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(libc.info)Hooks for Malloc

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Memory Allocation Hooks
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   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 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.

 - 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 (Note: Heap Consistency Checking) works by
installing such hooks.