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| <h5 class="subsubsection">3.2.2.10 Memory Allocation Hooks</h5> |
| |
| <p><a name="index-allocation-hooks_002c-for-_0040code_007bmalloc_007d-285"></a> |
| The GNU C library lets you modify the behavior of <code>malloc</code>, |
| <code>realloc</code>, and <code>free</code> by specifying appropriate hook |
| functions. You can use these hooks to help you debug programs that use |
| dynamic memory allocation, for example. |
| |
| <p>The hook variables are declared in <samp><span class="file">malloc.h</span></samp>. |
| <a name="index-malloc_002eh-286"></a> |
| <!-- malloc.h --> |
| <!-- GNU --> |
| |
| <div class="defun"> |
| — Variable: <b>__malloc_hook</b><var><a name="index-g_t_005f_005fmalloc_005fhook-287"></a></var><br> |
| <blockquote><p>The value of this variable is a pointer to the function that |
| <code>malloc</code> uses whenever it is called. You should define this |
| function to look like <code>malloc</code>; that is, like: |
| |
| <pre class="smallexample"> void *<var>function</var> (size_t <var>size</var>, const void *<var>caller</var>) |
| </pre> |
| <p>The value of <var>caller</var> is the return address found on the stack when |
| the <code>malloc</code> function was called. This value allows you to trace |
| the memory consumption of the program. |
| </p></blockquote></div> |
| |
| <!-- malloc.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Variable: <b>__realloc_hook</b><var><a name="index-g_t_005f_005frealloc_005fhook-288"></a></var><br> |
| <blockquote><p>The value of this variable is a pointer to function that <code>realloc</code> |
| uses whenever it is called. You should define this function to look |
| like <code>realloc</code>; that is, like: |
| |
| <pre class="smallexample"> void *<var>function</var> (void *<var>ptr</var>, size_t <var>size</var>, const void *<var>caller</var>) |
| </pre> |
| <p>The value of <var>caller</var> is the return address found on the stack when |
| the <code>realloc</code> function was called. This value allows you to trace the |
| memory consumption of the program. |
| </p></blockquote></div> |
| |
| <!-- malloc.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Variable: <b>__free_hook</b><var><a name="index-g_t_005f_005ffree_005fhook-289"></a></var><br> |
| <blockquote><p>The value of this variable is a pointer to function that <code>free</code> |
| uses whenever it is called. You should define this function to look |
| like <code>free</code>; that is, like: |
| |
| <pre class="smallexample"> void <var>function</var> (void *<var>ptr</var>, const void *<var>caller</var>) |
| </pre> |
| <p>The value of <var>caller</var> is the return address found on the stack when |
| the <code>free</code> function was called. This value allows you to trace the |
| memory consumption of the program. |
| </p></blockquote></div> |
| |
| <!-- malloc.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Variable: <b>__memalign_hook</b><var><a name="index-g_t_005f_005fmemalign_005fhook-290"></a></var><br> |
| <blockquote><p>The value of this variable is a pointer to function that <code>memalign</code> |
| uses whenever it is called. You should define this function to look |
| like <code>memalign</code>; that is, like: |
| |
| <pre class="smallexample"> void *<var>function</var> (size_t <var>alignment</var>, size_t <var>size</var>, const void *<var>caller</var>) |
| </pre> |
| <p>The value of <var>caller</var> is the return address found on the stack when |
| the <code>memalign</code> function was called. This value allows you to trace the |
| memory consumption of the program. |
| </p></blockquote></div> |
| |
| <p>You must make sure that the function you install as a hook for one of |
| these functions does not call that function recursively without restoring |
| the old value of the hook first! Otherwise, your program will get stuck |
| in an infinite recursion. Before calling the function recursively, one |
| should make sure to restore all the hooks to their previous value. When |
| coming back from the recursive call, all the hooks should be resaved |
