<refentry id="glib-Memory-Allocation">
<refmeta>
<refentrytitle>Memory Allocation</refentrytitle>
<manvolnum>3</manvolnum>
<refmiscinfo>GLIB Library</refmiscinfo>
</refmeta>
<refnamediv>
<refname>Memory Allocation</refname><refpurpose>general memory-handling.</refpurpose>
</refnamediv>
<refsynopsisdiv><title>Synopsis</title>
<synopsis>
#include <glib.h>
#define <link linkend="g-new">g_new</link> (struct_type, n_structs)
#define <link linkend="g-new0">g_new0</link> (struct_type, n_structs)
#define <link linkend="g-renew">g_renew</link> (struct_type, mem, n_structs)
<link linkend="gpointer">gpointer</link> <link linkend="g-malloc">g_malloc</link> (<link linkend="gulong">gulong</link> n_bytes);
<link linkend="gpointer">gpointer</link> <link linkend="g-malloc0">g_malloc0</link> (<link linkend="gulong">gulong</link> n_bytes);
<link linkend="gpointer">gpointer</link> <link linkend="g-realloc">g_realloc</link> (<link linkend="gpointer">gpointer</link> mem,
<link linkend="gulong">gulong</link> n_bytes);
<link linkend="gpointer">gpointer</link> <link linkend="g-try-malloc">g_try_malloc</link> (<link linkend="gulong">gulong</link> n_bytes);
<link linkend="gpointer">gpointer</link> <link linkend="g-try-realloc">g_try_realloc</link> (<link linkend="gpointer">gpointer</link> mem,
<link linkend="gulong">gulong</link> n_bytes);
<link linkend="void">void</link> <link linkend="g-free">g_free</link> (<link linkend="gpointer">gpointer</link> mem);
#define <link linkend="g-alloca">g_alloca</link> (size)
#define <link linkend="g-newa">g_newa</link> (struct_type, n_structs)
#define <link linkend="g-memmove">g_memmove</link> (d,s,n)
<link linkend="gpointer">gpointer</link> <link linkend="g-memdup">g_memdup</link> (<link linkend="gconstpointer">gconstpointer</link> mem,
<link linkend="guint">guint</link> byte_size);
struct <link linkend="GMemVTable">GMemVTable</link>;
<link linkend="void">void</link> <link linkend="g-mem-set-vtable">g_mem_set_vtable</link> (<link linkend="GMemVTable">GMemVTable</link> *vtable);
<link linkend="gboolean">gboolean</link> <link linkend="g-mem-is-system-malloc">g_mem_is_system_malloc</link> (void);
extern GMemVTable *<link linkend="glib-mem-profiler-table">glib_mem_profiler_table</link>;
<link linkend="void">void</link> <link linkend="g-mem-profile">g_mem_profile</link> (void);
</synopsis>
</refsynopsisdiv>
<refsect1>
<title>Description</title>
<para>
These functions provide support for allocating and freeing memory.
</para>
<note>
<para>
If any call to allocate memory fails, the application is terminated.
This also means that there is no need to check if the call succeeded.
</para>
</note>
</refsect1>
<refsect1>
<title>Details</title>
<refsect2>
<title><anchor id="g-new"/>g_new()</title>
<indexterm><primary>g_new</primary></indexterm><programlisting>#define g_new(struct_type, n_structs)</programlisting>
<para>
Allocates <parameter>n_structs</parameter> elements of type <parameter>struct_type</parameter>.
The returned pointer is cast to a pointer to the given type.
If <parameter>count</parameter> is 0 it returns <literal>NULL</literal>.
</para><variablelist role="params">
<varlistentry><term><parameter>struct_type</parameter> :</term>
<listitem><simpara>the type of the elements to allocate.
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>n_structs</parameter> :</term>
<listitem><simpara>the number of elements to allocate.
</simpara></listitem></varlistentry>
<varlistentry><term><emphasis>Returns</emphasis> :</term><listitem><simpara>a pointer to the allocated memory, cast to a pointer to <parameter>struct_type</parameter>.
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-new0"/>g_new0()</title>
<indexterm><primary>g_new0</primary></indexterm><programlisting>#define g_new0(struct_type, n_structs)</programlisting>
<para>
Allocates <parameter>n_structs</parameter> elements of type <parameter>struct_type</parameter>, initialized to 0's.
The returned pointer is cast to a pointer to the given type.
If <parameter>count</parameter> is 0 it returns <literal>NULL</literal>.
