MALLOC_HOOK

NAME

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 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;

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.

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

CONFORMING TO

EXAMPLE

#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\n",
            (unsigned int) size, caller, result);

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

    return result;
}

SEE ALSO

COLOPHON