">Doubly Linked Lists</link>
use memory chunks to allocate space for elements of the lists.
</para>
<para>
There are two types of memory chunks, #G_ALLOC_ONLY, and #G_ALLOC_AND_FREE.
<itemizedlist>
<listitem><para>
#G_ALLOC_ONLY chunks only allow allocation of atoms. The atoms can never
be freed individually. The memory chunk can only be free in its entirety.
</para></listitem>
<listitem><para>
#G_ALLOC_AND_FREE chunks do allow atoms to be freed individually.
The disadvantage of this is that the memory chunk has to keep track of which
atoms have been freed. This results in more memory being used and a slight
degradation in performance.
</para></listitem>

</itemizedlist>
</para>
<para>
To create a memory chunk use g_mem_chunk_new() or the convenience macro
g_mem_chunk_create().
</para>
<para>
To allocate a new atom use g_mem_chunk_alloc(), g_mem_chunk_alloc0(),
or the convenience macros g_chunk_new() or g_chunk_new0(). 
</para>
<para>
To free an atom use g_mem_chunk_free(), or the convenience macro
g_chunk_free(). (Atoms can only be freed if the memory chunk is created
with the type set to #G_ALLOC_AND_FREE.)
</para>
<para>
To free any blocks of memory which are no longer being used, use
g_mem_chunk_clean(). To clean all memory chunks, use g_blow_chunks().
</para>
<para>
To reset the memory chunk, freeing all of the atoms, use g_mem_chunk_reset().
</para>
<para>
To destroy a memory chunk, use g_mem_chunk_destroy().
</para>
<para>
To help debug memory chunks, use g_mem_chunk_info() and g_mem_chunk_print().
</para>

<example>
<title>Using a <structname>GMemChunk</structname></title>
<programlisting>
  GMemChunk *mem_chunk;
  gchar *mem[10000];
  gint i;

  /* Create a GMemChunk with atoms 50 bytes long, and memory blocks holding
     100 bytes. Note that this means that only 2 atoms fit into each memory
     block and so isn't very efficient. */
  mem_chunk = g_mem_chunk_new ("test mem chunk", 50, 100, G_ALLOC_AND_FREE);

  /* Now allocate 10000 atoms. */
  for (i = 0; i &lt; 10000; i++)
    {
      mem[i] = g_chunk_new (gchar, mem_chunk);

      /* Fill in the atom memory with some junk. */
      for (j = 0; j &lt; 50; j++)
	mem[i][j] = i * j;
    }

  /* Now free all of the atoms. Note that since we are going to destroy the
     GMemChunk, this wouldn't normally be used. */
  for (i = 0; i &lt; 10000; i++)
    {
      g_mem_chunk_free (mem_chunk, mem[i]);
    }

  /* We are finished with the GMemChunk, so we destroy it. */
  g_mem_chunk_destroy (mem_chunk);
</programlisting></example>

<example>
<title>Using a <structname>GMemChunk</structname> with data structures</title>
<programlisting>
  GMemChunk *array_mem_chunk;
  GRealArray *array;

  /* Create a GMemChunk to hold GRealArray structures, using the
     g_mem_chunk_create(<!-- -->) convenience macro. We want 1024 atoms in each
     memory block, and we want to be able to free individual atoms. */
  array_mem_chunk = g_mem_chunk_create (GRealArray, 1024, G_ALLOC_AND_FREE);

  /* Allocate one atom, using the g_chunk_new(<!-- -->) convenience macro. */
  array = g_chunk_new (GRealArray, array_mem_chunk);

  /* We can now use array just like a normal pointer to a structure. */
  array->data            = NULL;
  array->len             = 0;
  array->alloc           = 0;
  array->zero_terminated = (zero_terminated ? 1 : 0);
  array->clear           = (clear ? 1 : 0);
  array->elt_size        = elt_size;

  /* We can free the element, so it can be reused. */
  g_chunk_free (array, array_mem_chunk);

  /* We destroy the GMemChunk when we are finished with it. */
  g_mem_chunk_destroy (array_mem_chunk);
</programlisting></example>

<!-- ##### SECTION See_Also ##### -->
<para>

</para>

<!-- ##### SECTION Stability_Level ##### -->


<!-- ##### STRUCT GMemChunk ##### -->
<para>
The #GMemChunk struct is an opaque data structure representing a memory
chunk. It should be accessed only through the use of the following functions.
</para>


<!-- ##### MACRO G_ALLOC_AND_FREE ##### -->
<para>
Specifies the type of a #GMemChunk.
Used in g_mem_chunk_new() and g_mem_chunk_create() to specify that atoms
will be freed individually.
</para>



<!-- ##### MACRO G_ALLOC_ONLY ##### -->
<para>
Specifies the type of a #GMemChunk.
Used in g_mem_chunk_new() and g_mem_chunk_create() to specify that atoms
will never be freed individually.
</para>



<!-- ##### FUNCTION g_mem_chunk_new ##### -->
<para>
Creates a new #GMemChunk.
</para>

@name: a string to identify the #GMemChunk. It is not copied so it
should be valid for the lifetime of the #GMemChunk. It is only used in
g_mem_chunk_print(), which is used for debugging.
@atom_size: the size, in bytes, of each element in the #GMemChunk.
@area_size: the size, in bytes, of each block of memory allocated to contain
the atoms.
@type: the type of the #GMemChunk.
#G_ALLOC_AND_FREE is used if the atoms will be freed individually.
#G_ALLOC_ONLY should be used if atoms will never be freed individually.
#G_ALLOC_ONLY is quicker, since it does not need to track free atoms,
but it obviously wastes memory if you no longer need many of the atoms.
@Returns: the new #GMemChunk.


