__malloc_hook

MALLOC_HOOK(3)		  Linux Programmer’s Manual	       MALLOC_HOOK(3)



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

SYNOPSIS
       #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);

DESCRIPTION
       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 alloca-
       tion, 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 defi-
       nition 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 mal-
       loc(), 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.

EXAMPLE
       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;
       }

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



GNU				  2002-07-20		       MALLOC_HOOK(3)