##### 3.2.2.10 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`.

— Variable: __malloc_hook

> 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.

— Variable: __realloc_hook

> 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.

— Variable: __free_hook

> 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.

— Variable: __memalign_hook

> 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 alignment, size_t size, 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.

— Variable: __malloc_initialize_hook

> 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.