MALLOC_HOOK(3)		   Linux Programmers Manual	       MALLOC_HOOK(3)



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

SYNOPSIS
       #include 

       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(3),  real
       loc(3),	and free(3) 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(3),
       realloc(3), memalign(3), free(3), respectively, except that they have a
       final argument caller that gives the address  of  the  caller  of  mal
       loc(3), etc.

       The  variable __after_morecore_hook points at a function that is called
       each time after sbrk(2) was asked for more memory.

CONFORMING TO
       These functions are GNU extensions.

EXAMPLE
       Here is a short example of how to use these variables.

       #include 
       #include 

       /* 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 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;
	   __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 %p\n",
		   (unsigned int) size, caller, result);

	   /* Restore our own hooks */
	   __malloc_hook = my_malloc_hook;

	   return result;
       }

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

COLOPHON
       This page is part of release 3.05 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.



GNU				  2002-07-20			MALLOC_HOOK(3)