#include <malloc.h>
void *(*volatile __malloc_hook)(size_t size, const void *caller);
void *(*volatile __realloc_hook)(void *ptr, size_t size, const void *caller);
void *(*volatile __memalign_hook)(size_t alignment, size_t size,
const void *caller);
void (*volatile __free_hook)(void *ptr, const void *caller);
void (*__malloc_initialize_hook)(void);
void (*volatile __after_morecore_hook)(void);
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 to by __malloc_hook, __realloc_hook, __memalign_hook, __free_hook have a prototype like the functions malloc(3), realloc(3), memalign(3), free(3), respectively, except that they have a final argument caller that gives the address of the caller of malloc(3), etc.
The variable __after_morecore_hook points at a function that is called each time after sbrk(2) was asked for more memory.
#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 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;
__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(%zu) called from %p returns %p\n",
size, caller, result);
/* Restore our own hooks */
__malloc_hook = my_malloc_hook;