| since a hook might modify itself. |
| |
| <!-- malloc.h --> |
| <!-- GNU --> |
| <div class="defun"> |
| — Variable: <b>__malloc_initialize_hook</b><var><a name="index-g_t_005f_005fmalloc_005finitialize_005fhook-291"></a></var><br> |
| <blockquote><p>The value of this variable is a pointer to a function that is called |
| once when the malloc implementation is initialized. This is a weak |
| variable, so it can be overridden in the application with a definition |
| like the following: |
| |
| <pre class="smallexample"> void (*<var>__malloc_initialize_hook</var>) (void) = my_init_hook; |
| </pre> |
| </blockquote></div> |
| |
| <p>An issue to look out for is the time at which the malloc hook functions |
| can be safely installed. If the hook functions call the malloc-related |
| functions recursively, it is necessary that malloc has already properly |
| initialized itself at the time when <code>__malloc_hook</code> etc. is |
| assigned to. On the other hand, if the hook functions provide a |
| complete malloc implementation of their own, it is vital that the hooks |
| are assigned to <em>before</em> the very first <code>malloc</code> call has |
| completed, because otherwise a chunk obtained from the ordinary, |
| un-hooked malloc may later be handed to <code>__free_hook</code>, for example. |
| |
| <p>In both cases, the problem can be solved by setting up the hooks from |
| within a user-defined function pointed to by |
| <code>__malloc_initialize_hook</code>—then the hooks will be set up safely |
| at the right time. |
| |
| <p>Here is an example showing how to use <code>__malloc_hook</code> and |
| <code>__free_hook</code> properly. It installs a function that prints out |
| information every time <code>malloc</code> or <code>free</code> is called. We just |
| assume here that <code>realloc</code> and <code>memalign</code> are not used in our |
| program. |
| |
| <pre class="smallexample"> /* Prototypes for __malloc_hook, __free_hook */ |
| #include <malloc.h> |
| |
| /* Prototypes for our hooks. */ |
| static void my_init_hook (void); |
| static void *my_malloc_hook (size_t, const void *); |
| static void my_free_hook (void*, const void *); |
| |
| /* Override initializing hook from the C library. */ |
| void (*__malloc_initialize_hook) (void) = my_init_hook; |
| |
| static void |
| my_init_hook (void) |
| { |
| old_malloc_hook = __malloc_hook; |
| old_free_hook = __free_hook; |
| __malloc_hook = my_malloc_hook; |
| __free_hook = my_free_hook; |
| } |
| |
| static void * |
| my_malloc_hook (size_t size, const void *caller) |
| { |
| void *result; |
| /* Restore all old hooks */ |
| __malloc_hook = old_malloc_hook; |
| __free_hook = old_free_hook; |
| /* Call recursively */ |
| result = malloc (size); |
| /* Save underlying hooks */ |
| old_malloc_hook = __malloc_hook; |
| old_free_hook = __free_hook; |
| /* <code>printf</code><span class="roman"> might call </span><code>malloc</code><span class="roman">, so protect it too.</span> */ |
| printf ("malloc (%u) returns %p\n", (unsigned int) size, result); |
| /* Restore our own hooks */ |
| __malloc_hook = my_malloc_hook; |
| __free_hook = my_free_hook; |
| return result; |
| } |
| |
| static void |
| my_free_hook (void *ptr, const void *caller) |
| { |
| /* Restore all old hooks */ |
| __malloc_hook = old_malloc_hook; |
| __free_hook = old_free_hook; |
| /* Call recursively */ |
| free (ptr); |
| /* Save underlying hooks */ |
| old_malloc_hook = __malloc_hook; |
| old_free_hook = __free_hook; |
| /* <code>printf</code><span class="roman"> might call </span><code>free</code><span class="roman">, so protect it too.</span> */ |
| printf ("freed pointer %p\n", ptr); |
| /* Restore our own hooks */ |
| __malloc_hook = my_malloc_hook; |
| __free_hook = my_free_hook; |
| } |
| |
| main () |
| { |
| ... |
| } |
| </pre> |
| <p>The <code>mcheck</code> function (see <a href="Heap-Consistency-Checking.html#Heap-Consistency-Checking">Heap Consistency Checking</a>) works by |
| installing such hooks. |
| |
| <!-- __morecore, __after_morecore_hook are undocumented --> |
| <!-- It's not clear whether to document them. --> |
| </body></html> |
| |
