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(3), realloc(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 definition 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 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 <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 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
",
		   (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 3.27 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)