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 malloc(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.

NOTES

       The use of these hook functions is not safe in multithreaded programs, and they are now deprecated.  From
       glibc 2.24 onwards, the __malloc_initialize_hook variable has been removed  from  the  API.   Programmers
       should  instead  preempt  calls  to  the  relevant  functions by defining and exporting functions such as
       "malloc" and "free".

EXAMPLES

       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(%zu) called from %p returns %p\n",
                   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

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