</para><variablelist role="params">
<varlistentry><term><parameter>struct_type</parameter> :</term>
<listitem><simpara>the type of the elements to allocate.
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>n_structs</parameter> :</term>
<listitem><simpara>the number of elements to allocate.
</simpara></listitem></varlistentry>
<varlistentry><term><emphasis>Returns</emphasis> :</term><listitem><simpara>a pointer to the allocated memory, cast to a pointer to <parameter>struct_type</parameter>.
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-renew"/>g_renew()</title>
<indexterm><primary>g_renew</primary></indexterm><programlisting>#define g_renew(struct_type, mem, n_structs)</programlisting>
<para>
Reallocates the memory pointed to by <parameter>mem</parameter>, so that it now has space for
<parameter>n_struct</parameter> elements of type <parameter>struct_type</parameter>. It returns the new address of
the memory, which may have been moved.
</para><variablelist role="params">
<varlistentry><term><parameter>struct_type</parameter> :</term>
<listitem><simpara>the type of the elements to allocate.
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>mem</parameter> :</term>
<listitem><simpara>the currently allocated memory.
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>n_structs</parameter> :</term>
<listitem><simpara>the number of elements to allocate.
</simpara></listitem></varlistentry>
<varlistentry><term><emphasis>Returns</emphasis> :</term><listitem><simpara>a pointer to the new allocated memory, cast to a pointer to <parameter>struct_type</parameter>.
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-malloc"/>g_malloc ()</title>
<indexterm><primary>g_malloc</primary></indexterm><programlisting><link linkend="gpointer">gpointer</link> g_malloc (<link linkend="gulong">gulong</link> n_bytes);</programlisting>
<para>
Allocates <parameter>n_bytes</parameter> bytes of memory.
If <parameter>n_bytes</parameter> is 0 it returns <literal>NULL</literal>.
</para><variablelist role="params">
<varlistentry><term><parameter>n_bytes</parameter> :</term>
<listitem><simpara>the number of bytes to allocate.
</simpara></listitem></varlistentry>
<varlistentry><term><emphasis>Returns</emphasis> :</term><listitem><simpara>a pointer to the allocated memory.
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-malloc0"/>g_malloc0 ()</title>
<indexterm><primary>g_malloc0</primary></indexterm><programlisting><link linkend="gpointer">gpointer</link> g_malloc0 (<link linkend="gulong">gulong</link> n_bytes);</programlisting>
<para>
Allocates <parameter>n_bytes</parameter> bytes of memory, initialized to 0's.
If <parameter>n_bytes</parameter> is 0 it returns <literal>NULL</literal>.
</para><variablelist role="params">
<varlistentry><term><parameter>n_bytes</parameter> :</term>
<listitem><simpara>the number of bytes to allocate.
</simpara></listitem></varlistentry>
<varlistentry><term><emphasis>Returns</emphasis> :</term><listitem><simpara>a pointer to the allocated memory.
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-realloc"/>g_realloc ()</title>
<indexterm><primary>g_realloc</primary></indexterm><programlisting><link linkend="gpointer">gpointer</link> g_realloc (<link linkend="gpointer">gpointer</link> mem,
<link linkend="gulong">gulong</link> n_bytes);</programlisting>
<para>
Reallocates the memory pointed to by <parameter>mem</parameter>, so that it now has space for
<parameter>n_bytes</parameter> bytes of memory. It returns the new address of the memory, which may
have been moved. <parameter>mem</parameter> may be <literal>NULL</literal>, in which case it's considered to
have zero-length. <parameter>n_bytes</parameter> may be 0, in which case <literal>NULL</literal> will be returned.
</para><variablelist role="params">
<varlistentry><term><parameter>mem</parameter> :</term>
<listitem><simpara>the memory to reallocate.
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>n_bytes</parameter> :</term>
<listitem><simpara>new size of the memory in bytes.
</simpara></listitem></varlistentry>
<varlistentry><term><emphasis>Returns</emphasis> :</term><listitem><simpara>the new address of the allocated memory.
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-try-malloc"/>g_try_malloc ()</title>
<indexterm><primary>g_try_malloc</primary></indexterm><programlisting><link linkend="gpointer">gpointer</link> g_try_malloc (<link linkend="gulong">gulong</link> n_bytes);</programlisting>
<para>
Attempts to allocate <parameter>n_bytes</parameter>, and returns <literal>NULL</literal> on failure.
Contrast with <link linkend="g-malloc"><function>g_malloc()</function></link>, which aborts the program on failure.
</para><variablelist role="params">
<varlistentry><term><parameter>n_bytes</parameter> :</term>
<listitem><simpara>number of bytes to allocate.