<!-- ##### FUNCTION g_mem_chunk_alloc ##### -->
<para>
Allocates an atom of memory from a #GMemChunk.
</para>

@mem_chunk: a #GMemChunk.
@Returns: a pointer to the allocated atom.


<!-- ##### FUNCTION g_mem_chunk_alloc0 ##### -->
<para>
Allocates an atom of memory from a #GMemChunk, setting the memory to 0.
</para>

@mem_chunk: a #GMemChunk.
@Returns: a pointer to the allocated atom.


<!-- ##### FUNCTION g_mem_chunk_free ##### -->
<para>
Frees an atom in a #GMemChunk.
This should only be called if the #GMemChunk was created with
#G_ALLOC_AND_FREE. Otherwise it will simply return.
</para>

@mem_chunk: a #GMemChunk.
@mem: a pointer to the atom to free.


<!-- ##### FUNCTION g_mem_chunk_destroy ##### -->
<para>
Frees all of the memory allocated for a #GMemChunk.
</para>

@mem_chunk: a #GMemChunk.


<!-- ##### MACRO g_mem_chunk_create ##### -->
<para>
A convenience macro for creating a new #GMemChunk.
It calls g_mem_chunk_new(), using the given type to create the #GMemChunk
name. The atom size is determined using <function>sizeof()</function>, and the
area size is calculated by multiplying the @pre_alloc parameter with
the atom size.
</para>

@type: the type of the atoms, typically a structure name.
@pre_alloc: the number of atoms to store in each block of memory.
@alloc_type: the type of the #GMemChunk.
#G_ALLOC_AND_FREE is used if the atoms will be freed individually.
#G_ALLOC_ONLY should be used if atoms will never be freed individually.
#G_ALLOC_ONLY is quicker, since it does not need to track free atoms,
but it obviously wastes memory if you no longer need many of the atoms.
@Returns: the new #GMemChunk.


<!-- ##### MACRO g_chunk_new ##### -->
<para>
A convenience macro to allocate an atom of memory from a #GMemChunk.
It calls g_mem_chunk_alloc() and casts the returned atom to a pointer to
the given type, avoiding a type cast in the source code.
</para>

@type: the type of the #GMemChunk atoms, typically a structure name.
@chunk: a #GMemChunk.
@Returns: a pointer to the allocated atom, cast to a pointer to @type.


<!-- ##### MACRO g_chunk_new0 ##### -->
<para>
A convenience macro to allocate an atom of memory from a #GMemChunk.
It calls g_mem_chunk_alloc0() and casts the returned atom to a pointer to
the given type, avoiding a type cast in the source code.
</para>

@type: the type of the #GMemChunk atoms, typically a structure name.
@chunk: a #GMemChunk.
@Returns: a pointer to the allocated atom, cast to a pointer to @type.


<!-- ##### MACRO g_chunk_free ##### -->
<para>
A convenience macro to free an atom of memory from a #GMemChunk.
It simply switches the arguments and calls g_mem_chunk_free()
It is included simply to complement the other convenience macros, g_chunk_new()
and g_chunk_new0().
</para>

@mem: a pointer to the atom to be freed.
@mem_chunk: a #GMemChunk.


<!-- ##### FUNCTION g_mem_chunk_reset ##### -->
<para>
Resets a GMemChunk to its initial state.
It frees all of the currently allocated blocks of memory.
</para>

@mem_chunk: a #GMemChunk.


<!-- ##### FUNCTION g_mem_chunk_clean ##### -->
<para>
Frees any blocks in a #GMemChunk which are no longer being used.
</para>

@mem_chunk: a #GMemChunk.


<!-- ##### FUNCTION g_blow_chunks ##### -->
<para>
Calls g_mem_chunk_clean() on all #GMemChunk objects.
</para>



<!-- ##### FUNCTION g_mem_chunk_info ##### -->
<para>
Outputs debugging information for all #GMemChunk objects currently in use.
It outputs the number of #GMemChunk objects currently allocated,
and calls g_mem_chunk_print() to output information on each one.
</para>



<!-- ##### FUNCTION g_mem_chunk_print ##### -->
<para>
Outputs debugging information for a #GMemChunk.
It outputs the name of the #GMemChunk (set with g_mem_chunk_new()),
the number of bytes used, and the number of blocks of memory allocated.
</para>

@mem_chunk: a #GMemChunk.