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Memory Allocation Hooks

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 hook variables are declared in malloc.h.

__malloc_hook Variable
The value of this variable is a pointer to the function that malloc uses whenever it is called. You should define this function to look like malloc; that is, like: 
void *function (size_t size, const void *caller)

The value of caller is the return address found on the stack when the malloc function was called. This value allows you to trace the memory consumption of the program.

__realloc_hook Variable
The value of this variable is a pointer to function that realloc uses whenever it is called. You should define this function to look like realloc; that is, like: 
void *function (void *ptr, size_t size, const void *caller)

The value of caller is the return address found on the stack when the realloc function was called. This value allows you to trace the memory consumption of the program.

__free_hook Variable
The value of this variable is a pointer to function that free uses whenever it is called. You should define this function to look like free; that is, like: 
void function (void *ptr, const void *caller)

The value of caller is the return address found on the stack when the free function was called. This value allows you to trace the memory consumption of the program.

__memalign_hook Variable
The value of this variable is a pointer to function that memalign uses whenever it is called. You should define this function to look like memalign; that is, like: 
void *function (size_t size, size_t alignment, const void *caller)

The value of caller is the return address found on the stack when the memalign function was called. This value allows you to trace the memory consumption of the program.

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_initialize_hook Variable
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: 
void (*__malloc_initialize_hook) (void) = my_init_hook;

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 __malloc_hook 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 before the very first malloc call has completed, because otherwise a chunk obtained from the ordinary, un-hooked malloc may later be handed to __free_hook, for example.

In both cases, the problem can be solved by setting up the hooks from within a user-defined function pointed to by __malloc_initialize_hook--then the hooks will be set up safely at the right time.

Here is an example showing how to use __malloc_hook and __free_hook properly. It installs a function that prints out information every time malloc or free is called. We just assume here that realloc and memalign are not used in our program. 

/* 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;
  /* printf might call malloc, so protect it too. */
  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;
  /* printf might call free, so protect it too. */
  printf ("freed pointer %p\n", ptr);
  /* Restore our own hooks */
  __malloc_hook = my_malloc_hook;
  __free_hook = my_free_hook;
}

main ()
{
  ...
}

The mcheck function (see Heap Consistency Checking) works by installing such hooks.