</simpara></listitem></varlistentry>
<varlistentry><term><emphasis>Returns</emphasis> :</term><listitem><simpara>the allocated memory, or <literal>NULL</literal>.
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-try-realloc"/>g_try_realloc ()</title>
<indexterm><primary>g_try_realloc</primary></indexterm><programlisting><link linkend="gpointer">gpointer</link> g_try_realloc (<link linkend="gpointer">gpointer</link> mem,
<link linkend="gulong">gulong</link> n_bytes);</programlisting>
<para>
Attempts to realloc <parameter>mem</parameter> to a new size, <parameter>n_bytes</parameter>, and returns <literal>NULL</literal>
on failure. Contrast with <link linkend="g-realloc"><function>g_realloc()</function></link>, which aborts the program
on failure. If <parameter>mem</parameter> is <literal>NULL</literal>, behaves the same as <link linkend="g-try-malloc"><function>g_try_malloc()</function></link>.
</para><variablelist role="params">
<varlistentry><term><parameter>mem</parameter> :</term>
<listitem><simpara>previously-allocated memory, or <literal>NULL</literal>.
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>n_bytes</parameter> :</term>
<listitem><simpara>number of bytes to allocate.
</simpara></listitem></varlistentry>
<varlistentry><term><emphasis>Returns</emphasis> :</term><listitem><simpara>the allocated memory, or <literal>NULL</literal>.
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-free"/>g_free ()</title>
<indexterm><primary>g_free</primary></indexterm><programlisting><link linkend="void">void</link> g_free (<link linkend="gpointer">gpointer</link> mem);</programlisting>
<para>
Frees the memory pointed to by <parameter>mem</parameter>.
If <parameter>mem</parameter> is <literal>NULL</literal> it simply returns.
</para><variablelist role="params">
<varlistentry><term><parameter>mem</parameter> :</term>
<listitem><simpara>the memory to free.
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-alloca"/>g_alloca()</title>
<indexterm><primary>g_alloca</primary></indexterm><programlisting>#define g_alloca(size)</programlisting>
<para>
Allocates <parameter>size</parameter> bytes on the stack; these bytes will be freed when the current
stack frame is cleaned up. This macro essentially just wraps the
<function><link linkend="alloca"><function>alloca()</function></link></function> function present on most UNIX variants.
Thus it provides the same advantages and pitfalls as <function><link linkend="alloca"><function>alloca()</function></link></function>:
<variablelist>
<varlistentry><term></term><listitem><para>
+ <function><link linkend="alloca"><function>alloca()</function></link></function> is very fast, as on most systems it's implemented by just adjusting
the stack pointer register.
</para></listitem></varlistentry>
<varlistentry><term></term><listitem><para>
+ It doesn't cause any memory fragmentation, within its scope, separate <function><link linkend="alloca"><function>alloca()</function></link></function>
blocks just build up and are released together at function end.
</para></listitem></varlistentry>
<varlistentry><term></term><listitem><para>
- Allocation sizes have to fit into the current stack frame. For instance in a
threaded environment on Linux, the per-thread stack size is limited to 2 Megabytes,
so be sparse with <function><link linkend="alloca"><function>alloca()</function></link></function> uses.
</para></listitem></varlistentry>
<varlistentry><term></term><listitem><para>
- Allocation failure due to insufficient stack space is not indicated with a <literal>NULL</literal>
return like e.g. with <function><link linkend="malloc"><function>malloc()</function></link></function>. Instead, most systems probably handle it the same
way as out of stack space situations from infinite function recursion, i.e.
with a segmentation fault.
</para></listitem></varlistentry>
<varlistentry><term></term><listitem><para>
- Special care has to be taken when mixing <function><link linkend="alloca"><function>alloca()</function></link></function> with GNU C variable sized arrays.
Stack space allocated with <function><link linkend="alloca"><function>alloca()</function></link></function> in the same scope as a variable sized array
will be freed together with the variable sized array upon exit of that scope, and
not upon exit of the enclosing function scope.
</para></listitem></varlistentry>
</variablelist>
</para><variablelist role="params">
<varlistentry><term><parameter>size</parameter> :</term>
<listitem><simpara> number of bytes to allocate.
</simpara></listitem></varlistentry>
<varlistentry><term><emphasis>Returns</emphasis> :</term><listitem><simpara>space for <parameter>size</parameter> bytes, allocated on the stack
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-newa"/>g_newa()</title>
<indexterm><primary>g_newa</primary></indexterm><programlisting>#define g_newa(struct_type, n_structs)</programlisting>
<para>
Wraps <link linkend="g-alloca"><function>g_alloca()</function></link> in a more typesafe manner.
