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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 allocation,
       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, __mema‐
       lign_hook, __free_hook have a prototype like  the  functions  malloc(),
       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 memory.

       Hook variables are not thread-safe so they are  deprecated  now.	  Pro‐
       grammers	 should	 instead  preempt  calls  to the relevant functions by
       defining and exporting functions like "malloc" and "free".

EXAMPLE
       Here is a short example of 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;
       }

CONFORMING TO
       These functions are GNU extensions.

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

GNU				  2002-07-20			MALLOC_HOOK(3)
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