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__malloc_hook, __malloc_initialize_hook, __memalign_hook, __free_hook, __realloc_hook, __after_morecore_hook - malloc debugging variables




"#include <malloc.h>"

"void *(*__malloc_hook)(size_t "size, "const void *"caller);

"void *(*__realloc_hook)(void *"ptr, "size_t "size, "const void *"caller);

"void *(*__memalign_hook)(size_t "alignment, "size_t "size, "const void *"caller);

"void (*__free_hook)(void *"ptr, "const void *"caller);

"void (*__malloc_initialize_hook)(void);"

"void (*__after_morecore_hook)(void);"


The GNU C library lets you modify the behavior of malloc(), realloc(), and free() by specifying appropriate hook functions. You can use these hooks to help you debug programs that use dynamic memory allocation, for example.

The variable __malloc_initialize_hook points at 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:

void (*__malloc_initialize_hook)(void) = my_init_hook;

Now the function my_init_hook() can do the initialization of all hooks.

The four functions pointed at by __malloc_hook, __realloc_hook, __memalign_hook, __free_hook have a prototype like the functions malloc(), realloc(), memalign(), free(), respectively, except that they have a final argument caller that gives the address of the caller of malloc(), etc.

The variable __after_morecore_hook points at a function that is called each time after sbrk() was asked for more core.


Here a short example how to use these variables.

#include <stdio.h>
#include <malloc.h>

/* Prototypes for our hooks. */
static void my_init_hook(void);
static void *my_malloc_hook(size_t, const void *);

/* Variables to save original hooks. */
static void *(*old_malloc_hook)(size_t, const void *);

/* Override initialising hook from the C library. */
void (*__malloc_initialize_hook) (void) = my_init_hook;

static void
my_init_hook(void) {
old_malloc_hook = __malloc_hook;
__malloc_hook = my_malloc_hook;

static void *
my_malloc_hook (size_t size, const void *caller) {
void *result;

/* Restore all old hooks */
__malloc_hook = old_malloc_hook;

/* Call recursively */
result = malloc (size);

/* Save underlying hooks */
old_malloc_hook = __malloc_hook;

/* ‘printf’ might call ‘malloc’, so protect it too. */
printf ("malloc(%u) called from %p returns %p0,
(unsigned int) size, caller, result);

/* Restore our own hooks */
__malloc_hook = my_malloc_hook;

return result;


mallinfo(3), malloc(3), mtrace(3), mcheck(3)

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