</para><variablelist role="params">
<varlistentry><term><parameter>struct_type</parameter> :</term>
<listitem><simpara>Type of memory chunks to be allocated
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>n_structs</parameter> :</term>
<listitem><simpara> Number of chunks to be allocated
</simpara></listitem></varlistentry>
<varlistentry><term><emphasis>Returns</emphasis> :</term><listitem><simpara> Pointer to stack space for <parameter>n_structs</parameter> chunks of type <parameter>struct_type</parameter>
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-memmove"/>g_memmove()</title>
<indexterm><primary>g_memmove</primary></indexterm><programlisting>#define g_memmove(d,s,n)</programlisting>
<para>
Copies a block of memory <parameter>n</parameter> bytes long, from <parameter>s</parameter> to <parameter>d</parameter>.
The source and destination areas may overlap.
</para>
<para>
In order to use this function, you must include <filename>string.h</filename>
yourself, because this macro will typically simply resolve
to <function><link linkend="memmove"><function>memmove()</function></link></function> and GLib does not include <filename>string.h</filename> for you.
</para><variablelist role="params">
<varlistentry><term><parameter>d</parameter> :</term>
<listitem><simpara>the destination address to copy the bytes to.
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>s</parameter> :</term>
<listitem><simpara>the source address to copy the bytes from.
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>n</parameter> :</term>
<listitem><simpara>the number of bytes to copy.
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-memdup"/>g_memdup ()</title>
<indexterm><primary>g_memdup</primary></indexterm><programlisting><link linkend="gpointer">gpointer</link> g_memdup (<link linkend="gconstpointer">gconstpointer</link> mem,
<link linkend="guint">guint</link> byte_size);</programlisting>
<para>
Allocates <parameter>byte_size</parameter> bytes of memory, and copies <parameter>byte_size</parameter> bytes into it
from <parameter>mem</parameter>. If <parameter>mem</parameter> is <literal>NULL</literal> it returns <literal>NULL</literal>.
</para><variablelist role="params">
<varlistentry><term><parameter>mem</parameter> :</term>
<listitem><simpara>the memory to copy.
</simpara></listitem></varlistentry>
<varlistentry><term><parameter>byte_size</parameter> :</term>
<listitem><simpara>the number of bytes to copy.
</simpara></listitem></varlistentry>
<varlistentry><term><emphasis>Returns</emphasis> :</term><listitem><simpara>a pointer to the newly-allocated copy of the memory, or <literal>NULL</literal> if <parameter>mem</parameter>
is <literal>NULL</literal>.
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="GMemVTable"/>struct GMemVTable</title>
<indexterm><primary>GMemVTable</primary></indexterm><programlisting>struct GMemVTable {
gpointer (*malloc) (gsize n_bytes);
gpointer (*realloc) (gpointer mem,
gsize n_bytes);
void (*free) (gpointer mem);
/* optional; set to NULL if not used ! */
gpointer (*calloc) (gsize n_blocks,
gsize n_block_bytes);
gpointer (*try_malloc) (gsize n_bytes);
gpointer (*try_realloc) (gpointer mem,
gsize n_bytes);
};
</programlisting>
<para>
A set of functions used to perform memory allocation. The same <link linkend="GMemVTable"><type>GMemVTable</type></link> must
be used for all allocations in the same program; a call to <link linkend="g-mem-set-vtable"><function>g_mem_set_vtable()</function></link>,
if it exists, should be prior to any use of GLib.
</para><variablelist role="struct">
<varlistentry>
<term><link linkend="gpointer">gpointer</link> (*<structfield>malloc</structfield>) (gsize n_bytes)</term>
<listitem><simpara>function to use for allocating memory.
</simpara></listitem>
</varlistentry>
<varlistentry>
<term><link linkend="gpointer">gpointer</link> (*<structfield>realloc</structfield>) (gpointer mem,
gsize n_bytes)</term>
<listitem><simpara>function to use for reallocating memory.
</simpara></listitem>
</varlistentry>
<varlistentry>
<term><link linkend="void">void</link> (*<structfield>free</structfield>) (gpointer mem)</term>
<listitem><simpara>function to use to free memory.
</simpara></listitem>
</varlistentry>
<varlistentry>
<term><link linkend="gpointer">gpointer</link> (*<structfield>calloc</structfield>) (gsize n_blocks,
gsize n_block_bytes)</term>
<listitem><simpara>function to use for allocating zero-filled memory.
</simpara></listitem>
</varlistentry>
<varlistentry>
<term><link linkend="gpointer">gpointer</link> (*<structfield>try_malloc</structfield>) (gsize n_bytes)</term>
<listitem><simpara>function to use for allocating memory without a default error handler.
</simpara></listitem>
</varlistentry>
<varlistentry>
<term><link linkend="gpointer">gpointer</link> (*<structfield>try_realloc</structfield>) (gpointer mem,
gsize n_bytes)</term>
<listitem><simpara>function to use for reallocating memory without a default error handler.
</simpara></listitem>
</varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-mem-set-vtable"/>g_mem_set_vtable ()</title>
<indexterm><primary>g_mem_set_vtable</primary></indexterm><programlisting><link linkend="void">void</link> g_mem_set_vtable (<link linkend="GMemVTable">GMemVTable</link> *vtable);</programlisting>
<para>
Sets the <link linkend="GMemVTable"><type>GMemVTable</type></link> to use for memory allocation. You can use this to provide
custom memory allocation routines. <emphasis>This function must be called before using any other GLib functions.</emphasis> The <parameter>vtable</parameter> only needs to provide <function><link linkend="malloc"><function>malloc()</function></link></function>, <function><link linkend="realloc"><function>realloc()</function></link></function>, and <function><link linkend="free"><function>free()</function></link></function>
functions; GLib can provide default implementations of the others. The <function><link linkend="malloc"><function>malloc()</function></link></function>
and <function><link linkend="realloc"><function>realloc()</function></link></function> implementations should return <literal>NULL</literal> on failure, GLib will handle
error-checking for you. <parameter>vtable</parameter> is copied, so need not persist after this
function has been called.
</para><variablelist role="params">
<varlistentry><term><parameter>vtable</parameter> :</term>
<listitem><simpara>table of memory allocation routines.
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="g-mem-is-system-malloc"/>g_mem_is_system_malloc ()</title>
<indexterm><primary>g_mem_is_system_malloc</primary></indexterm><programlisting><link linkend="gboolean">gboolean</link> g_mem_is_system_malloc (void);</programlisting>
<para>
Checks whether the allocator used by <link linkend="g-malloc"><function>g_malloc()</function></link> is the system's
malloc implementation. If it returns <literal>TRUE</literal> memory allocated with
<link linkend="malloc"><function>malloc()</function></link> can be used interchangeable with memory allocated using <link linkend="g-malloc"><function>g_malloc()</function></link>.
This function is useful for avoiding an extra copy of allocated memory returned
by a non-GLib-based API.
</para>
<para>
A different allocator can be set using <link linkend="g-mem-set-vtable"><function>g_mem_set_vtable()</function></link>.</para>
<para>
</para><variablelist role="params">
<varlistentry><term><emphasis>Returns</emphasis> :</term><listitem><simpara> if <literal>TRUE</literal>, <link linkend="malloc"><function>malloc()</function></link> and <link linkend="g-malloc"><function>g_malloc()</function></link> can be mixed.
</simpara></listitem></varlistentry>
</variablelist></refsect2>
<refsect2>
<title><anchor id="glib-mem-profiler-table"/>glib_mem_profiler_table</title>
<indexterm><primary>glib_mem_profiler_table</primary></indexterm><programlisting>extern GMemVTable *glib_mem_profiler_table;
</programlisting>
<para>
A <link linkend="GMemVTable"><type>GMemVTable</type></link> containing profiling variants of the memory
allocation functions. Use them together with <link linkend="g-mem-profile"><function>g_mem_profile()</function></link>
in order to get information about the memory allocation pattern
of your program.
</para></refsect2>
<refsect2>
<title><anchor id="g-mem-profile"/>g_mem_profile ()</title>
<indexterm><primary>g_mem_profile</primary></indexterm><programlisting><link linkend="void">void</link> g_mem_profile (void);</programlisting>
<para>
Outputs a summary of memory usage.
</para>
<para>
It outputs the frequency of allocations of different sizes,
the total number of bytes which have been allocated,
the total number of bytes which have been freed,
and the difference between the previous two values, i.e. the number of bytes
still in use.
</para>
<para>
Note that this function will not output anything unless you have
previously installed the <link linkend="glib-mem-profiler-table"><type>glib_mem_profiler_table</type></link> with <link linkend="g-mem-set-vtable"><function>g_mem_set_vtable()</function></link>.
</para></refsect2>
</refsect1>
</refentry>